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Induction coil
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This article is about the type of transformer that produces high-voltage pulses. For the more general electrical component, see Inductor. For the high-frequency heating coil, see induction heating. For the device in telephone sets, see hybrid coil.
Antique induction coil used in schools, from around 1900, Bremerhaven, Germany
Induction coil showing construction, from 1920.
An induction coil or "spark coil" (archaically known as an inductorium or Ruhmkorff coil[1] after Heinrich Ruhmkorff) is a type of electrical transformer used to produce high-voltage pulses from a low-voltage direct current (DC) supply.[1][2] To create the flux changes necessary to induce voltage in the secondary coil, the direct current in the primary coil is repeatedly interrupted by a vibrating mechanical contact called an interrupter.[1] Invented in 1836 by Nicholas Callan, Charles Page and others,[1] the induction coil was the first type of transformer. It was widely used in x-ray machines,[1][3] spark-gap radio transmitters,[1][3] arc lighting and quack medical electrotherapy devices from the 1880s to the 1920s. Today its only common use is as the ignition coils in internal combustion engines and in physics education to demonstrate induction.
Contents
[hide]
1 Construction and function
1.1 The interrupter
1.2 Capacitor
1.3 Construction details
2 Mercury and electrolytic interrupters
3 History
4 See also
5 Footnotes
6 Further reading
7 External links
Construction and function[edit]
Schematic diagram
See schematic diagram. An induction coil consists of two coils of insulated copper wire wound around a common iron core (M).[1][4] One coil, called the primary winding (P), is made from relatively few (tens or hundreds) turns of coarse wire.[4] The other coil, the secondary winding, (S) typically consists of many (thousands) turns of fine wire.[1][4]
An electric current is passed through the primary, creating a magnetic field.[1][4] Because of the common core, most of the primary's magnetic field couples with the secondary winding.[citation needed] The primary behaves as an inductor, storing energy in the associated magnetic field. When the primary current is suddenly interrupted, the magnetic field rapidly collapses. This causes a high voltage pulse to be developed across the secondary terminals through electromagnetic induction. Because of the large number of turns in the secondary coil, the secondary voltage pulse is typically many thousands of volts. This voltage is often sufficient to cause an electric spark, to jump across an air gap (G) separating the secondary's output terminals. For this reason, induction coils were called spark coils.
The size of induction coils is usually specified by the length of spark it can produce; a '4 inch' (10 cm) induction coil is one that could produce a 4 inch spark. This form of specification was, though imprecise, until the advent of the cathode ray oscilloscope the most reliable measure of peak voltage of such asymmetric waveforms. The relationship between spark length and voltage is not linear:
4 inches (10 cm) = 110kV ; 8 inches (20 cm) = 150kV ; 12 inches (30 cm) = 190kV ; 16 inches (41 cm) = 230kV[5]
The interrupter[edit]
Without capacitor
With capacitor
Waveforms in the induction coil. i1 (blue ) is the current in the coil's primary winding, v2 (red ) is the voltage across the secondary. Not to common scale; v2 is much larger in bottom drawing.[dubious – discuss]
To operate the coil continually, the DC supply current must be repeatedly connected and disconnected to create the magnetic field changes needed for induction.[1] To do that, induction coils use a magnetically activated vibrating arm called an interrupter or break (A) to rapidly connect and break the current flowing into the primary coil.[1] The interrupter is mounted on the end of the coil next to the iron core. When the power is turned on, the increasing current in the primary coil produces an increasing magnetic field, the magnetic field attracts the interrupter's iron armature (A). After a time, the magnetic attraction overcomes the armature's spring force, and the armature begins to move. When the armature has moved far enough, the pair of contacts (K) in the primary circuit open and disconnect the primary current. Disconnecting the current causes the magnetic field to collapse and create the spark. Also, the collapsed field no longer attracts the armature, so the spring force accelerates the armature toward its initial position. A short time later the contacts reconnect, and the current starts building the magnetic field again. The whole process starts over and repeats many times per second. The secondary voltage v2 (red, left), is roughly proportional to the rate of change of primary current i1 (blue).
Opposite potentials are induced in the secondary when the interrupter 'breaks' the circuit and 'closes' the circuit. However, the current change in the primary is much more abrupt when the interrupter 'breaks'. When the contacts close, the current builds up slowly in the primary because the supply voltage has a limited ability to force current through the coil's inductance. In contrast, when the interrupter contacts open, the current falls to zero suddenly. So the pulse of voltage induced in the secondary at 'break' is much larger than the pulse induced at 'close', it is the 'break' that generates the coil's high voltage output.
Capacitor[edit]
An arc forms at the interrupter contacts at 'break' which consumes energy stored in the coil, slowing the rate of change of primary current, reducing the output voltage. To prevent this a capacitor (C) of 0.5 to 15 μF is connected across the contacts to increase the speed of switching on 'break', producing much higher voltages.[dubious – discuss] It also prevents damage to the contacts by the arc. The capacitor and primary winding together form a tuned circuit, so on break an oscillating decaying sinusoidal current flows in the primary.[dubious – discuss] This induces a sinusoidal voltage in the secondary. As a result: the high voltage output pulse at each break actually consists of a rapidly alternating series of positive and negative pulses (left) which decay rapidly to zero.[citation needed]
Construction details[edit]
To prevent the high voltages generated in the coil from breaking down the thin insulation and arcing between the secondary wires, the secondary coil uses special construction so as to avoid having wires carrying large voltage differences lying next to each other. In one widely used technique, the secondary coil is wound in many thin flat pancake-shaped sections (called "pies"), connected in series.[1] The primary coil is first wound on the iron core and insulated from the secondary with a thick paper or rubber coating.[1] Then each secondary subcoil is connected to the coil next to it and slid onto the iron core, insulated from adjoining coils with waxed cardboard disks. The voltage developed in each subcoil isn't large enough to jump between the wires in the subcoil.[1] Large voltages are only developed across many subcoils in series, which are too widely separated to arc over. To give the entire coil a final insulating coating, it is immersed in melted paraffin wax or rosin; the air evacuated to ensure there are no air bubbles left inside and the paraffin allowed to solidify, so the entire coil is encased in wax.
To prevent eddy currents, which cause energy losses, the iron core is made of a bundle of parallel iron wires, individually coated with shellac to insulate them electrically.[1] The eddy currents, which flow in loops in the core perpendicular to the magnetic axis, are blocked by the layers of insulation. The ends of the insulated primary coil often protruded several inches from either end of the secondary coil, to prevent arcs from the secondary to the primary or the core.
Mercury and electrolytic interrupters[edit]
(left) 3-electrode Wehnelt interrupter used in high power coils. (right) Mercury turbine interrupter. The motor turns the toothed wheel while a stream of mercury is sprayed on the teeth. By adjusting the wheel up and down the duty cycle of the primary current can be changed.
Although modern induction coils used for educational purposes all use the vibrating arm 'hammer' type interrupter described above, these were inadequate for powering the large induction coils used in spark-gap radio transmitters and x-ray machines around the turn of the 20th century. In powerful coils the high primary current created arcs at the interrupter contacts which quickly destroyed the contacts.[1] Also, since each "break" produces a pulse of voltage from the coil, the more breaks per second the greater the power output. Hammer interrupters were not capable of interruption rates over 200 breaks per second and the ones used on powerful coils were limited to 20 – 40 breaks per second.
Therefore much research went into improving interrupters and improved designs were used in high power coils, with the hammer interrupters only used on small coils under 8" sparks.[6] Léon Foucault and others developed interrupters consisting of an oscillating needle dipping into and out of a container of mercury.[1] The mercury was covered with a layer of spirits which extinguished the arc quickly, causing faster switching. These were often driven by a separate electromagnet or motor,[1] which allowed the interruption rate and "dwell" time to be adjusted separately from the primary current.
The largest coils used either electrolytic or mercury turbine interrupters.[1] The electrolytic or Wehnelt interrupter, invented by Arthur Wehnelt in 1899, consisted of a short platinum needle anode immersed in an electrolyte of dilute sulfuric acid, with the other side of the circuit connected to a lead plate cathode.[1][7] When the primary current passed through it, hydrogen gas bubbles formed on the needle which repeatedly broke the circuit. This resulted in a primary current broken randomly at rates up to 2000 breaks per second. They were preferred for powering X-ray tubes. They produced a lot of heat and due to the hydrogen could explode. Mercury turbine interrupters had a centrifugal pump which sprayed a stream of liquid mercury onto rotating metal contacts.[1] They could achieve interruption rates up to 10,000 breaks per second and were the most widely used type of interrupter in commercial wireless stations.[1][7]
History[edit]
Early coil by William Sturgeon, 1837. The sawtooth zinc interrupter wheel (D) was turned by hand. The first coil to use a divided core of iron wires (F) to prevent eddy currents.
Early coil by Charles G. Page, 1838, had one of the first automatic interrupters. The cup was filled with mercury. The magnetic field attracted the iron piece on the arm (left), lifting the wire out of the cup, breaking the primary circuit.
Induction coil by Heinrich Ruhmkorff, 1850s. In addition to the hammer interrupter (right), it had a mercury interrupter by Fizeau (left) that could be adjusted to change the dwell time.
One of the largest coils ever constructed, built in 1877 by Alfred Apps for William Spottiswoode. Wound with 280 miles of wire, could produce a 42 in. (106 cm) spark, corresponding to roughly one million volts. Powered by 30 quart size liquid batteries and a separate interrupter (not shown).
The first induction coil, built by Nicholas Callan, 1836.
The induction coil was the first type of electrical transformer. During its development between 1836 and the 1860s, mostly by trial and error, researchers discovered many of the principles that governed all transformers, such as the proportionality between turns and output voltage and the use of a "divided" iron core to reduce eddy current losses.
Michael Faraday discovered the principle of induction, Faraday's induction law, in 1831 and did the first experiments with induction between coils of wire.[8] The induction coil was invented by the American physician Charles Grafton Page in 1836[9][10] and, later on, independently by Irish scientist and Catholic priest Nicholas Callan in the same year at the St. Patrick's College, Maynooth[1][11][12][13][14] and improved by William Sturgeon and Charles Grafton Page.[1] George Henry Bachhoffner[1] and Sturgeon (1837) independently discovered that a "divided" iron core of iron wires reduced power losses.[15] The early coils had hand cranked interrupters, invented by Callan and Antoine Philibert Masson (1837).[16][17][18] The automatic 'hammer' interrupter was invented by Rev. Prof. James William MacGauley (1838) of Dublin, Ireland,[9][19] Johann Philipp Wagner (1839), and Christian Ernst Neeff (1847).[1][20][21] Hippolyte Fizeau (1853) introduced the use of the quenching capacitor.[1][22][23] Heinrich Ruhmkorff generated higher voltages by greatly increasing the length of the secondary,[1] in some coils using 5 or 6 miles (10 km) of wire and produced sparks up to 16 inches. In the early 1850s, American inventor Edward Samuel Ritchie introduced the divided secondary construction to improve insulation.[24][25] Jonathan Nash Hearder worked on induction coils.[26][27][28][29][30] Callan's induction coil was named an IEEE Milestone in 2006.[31]
Induction coils were used to provide high voltage for early gas discharge and Crookes tubes and other high voltage research. They were also used to provide entertainment (lighting Geissler tubes, for example) and to drive small "shocking coils", Tesla coils and violet ray devices used in quack medicine. They were used by Hertz to demonstrate the existence of electromagnetic waves, as predicted by James Clerk Maxwell and by Lodge and Marconi in the first research into radio waves. Their largest industrial use was probably in early wireless telegraphy spark-gap radio transmitters and to power early cold cathode x-ray tubes from the 1890s to the 1920s, after which they were supplanted in both these applications by AC transformers and vacuum tubes. However their largest use was as the ignition coil or spark coil in the ignition system of internal combustion engines, where they are still used, although the interrupter contacts are now replaced by solid state switches. A smaller version is used to trigger the flash tubes used in cameras and strobe lights.
Induction coil (top) powering 1915 wall mounted x-ray unit, with electrolytic interrupter (bottom).
Vibrator ignition coil used in early automobiles such as the Ford Model T around 1910
Modern automobile ignition coil, the largest remaining use for induction coils
See also[edit]
Ignition coil
Trembler coil
Spark gap transmitter
Transformer
Tesla coil
Faraday's law of induction
Ignition system
Inductor
Magnetic field
Nicholas Callan
Footnotes[edit]
^ Jump up to: a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab John Archibald Fleming "Induction Coil". Encyclopædia Britannica, 11th Ed. 13. The Encyclopædia Britannica Co. 1911. pp. 502–505. Retrieved October 13, 2014.
Jump up ^ Collins, Archie F. (1908). The Design and Construction of Induction Coils. New York: Munn & Co. p.98
^ Jump up to: a b Collins, 1908, p. iii
^ Jump up to: a b c d Collins, 1908, p. 16-19
Jump up ^ Schall & Son Electro-medical Instruments and their Management London 1914
Jump up ^ Collins, 1908, p. 98
^ Jump up to: a b Moore, Arthur (1911). How to make a wireless set. Chicago: The Popular Mechanics Co. ISBN 1440048746. The electrolytic interrupter consists of a vessel containing a solution of dilute sulphuric acid with two terminals immersed in this solution. The positive terminal or anode is made of platinum and should have a surface of about 3/16 in.[sic] The negative terminal or cathode is made of lead and should have an area of something like 1 sq. ft. When this interrupter is connected in series with the primary of an induction coil and a source of electromotive force of about 40 volts, the circuit will be interrupted, due to the formation and collapse of bubbles on the platinum electrode. Page 31 describes electrolytic interrupter, but does not identify as Wehnelt interrupter.
Jump up ^ Faraday, Michael (1834). "Experimental Researches in Electricity. Seventh Series". Philosophical Transactions of the Royal Society of London. 124: 77–122. doi:10.1098/rstl.1834.0008.
^ Jump up to: a b Page, Charles Grafton (1867). History of Induction: The American Claim to the Induction Coil and Its Electrostatic Developments. Washington, D.C.: Intelligencer Printing House. pp. 26–27, 57.
Jump up ^ Czarnik, Stanley A. (March 1993). "The Classic Induction Coil" (PDF). Popular Electronics. New York: Gernsback Publications Inc. 9 (3): 35–40. ISSN 1042-170X. Retrieved September 3, 2015. , archived Archived 2016-10-30 at the Wayback Machine.
Jump up ^ Callan, N. J. (December 1836). "On a new galvanic battery". Philosophical Magazine. 9 (3): 472–478. doi:10.1080/14786443608649044. Retrieved February 14, 2013.
Jump up ^ Callan, N. J. A Description of an Electromagnetic Repeater in Sturgeon, Ed., William (1837). The Annals of Electricity, Magnetism, and Chemistry, Vol. 1. London: Sherwood, Gilbert, and Piper. pp. 229–230. and p.522 fig. 52
Jump up ^ Fleming, John Ambrose (1896). The Alternate Current Transformer in Theory and Practice, Vol. 2. London: The Electrician Publishing Co. pp. 16–18.
Jump up ^ McKeith, Niall. "Reverend Professor Nicholas Callan". National Science Museum. St. Patrick's College, Maynooth. Archived from the original on February 25, 2013. Retrieved February 14, 2013.
Jump up ^ Fleming (1896) The Alternate Current Transformer in Theory and Practice, Vol. 2, p. 10-11
Jump up ^ Masson, Antoine Philibert (1837). "Rapport sur plusieurs mémoires, relatifs à un mode particulier d'action des courants électriques (Report on several memoirs regarding a particular mode of action of electric currents)". Comptes rendus. Paris: Elsevier. 4: 456–460. Retrieved February 14, 2013. On page 458, an interrupter consisting of a toothed wheel is described.
Jump up ^ Masson, A. (1837). "De l'induction d'un courant sur lui-même (On the induction of a current in itself)". Annales de Chimie et de Physique. Paris: Elsevier. 66: 5–36. Retrieved February 14, 2013.
Jump up ^ Masson, Antoine Philibert; Louis Breguet (1841). "Mémoire sur l'induction". Annales de chimie et de physique. Paris: Elsevier. 4 (3): 129–152. Retrieved February 14, 2013. On page 134, Masson describes the toothed wheels that functioned as an interrupter.
Jump up ^ McGauley, J. W. (1838). "Electro-magnetic apparatus for the production of electricity of high intensity". Proceedings of the British Association for the Advancement of Science. BAAS. 7: 25. presented at meeting of September 1837 in Liverpool, England
Jump up ^ Neeff, Christian Ernst (1839). "Ueber einen neuen Magnetelektromotor (On a new electromagnetic motor)". Annalen der Physik und Chemie. Berlin. 46: 104–127. Retrieved February 14, 2013.
Jump up ^ Neeff, C. (1835). "Das Blitzrad, ein Apparat zu rasch abwechselnden galvanischen Schliessungen und Trennungen (The spark wheel, an apparatus for rapidly alternating closings and openings of galvanic circuits)". Annalen der Physik und Chemie. 36: 352–366. Retrieved February 14, 2013. Description of Neeff and Wagner's earlier toothed wheel interrupter
Jump up ^ Fizeau, H. (1853). "Note sur les machines électriques inductives et sur un moyen facile d'accroître leurs effets" [Note on electric induction machines and on an easy way to increase their effects]. Comptes rendus (in French). Elsevier. 36: 418–421. Retrieved February 14, 2013.
Jump up ^ Severns, Rudy. "History of soft switching, Part 2" (PDF). Design Resource Center. Switching Power Magazine. Archived from the original (PDF) on 2011-07-16. Retrieved 2008-05-16.
Jump up ^ American Academy of Arts and Sciences, Proceedings of the American Academy of Arts and Sciences, Vol. XXIII, May 1895 - May 1896, Boston: University Press, John Wilson and Son (1896), pp. 359-360
Jump up ^ Page, Charles G., History of Induction: The American Claim to the Induction Coil and Its Electrostatic Developments, Washington, D.C.: Intelligencer Printing House (1867), pp. 104-106
Jump up ^ Fleming, J. A. (1891). "The Historical Development of the Induction Coil and Transformer". The Electrician. 26–27: V26:––417, V27: 211–213, 246–248, 300–302, 359–361, 433–435. at page 360.
Jump up ^ "Hearder's induction coil". Journal of the Franklin Institute. 63 (3): 179–81. 1857. doi:10.1016/0016-0032(57)90712-3.
Jump up ^ "The improved induction coil". Philosophical Magazine. Series 4. 13 (88): 471. 1857. doi:10.1080/14786445708642330 (inactive 2017-01-26).
Jump up ^ "The improved induction coil". Philosophical Magazine. Series 4. 14 (93): 319–20. 1857. doi:10.1080/14786445708642396 (inactive 2017-01-26).
Jump up ^ Hearder, Ian G. (September 2004). "Hearder, Jonathan Nash (1809–1876)". Oxford Dictionary of National Biography. Oxford University Press. Retrieved 7 April 2010.
Jump up ^ "Milestones:Callan's Pioneering Contributions to Electrical Science and Technology, 1836". IEEE Global History Network. IEEE. Retrieved 26 July 2011.
Further reading[edit]
Norrie, H. S., "Induction Coils: How to Make, Use, and Repair Them". Norman H. Schneider, 1907, New York. 4th edition.
Collins, Archie F. (1908). The Design and Construction of Induction Coils. New York: Munn & Co.
Fleming, John Ambrose (1896). The Alternate Current Transformer in Theory and Practice, Vol.2. The Electrician Publishing Co. Has detailed history of invention of induction coil
External links[edit]
Battery powered Driver circuit for Induction Coils
The Cathode Ray Tube site
Newman, F. H. (1921). "A New Form of Wehnelt Interrupter". Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences. 99 (699): 324–30. Bibcode:1921RSPSA..99..324N. doi:10.1098/rspa.1921.0045. JSTOR 93959.
http://media.digikey.com/pdf/other%20related%20documents/panasonic%20other%20doc/small%20signal%20relay%20techincal%20info.pdf See section "Contact Protection – Counter EMF".
https://web.archive.org/web/20161009151329/http://www.worldphaco.net/uploads/CAPACITIVE_DISCHARGE_IGNITION_vs_MAGNETIC_DISCHARGE_IGNITION..pdf See figure 9 for actual discharge.
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text:
Nash equilibrium
From Wikipedia, the free encyclopedia
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Nash equilibrium
A solution concept in game theory
Relationship
Subset of
Rationalizability, Epsilon-equilibrium, Correlated equilibrium
Superset of
Evolutionarily stable strategy, Subgame perfect equilibrium, Perfect Bayesian equilibrium, Trembling hand perfect equilibrium, Stable Nash equilibrium, Strong Nash equilibrium
Significance
Proposed by
John Forbes Nash Jr.
Used for
All non-cooperative games
In game theory, the Nash equilibrium, named after American mathematician John Forbes Nash Jr., is a solution concept of a non-cooperative game involving two or more players in which each player is assumed to know the equilibrium strategies of the other players, and no player has anything to gain by changing only their own strategy.[1] If each player has chosen a strategy and no player can benefit by changing strategies while the other players keep theirs unchanged, then the current set of strategy choices and the corresponding payoffs constitutes a Nash equilibrium. The Nash equilibrium is one of the foundational concepts in game theory. The reality of the Nash equilibrium of a game can be tested using experimental economics methods.
Stated simply, Alice and Bob are in Nash equilibrium if Alice is making the best decision she can, taking into account Bob's decision while Bob's decision remains unchanged, and Bob is making the best decision he can, taking into account Alice's decision while Alice's decision remains unchanged. Likewise, a group of players are in Nash equilibrium if each one is making the best decision possible, taking into account the decisions of the others in the game as long as the other parties' decisions remain unchanged.
Contents
[hide]
1 Applications
2 History
3 Definitions
3.1 Informal definition
3.2 Formal definition
3.3 Nash's Existence Theorem
4 Examples
4.1 Coordination game
4.2 Prisoner's dilemma
4.3 Network traffic
4.4 Competition game
4.5 Nash equilibria in a payoff matrix
5 Stability
6 Occurrence
6.1 Where the conditions are not met
6.2 Where the conditions are met
7 NE and non-credible threats
8 Proof of existence
8.1 Proof using the Kakutani fixed-point theorem
8.2 Alternate proof using the Brouwer fixed-point theorem
9 Computing Nash equilibria
9.1 Examples
10 See also
11 Notes
12 References
12.1 Game theory textbooks
12.2 Original Nash papers
12.3 Other references
13 External links
Applications[edit]
Game theorists use the Nash equilibrium concept to analyze the outcome of the strategic interaction of several decision makers. In other words, it provides a way of predicting what will happen if several people or several institutions are making decisions at the same time, and if the outcome depends on the decisions of the others. The simple insight underlying John Nash's idea is that one cannot predict the result of the choices of multiple decision makers if one analyzes those decisions in isolation. Instead, one must ask what each player would do, taking into account the decision-making of the others.
Nash equilibrium has been used to analyze hostile situations like war and arms races[2] (see prisoner's dilemma), and also how conflict may be mitigated by repeated interaction (see tit-for-tat). It has also been used to study to what extent people with different preferences can cooperate (see battle of the sexes), and whether they will take risks to achieve a cooperative outcome (see stag hunt). It has been used to study the adoption of technical standards,[citation needed] and also the occurrence of bank runs and currency crises (see coordination game). Other applications include traffic flow (see Wardrop's principle), how to organize auctions (see auction theory), the outcome of efforts exerted by multiple parties in the education process,[3] regulatory legislation such as environmental regulations (see tragedy of the Commons),[4] natural resource management,[5] analysing strategies in marketing,[6] and even penalty kicks in football (see matching pennies).[7]
History[edit]
The Nash equilibrium was named after American mathematician John Forbes Nash, Jr. A version of the Nash equilibrium concept was first known to be used in 1838 by Antoine Augustin Cournot in his theory of oligopoly.[8] In Cournot's theory, firms choose how much output to produce to maximize their own profit. However, the best output for one firm depends on the outputs of others. A Cournot equilibrium occurs when each firm's output maximizes its profits given the output of the other firms, which is a pure-strategy Nash equilibrium. Cournot also introduced the concept of best response dynamics in his analysis of the stability of equilibrium. However, Nash's definition of equilibrium is broader than Cournot's. It is also broader than the definition of a Pareto-efficient equilibrium, since the Nash definition makes no judgements about the optimality of the equilibrium being generated.
The modern game-theoretic concept of Nash equilibrium is instead defined in terms of mixed strategies, where players choose a probability distribution over possible actions. The concept of the mixed-strategy Nash equilibrium was introduced by John von Neumann and Oskar Morgenstern in their 1944 book The Theory of Games and Economic Behavior. However, their analysis was restricted to the special case of zero-sum games. They showed that a mixed-strategy Nash equilibrium will exist for any zero-sum game with a finite set of actions.[9] The contribution of Nash in his 1951 article Non-Cooperative Games was to define a mixed-strategy Nash equilibrium for any game with a finite set of actions and prove that at least one (mixed-strategy) Nash equilibrium must exist in such a game. The key to Nash's ability to prove existence far more generally than von Neumann lay in his definition of equilibrium. According to Nash, "an equilibrium point is an n-tuple such that each player's mixed strategy maximizes his payoff if the strategies of the others are held fixed. Thus each player's strategy is optimal against those of the others." Just putting the problem in this framework allowed Nash to employ the Kakutani fixed-point theorem in his 1950 paper, and a variant upon it in his 1951 paper used the Brouwer fixed-point theorem to prove that there had to exist at least one mixed strategy profile that mapped back into itself for finite-player (not necessarily zero-sum) games, namely, a strategy profile that did not call for a shift in strategies that could improve payoffs.[10]
Since the development of the Nash equilibrium concept, game theorists have discovered that it makes misleading predictions (or fails to make a unique prediction) in certain circumstances. They have proposed many related solution concepts (also called 'refinements' of Nash equilibria) designed to overcome perceived flaws in the Nash concept. One particularly important issue is that some Nash equilibria may be based on threats that are not 'credible'. In 1965 Reinhard Selten proposed subgame perfect equilibrium as a refinement that eliminates equilibria which depend on non-credible threats. Other extensions of the Nash equilibrium concept have addressed what happens if a game is repeated, or what happens if a game is played in the absence of complete information. However, subsequent refinements and extensions of the Nash equilibrium concept share the main insight on which Nash's concept rests: all equilibrium concepts analyze what choices will be made when each player takes into account the decision-making of others.
Definitions[edit]
Informal definition[edit]
Informally, a strategy profile is a Nash equilibrium if no player can do better by unilaterally changing his or her strategy. To see what this means, imagine that each player is told the strategies of the others. Suppose then that each player asks themselves: "Knowing the strategies of the other players, and treating the strategies of the other players as set in stone, can I benefit by changing my strategy?"
If any player could answer "Yes", then that set of strategies is not a Nash equilibrium. But if every player prefers not to switch (or is indifferent between switching and not) then the strategy profile is a Nash equilibrium. Thus, each strategy in a Nash equilibrium is a best response to all other strategies in that equilibrium.[11]
The Nash equilibrium may sometimes appear non-rational in a third-person perspective. This is because a Nash equilibrium is not necessarily Pareto optimal.
The Nash equilibrium may also have non-rational consequences in sequential games because players may "threaten" each other with non-rational moves. For such games the subgame perfect Nash equilibrium may be more meaningful as a tool of analysis.
Formal definition[edit]
Let ( S , f ) {\displaystyle (S,f)} be a game with n {\displaystyle n} players, where S i {\displaystyle S_{i}} is the strategy set for player i {\displaystyle i} , S = S 1 × S 2 × ⋯ × S n {\displaystyle S=S_{1}\times S_{2}\times \dotsb \times S_{n}} is the set of strategy profiles and f ( x ) = ( f 1 ( x ) , … , f n ( x ) ) {\displaystyle f(x)=(f_{1}(x),\dotsc ,f_{n}(x))} is its payoff function evaluated at x ∈ S {\displaystyle x\in S} . Let x i {\displaystyle x_{i}} be a strategy profile of player i {\displaystyle i} and x − i {\displaystyle x_{-i}} be a strategy profile of all players except for player i {\displaystyle i} . When each player i ∈ { 1 , … , n } {\displaystyle i\in \{1,\dotsc ,n\}} chooses strategy x i {\displaystyle x_{i}} resulting in strategy profile x = ( x 1 , … , x n ) {\displaystyle x=(x_{1},\dotsc ,x_{n})} then player i {\displaystyle i} obtains payoff f i ( x ) {\displaystyle f_{i}(x)} . Note that the payoff depends on the strategy profile chosen, i.e., on the strategy chosen by player i {\displaystyle i} as well as the strategies chosen by all the other players. A strategy profile x ∗ ∈ S {\displaystyle x^{*}\in S} is a Nash equilibrium (NE) if no unilateral deviation in strategy by any single player is profitable for that player, that is
∀ i , x i ∈ S i : f i ( x i ∗ , x − i ∗ ) ≥ f i ( x i , x − i ∗ ) . {\displaystyle \forall i,x_{i}\in S_{i}:f_{i}(x_{i}^{*},x_{-i}^{*})\geq f_{i}(x_{i},x_{-i}^{*}).}
When the inequality above holds strictly (with > instead of ≥) for all players and all feasible alternative strategies, then the equilibrium is classified as a strict Nash equilibrium. If instead, for some player, there is exact equality between x i ∗ {\displaystyle x_{i}^{*}} and some other strategy in the set S {\displaystyle S} , then the equilibrium is classified as a weak Nash equilibrium.
A game can have a pure-strategy or a mixed-strategy Nash equilibrium. (In the latter a pure strategy is chosen stochastically with a fixed probability).
Nash's Existence Theorem[edit]
Nash proves that if we allow mixed strategies, then every game with a finite number of players in which each player can choose from finitely many pure strategies has at least one Nash equilibrium.
Nash equilibrium need not exist if the set of choices is infinite and noncompact. An example is when two players simultaneously name a natural number with the player naming the larger number wins. However, Nash equilibrium exists if the set of choices is compact with continuous payoff.[12] An example (with the equilibrium being a mixture of continuously many pure strategies) is if two players simultaneously pick a real number between 0 and 1 (inclusive) with player one winnings (paid by the second player) equaling square root of the distance between the two numbers.
Examples[edit]
Coordination game[edit]
Main article: Coordination game
A sample coordination game showing relative payoff for player 2 (row) / player 1 (column) with each combination
Player 2
Player 1
Player 2 adopts strategy A
Player 2 adopts strategy B
Player 1 adopts strategy A
4 / 4 1 / 3
Player 1 adopts strategy B
3 / 1 2 / 2
The coordination game is a classic (symmetric) two player, two strategy game, with an example payoff matrix shown to the right. The players should thus coordinate, both adopting strategy A, to receive the highest payoff; i.e., 4. If both players chose strategy B though, there is still a Nash equilibrium. Although each player is awarded less than optimal payoff, neither player has incentive to change strategy due to a reduction in the immediate payoff (from 2 to 1).
A famous example of this type of game was called the stag hunt; in the game two players may choose to hunt a stag or a rabbit, the former providing more meat (4 utility units) than the latter (1 utility unit). The caveat is that the stag must be cooperatively hunted, so if one player attempts to hunt the stag, while the other hunts the rabbit, he will fail in hunting (0 utility units), whereas if they both hunt it they will split the payload (2, 2). The game hence exhibits two equilibria at (stag, stag) and (rabbit, rabbit) and hence the players' optimal strategy depend on their expectation on what the other player may do. If one hunter trusts that the other will hunt the stag, they should hunt the stag; however if they suspect that the other will hunt the rabbit, they should hunt the rabbit. This game was used as an analogy for social cooperation, since much of the benefit that people gain in society depends upon people cooperating and implicitly trusting one another to act in a manner corresponding with cooperation.
Another example of a coordination game is the setting where two technologies are available to two firms with comparable products, and they have to elect a strategy to become the market standard. If both firms agree on the chosen technology, high sales are expected for both firms. If the firms do not agree on the standard technology, few sales result. Both strategies are Nash equilibria of the game.
Driving on a road against an oncoming car, and having to choose either to swerve on the left or to swerve on the right of the road, is also a coordination game. For example, with payoffs 10 meaning no crash and 0 meaning a crash, the coordination game can be defined with the following payoff matrix:
The driving game
Driver 2
Driver 1
Drive on the Left
Drive on the Right
Drive on the Left
10, 10 0, 0
Drive on the Right
0, 0 10, 10
In this case there are two pure-strategy Nash equilibria, when both choose to either drive on the left or on the right. If we admit mixed strategies (where a pure strategy is chosen at random, subject to some fixed probability), then there are three Nash equilibria for the same case: two we have seen from the pure-strategy form, where the probabilities are (0%,100%) for player one, (0%, 100%) for player two; and (100%, 0%) for player one, (100%, 0%) for player two respectively. We add another where the probabilities for each player is (50%, 50%).
Prisoner's dilemma[edit]
Main article: Prisoner's dilemma
Example PD payoff matrix
Prisoner 2
Prisoner 1
Cooperate (with other)
Defect (betray other)
Cooperate (with other)
2, 2 0, 3
Defect (betray other)
3, 0 1, 1
Imagine two prisoners held in separate cells, interrogated simultaneously, and offered deals (lighter jail sentences) for betraying their fellow criminal. They can "cooperate" (with the other prisoner) by not snitching, or "defect" by betraying the other. However, there is a catch; if both players defect, then they both serve a longer sentence than if neither said anything. Lower jail sentences are interpreted as higher payoffs (shown in the table).
The prisoner's dilemma has a similar matrix as depicted for the coordination game, but the maximum reward for each player (in this case, a minimum loss of 0) is obtained only when the players' decisions are different. Each player improves their own situation by switching from "cooperating" to "defecting", given knowledge that the other player's best decision is to "defect". The prisoner's dilemma thus has a single Nash equilibrium: both players choosing to defect.
What has long made this an interesting case to study is the fact that this scenario is globally inferior to "both cooperating". That is, both players would be better off if they both chose to "cooperate" instead of both choosing to defect. However, each player could improve their own situation by breaking the mutual cooperation, no matter how the other player possibly (or certainly) changes their decision.
Network traffic[edit]
See also: Braess's paradox
Sample network graph. Values on edges are the travel time experienced by a 'car' traveling down that edge. x is the number of cars traveling via that edge.
An application of Nash equilibria is in determining the expected flow of traffic in a network. Consider the graph on the right. If we assume that there are x "cars" traveling from A to D, what is the expected distribution of traffic in the network?
This situation can be modeled as a "game" where every traveler has a choice of 3 strategies, where each strategy is a route from A to D (either ABD, ABCD, or ACD). The "payoff" of each strategy is the travel time of each route. In the graph on the right, a car travelling via ABD experiences travel time of (1+x/100)+2, where x is the number of cars traveling on edge AB. Thus, payoffs for any given strategy depend on the choices of the other players, as is usual. However, the goal, in this case, is to minimize travel time, not maximize it. Equilibrium will occur when the time on all paths is exactly the same. When that happens, no single driver has any incentive to switch routes, since it can only add to their travel time. For the graph on the right, if, for example, 100 cars are travelling from A to D, then equilibrium will occur when 25 drivers travel via ABD, 50 via ABCD, and 25 via ACD. Every driver now has a total travel time of 3.75 (to see this, note that a total of 75 cars take the AB edge, and likewise, 75 cars take the CD edge).
Notice that this distribution is not, actually, socially optimal. If the 100 cars agreed that 50 travel via ABD and the other 50 through ACD, then travel time for any single car would actually be 3.5, which is less than 3.75. This is also the Nash equilibrium if the path between B and C is removed, which means that adding another possible route can decrease the efficiency of the system, a phenomenon known as Braess's paradox.
Competition game[edit]
A competition game
Player 2
Player 1
Choose '0'
Choose '1'
Choose '2'
Choose '3'
Choose '0'
0, 0 2, −2 2, −2 2, −2
Choose '1'
−2, 2 1, 1 3, −1 3, −1
Choose '2'
−2, 2 −1, 3 2, 2 4, 0
Choose '3'
−2, 2 −1, 3 0, 4 3, 3
This can be illustrated by a two-player game in which both players simultaneously choose an integer from 0 to 3 and they both win the smaller of the two numbers in points. In addition, if one player chooses a larger number than the other, then they have to give up two points to the other.
This game has a unique pure-strategy Nash equilibrium: both players choosing 0 (highlighted in light red). Any other strategy can be improved by a player switching their number to one less than that of the other player. In the adjacent table, if the game begins at the green square, it is in player 1's interest to move to the purple square and it is in player 2's interest to move to the blue square. Although it would not fit the definition of a competition game, if the game is modified so that the two players win the named amount if they both choose the same number, and otherwise win nothing, then there are 4 Nash equilibria: (0,0), (1,1), (2,2), and (3,3).
Nash equilibria in a payoff matrix[edit]
There is an easy numerical way to identify Nash equilibria on a payoff matrix. It is especially helpful in two-person games where players have more than two strategies. In this case formal analysis may become too long. This rule does not apply to the case where mixed (stochastic) strategies are of interest. The rule goes as follows: if the first payoff number, in the payoff pair of the cell, is the maximum of the column of the cell and if the second number is the maximum of the row of the cell - then the cell represents a Nash equilibrium.
A payoff matrix – Nash equilibria in bold
Player 2
Player 1
Option A
Option B
Option C
Option A
0, 0 25, 40 5, 10
Option B
40, 25 0, 0 5, 15
Option C
10, 5 15, 5 10, 10
We can apply this rule to a 3×3 matrix:
Using the rule, we can very quickly (much faster than with formal analysis) see that the Nash equilibria cells are (B,A), (A,B), and (C,C). Indeed, for cell (B,A) 40 is the maximum of the first column and 25 is the maximum of the second row. For (A,B) 25 is the maximum of the second column and 40 is the maximum of the first row. Same for cell (C,C). For other cells, either one or both of the duplet members are not the maximum of the corresponding rows and columns.
This said, the actual mechanics of finding equilibrium cells is obvious: find the maximum of a column and check if the second member of the pair is the maximum of the row. If these conditions are met, the cell represents a Nash equilibrium. Check all columns this way to find all NE cells. An N×N matrix may have between 0 and N×N pure-strategy Nash equilibria.
Stability[edit]
The concept of stability, useful in the analysis of many kinds of equilibria, can also be applied to Nash equilibria.
A Nash equilibrium for a mixed-strategy game is stable if a small change (specifically, an infinitesimal change) in probabilities for one player leads to a situation where two conditions hold:
the player who did not change has no better strategy in the new circumstance
the player who did change is now playing with a strictly worse strategy.
If these cases are both met, then a player with the small change in their mixed strategy will return immediately to the Nash equilibrium. The equilibrium is said to be stable. If condition one does not hold then the equilibrium is unstable. If only condition one holds then there are likely to be an infinite number of optimal strategies for the player who changed.
In the "driving game" example above there are both stable and unstable equilibria. The equilibria involving mixed strategies with 100% probabilities are stable. If either player changes their probabilities slightly, they will be both at a disadvantage, and their opponent will have no reason to change their strategy in turn. The (50%,50%) equilibrium is unstable. If either player changes their probabilities, then the other player immediately has a better strategy at either (0%, 100%) or (100%, 0%).
Stability is crucial in practical applications of Nash equilibria, since the mixed strategy of each player is not perfectly known, but has to be inferred from statistical distribution of their actions in the game. In this case unstable equilibria are very unlikely to arise in practice, since any minute change in the proportions of each strategy seen will lead to a change in strategy and the breakdown of the equilibrium.
The Nash equilibrium defines stability only in terms of unilateral deviations. In cooperative games such a concept is not convincing enough. Strong Nash equilibrium allows for deviations by every conceivable coalition.[13] Formally, a strong Nash equilibrium is a Nash equilibrium in which no coalition, taking the actions of its complements as given, can cooperatively deviate in a way that benefits all of its members.[14] However, the strong Nash concept is sometimes perceived as too "strong" in that the environment allows for unlimited private communication. In fact, strong Nash equilibrium has to be Pareto efficient. As a result of these requirements, strong Nash is too rare to be useful in many branches of game theory. However, in games such as elections with many more players than possible outcomes, it can be more common than a stable equilibrium.
A refined Nash equilibrium known as coalition-proof Nash equilibrium (CPNE)[13] occurs when players cannot do better even if they are allowed to communicate and make "self-enforcing" agreement to deviate. Every correlated strategy supported by iterated strict dominance and on the Pareto frontier is a CPNE.[15] Further, it is possible for a game to have a Nash equilibrium that is resilient against coalitions less than a specified size, k. CPNE is related to the theory of the core.
Finally in the eighties, building with great depth on such ideas Mertens-stable equilibria were introduced as a solution concept. Mertens stable equilibria satisfy both forward induction and backward induction. In a game theory context stable equilibria now usually refer to Mertens stable equilibria.
Occurrence[edit]
If a game has a unique Nash equilibrium and is played among players under certain conditions, then the NE strategy set will be adopted. Sufficient conditions to guarantee that the Nash equilibrium is played are:
The players all will do their utmost to maximize their expected payoff as described by the game.
The players are flawless in execution.
The players have sufficient intelligence to deduce the solution.
The players know the planned equilibrium strategy of all of the other players.
The players believe that a deviation in their own strategy will not cause deviations by any other players.
There is common knowledge that all players meet these conditions, including this one. So, not only must each player know the other players meet the conditions, but also they must know that they all know that they meet them, and know that they know that they know that they meet them, and so on.
Where the conditions are not met[edit]
Examples of game theory problems in which these conditions are not met:
The first condition is not met if the game does not correctly describe the quantities a player wishes to maximize. In this case there is no particular reason for that player to adopt an equilibrium strategy. For instance, the prisoner’s dilemma is not a dilemma if either player is happy to be jailed indefinitely.
Intentional or accidental imperfection in execution. For example, a computer capable of flawless logical play facing a second flawless computer will result in equilibrium. Introduction of imperfection will lead to its disruption either through loss to the player who makes the mistake, or through negation of the common knowledge criterion leading to possible victory for the player. (An example would be a player suddenly putting the car into reverse in the game of chicken, ensuring a no-loss no-win scenario).
In many cases, the third condition is not met because, even though the equilibrium must exist, it is unknown due to the complexity of the game, for instance in Chinese chess.[16] Or, if known, it may not be known to all players, as when playing tic-tac-toe with a small child who desperately wants to win (meeting the other criteria).
The criterion of common knowledge may not be met even if all players do, in fact, meet all the other criteria. Players wrongly distrusting each other's rationality may adopt counter-strategies to expected irrational play on their opponents’ behalf. This is a major consideration in "chicken" or an arms race, for example.
Where the conditions are met[edit]
In his Ph.D. dissertation, John Nash proposed two interpretations of his equilibrium concept, with the objective of showing how equilibrium points ``(...) can be connected with observable phenomenon. One interpretation is rationalistic: if we assume that players are rational, know the full structure of the game, the game is played just once, and there is just one Nash equilibrium, then players will play according to that equilibrium. This idea was formalized by Aumann, R. and A. Brandenburger, 1995, ``Epistemic Conditions for Nash Equilibrium, Econometrica, 63, 1161-1180 who interpreted each player's mixed strategy as a conjecture about the behaviour of other players and have shown that if the game and the rationality of players is mutually known and these conjectures are commonly know, then the conjectures must be a Nash equilibrium (a common prior assumption is needed for this result in general, but not in the case of two players. In this case, the conjectures need only be mutually known).
A second interpretation, that Nash referred to by the mass action interpretation, is less demanding on players: ``[i]t is unnecessary to assume that the participants have full knowledge of the total structure of the game, or the ability and inclination to go through any complex reasoning processes. What is assumed is that there is a population of participants for each position in the game, which will be played throughout time by participants drawn at random from the different populations. If there is a stable average frequency with which each pure strategy is employed by the ``average member of the appropriate population, then this stable average frequency constitutes a mixed strategy Nash equilibrium. For a formal result along these lines, see Kuhn, H. and et al., 1996, The Work of John Nash in Game Theory, Journal of Economic Theory, 69, 153-185.
Due to the limited conditions in which NE can actually be observed, they are rarely treated as a guide to day-to-day behaviour, or observed in practice in human negotiations. However, as a theoretical concept in economics and evolutionary biology, the NE has explanatory power. The payoff in economics is utility (or sometimes money), and in evolutionary biology is gene transmission; both are the fundamental bottom line of survival. Researchers who apply games theory in these fields claim that strategies failing to maximize these for whatever reason will be competed out of the market or environment, which are ascribed the ability to test all strategies. This conclusion is drawn from the "stability" theory above. In these situations the assumption that the strategy observed is actually a NE has often been borne out by research.[17]
NE and non-credible threats[edit]
Extensive and Normal form illustrations that show the difference between SPNE and other NE. The blue equilibrium is not subgame perfect because player two makes a non-credible threat at 2(2) to be unkind (U).
The Nash equilibrium is a superset of the subgame perfect Nash equilibrium. The subgame perfect equilibrium in addition to the Nash equilibrium requires that the strategy also is a Nash equilibrium in every subgame of that game. This eliminates all non-credible threats, that is, strategies that contain non-rational moves in order to make the counter-player change their strategy.
The image to the right shows a simple sequential game that illustrates the issue with subgame imperfect Nash equilibria. In this game player one chooses left(L) or right(R), which is followed by player two being called upon to be kind (K) or unkind (U) to player one, However, player two only stands to gain from being unkind if player one goes left. If player one goes right the rational player two would de facto be kind to him in that subgame. However, The non-credible threat of being unkind at 2(2) is still part of the blue (L, (U,U)) Nash equilibrium. Therefore, if rational behavior can be expected by both parties the subgame perfect Nash equilibrium may be a more meaningful solution concept when such dynamic inconsistencies arise.
Proof of existence[edit]
Proof using the Kakutani fixed-point theorem[edit]
Nash's original proof (in his thesis) used Brouwer's fixed-point theorem (e.g., see below for a variant). We give a simpler proof via the Kakutani fixed-point theorem, following Nash's 1950 paper (he credits David Gale with the observation that such a simplification is possible).
To prove the existence of a Nash equilibrium, let r i ( σ − i ) {\displaystyle r_{i}(\sigma _{-i})} be the best response of player i to the strategies of all other players.
r i ( σ − i ) = arg  max σ i u i ( σ i , σ − i ) {\displaystyle r_{i}(\sigma _{-i})=\arg \max _{\sigma _{i}}u_{i}(\sigma _{i},\sigma _{-i})}
Here, σ ∈ Σ {\displaystyle \sigma \in \Sigma } , where Σ = Σ i × Σ − i {\displaystyle \Sigma =\Sigma _{i}\times \Sigma _{-i}} , is a mixed-strategy profile in the set of all mixed strategies and u i {\displaystyle u_{i}} is the payoff function for player i. Define a set-valued function r : Σ → 2 Σ {\displaystyle r\colon \Sigma \rightarrow 2^{\Sigma }} such that r = ( r i ( σ − i ) , r − i ( σ i ) ) {\displaystyle r=(r_{i}(\sigma _{-i}),r_{-i}(\sigma _{i}))} . The existence of a Nash equilibrium is equivalent to r {\displaystyle r} having a fixed point.
Kakutani's fixed point theorem guarantees the existence of a fixed point if the following four conditions are satisfied.
Σ {\displaystyle \Sigma } is compact, convex, and nonempty.
r ( σ ) {\displaystyle r(\sigma )} is nonempty.
r ( σ ) {\displaystyle r(\sigma )} is convex.
r ( σ ) {\displaystyle r(\sigma )} is upper hemicontinuous
Condition 1. is satisfied from the fact that Σ {\displaystyle \Sigma } is a simplex and thus compact. Convexity follows from players' ability to mix strategies. Σ {\displaystyle \Sigma } is nonempty as long as players have strategies.
Condition 2. is satisfied because players maximize expected payoffs which is a continuous function over a compact set. The Weierstrass extreme value theorem guarantees that there is always a maximum value.
Condition 3. is satisfied as a result of mixed strategies. Suppose σ i , σ i ′ ∈ r ( σ − i ) {\displaystyle \sigma _{i},\sigma '_{i}\in r(\sigma _{-i})} , then λ σ i + ( 1 − λ ) σ i ′ ∈ r ( σ − i ) {\displaystyle \lambda \sigma _{i}+(1-\lambda )\sigma '_{i}\in r(\sigma _{-i})} . i.e. if two strategies maximize payoffs, then a mix between the two strategies will yield the same payoff.
Condition 4. is satisfied by way of Berge's maximum theorem. Because u i {\displaystyle u_{i}} is continuous and compact, r ( σ i ) {\displaystyle r(\sigma _{i})} is upper hemicontinuous.
Therefore, there exists a fixed point in r {\displaystyle r} and a Nash equilibrium.[18]
When Nash made this point to John von Neumann in 1949, von Neumann famously dismissed it with the words, "That's trivial, you know. That's just a fixed-point theorem." (See Nasar, 1998, p. 94.)
Alternate proof using the Brouwer fixed-point theorem[edit]
We have a game G = ( N , A , u ) {\displaystyle G=(N,A,u)} where N {\displaystyle N} is the number of players and A = A 1 × ⋯ × A N {\displaystyle A=A_{1}\times \cdots \times A_{N}} is the action set for the players. All of the action sets A i {\displaystyle A_{i}} are finite. Let Δ = Δ 1 × ⋯ × Δ N {\displaystyle \Delta =\Delta _{1}\times \cdots \times \Delta _{N}} denote the set of mixed strategies for the players. The finiteness of the A i {\displaystyle A_{i}} s ensures the compactness of Δ {\displaystyle \Delta } .
We can now define the gain functions. For a mixed strategy σ ∈ Δ {\displaystyle \sigma \in \Delta } , we let the gain for player i {\displaystyle i} on action a ∈ A i {\displaystyle a\in A_{i}} be
Gain i ( σ , a ) = max { 0 , u i ( a , σ − i ) − u i ( σ i , σ − i ) } . {\displaystyle {\text{Gain}}_{i}(\sigma ,a)=\max\{0,u_{i}(a,\sigma _{-i})-u_{i}(\sigma _{i},\sigma _{-i})\}.}
The gain function represents the benefit a player gets by unilaterally changing their strategy. We now define g = ( g 1 , … , g N ) {\displaystyle g=(g_{1},\dotsc ,g_{N})} where
g i ( σ ) ( a ) = σ i ( a ) + Gain i ( σ , a ) {\displaystyle g_{i}(\sigma )(a)=\sigma _{i}(a)+{\text{Gain}}_{i}(\sigma ,a)}
for σ ∈ Δ , a ∈ A i {\displaystyle \sigma \in \Delta ,a\in A_{i}} . We see that
∑ a ∈ A i g i ( σ ) ( a ) = ∑ a ∈ A i σ i ( a ) + Gain i ( σ , a ) = 1 + ∑ a ∈ A i Gain i ( σ , a ) > 0. {\displaystyle \sum _{a\in A_{i}}g_{i}(\sigma )(a)=\sum _{a\in A_{i}}\sigma _{i}(a)+{\text{Gain}}_{i}(\sigma ,a)=1+\sum _{a\in A_{i}}{\text{Gain}}_{i}(\sigma ,a)>0.}
Next we define:
{ f = ( f 1 , ⋯ , f N ) : Δ → Δ f i ( σ ) ( a ) = g i ( σ ) ( a ) ∑ b ∈ A i g i ( σ ) ( b ) a ∈ A i {\displaystyle {\begin{cases}f=(f_{1},\cdots ,f_{N}):\Delta \to \Delta \\f_{i}(\sigma )(a)={\frac {g_{i}(\sigma )(a)}{\sum _{b\in A_{i}}g_{i}(\sigma )(b)}}&a\in A_{i}\end{cases}}}
It is easy to see that each f i {\displaystyle f_{i}} is a valid mixed strategy in Δ i {\displaystyle \Delta _{i}} . It is also easy to check that each f i {\displaystyle f_{i}} is a continuous function of σ {\displaystyle \sigma } , and hence f {\displaystyle f} is a continuous function. As the cross product of a finite number of compact convex sets, Δ {\displaystyle \Delta } is also compact and convex. Applying the Brouwer fixed point theorem to f {\displaystyle f} and Δ {\displaystyle \Delta } we conclude that So f {\displaystyle f} has a fixed point in Δ {\displaystyle \Delta } , call it σ ∗ {\displaystyle \sigma ^{*}} . We claim that σ ∗ {\displaystyle \sigma ^{*}} is a Nash equilibrium in G {\displaystyle G} . For this purpose, it suffices to show that
∀ i ∈ { 1 , ⋯ , N } , ∀ a ∈ A i : Gain i ( σ ∗ , a ) = 0. {\displaystyle \forall i\in \{1,\cdots ,N\},\forall a\in A_{i}:\quad {\text{Gain}}_{i}(\sigma ^{*},a)=0.}
This simply states that each player gains no benefit by unilaterally changing their strategy, which is exactly the necessary condition for a Nash equilibrium.
Now assume that the gains are not all zero. Therefore, ∃ i ∈ { 1 , ⋯ , N } , {\displaystyle \exists i\in \{1,\cdots ,N\},} and a ∈ A i {\displaystyle a\in A_{i}} such that Gain i ( σ ∗ , a ) > 0 {\displaystyle {\text{Gain}}_{i}(\sigma ^{*},a)>0} . Note then that
∑ a ∈ A i g i ( σ ∗ , a ) = 1 + ∑ a ∈ A i Gain i ( σ ∗ , a ) > 1. {\displaystyle \sum _{a\in A_{i}}g_{i}(\sigma ^{*},a)=1+\sum _{a\in A_{i}}{\text{Gain}}_{i}(\sigma ^{*},a)>1.}
So let
C = ∑ a ∈ A i g i ( σ ∗ , a ) . {\displaystyle C=\sum _{a\in A_{i}}g_{i}(\sigma ^{*},a).}
Also we shall denote Gain ( i , ⋅ ) {\displaystyle {\text{Gain}}(i,\cdot )} as the gain vector indexed by actions in A i {\displaystyle A_{i}} . Since σ ∗ {\displaystyle \sigma ^{*}} is the fixed point we have:
σ ∗ = f ( σ ∗ ) ⇒ σ i ∗ = f i ( σ ∗ ) ⇒ σ i ∗ = g i ( σ ∗ ) ∑ a ∈ A i g i ( σ ∗ ) ( a ) ⇒ σ i ∗ = 1 C ( σ i ∗ + Gain i ( σ ∗ , ⋅ ) ) ⇒ C σ i ∗ = σ i ∗ + Gain i ( σ ∗ , ⋅ ) ⇒ ( C − 1 ) σ i ∗ = Gain i ( σ ∗ , ⋅ ) ⇒ σ i ∗ = ( 1 C − 1 ) Gain i ( σ ∗ , ⋅ ) . {\displaystyle {\begin{aligned}\sigma ^{*}=f(\sigma ^{*})&\Rightarrow \sigma _{i}^{*}=f_{i}(\sigma ^{*})\\&\Rightarrow \sigma _{i}^{*}={\frac {g_{i}(\sigma ^{*})}{\sum _{a\in A_{i}}g_{i}(\sigma ^{*})(a)}}\\[6pt]&\Rightarrow \sigma _{i}^{*}={\frac {1}{C}}\left(\sigma _{i}^{*}+{\text{Gain}}_{i}(\sigma ^{*},\cdot )\right)\\[6pt]&\Rightarrow C\sigma _{i}^{*}=\sigma _{i}^{*}+{\text{Gain}}_{i}(\sigma ^{*},\cdot )\\&\Rightarrow \left(C-1\right)\sigma _{i}^{*}={\text{Gain}}_{i}(\sigma ^{*},\cdot )\\&\Rightarrow \sigma _{i}^{*}=\left({\frac {1}{C-1}}\right){\text{Gain}}_{i}(\sigma ^{*},\cdot ).\end{aligned}}}
Since C > 1 {\displaystyle C>1} we have that σ i ∗ {\displaystyle \sigma _{i}^{*}} is some positive scaling of the vector Gain i ( σ ∗ , ⋅ ) {\displaystyle {\text{Gain}}_{i}(\sigma ^{*},\cdot )} . Now we claim that
∀ a ∈ A i : σ i ∗ ( a ) ( u i ( a i , σ − i ∗ ) − u i ( σ i ∗ , σ − i ∗ ) ) = σ i ∗ ( a ) Gain i ( σ ∗ , a ) {\displaystyle \forall a\in A_{i}:\quad \sigma _{i}^{*}(a)(u_{i}(a_{i},\sigma _{-i}^{*})-u_{i}(\sigma _{i}^{*},\sigma _{-i}^{*}))=\sigma _{i}^{*}(a){\text{Gain}}_{i}(\sigma ^{*},a)}
To see this, we first note that if Gain i ( σ ∗ , a ) > 0 {\displaystyle {\text{Gain}}_{i}(\sigma ^{*},a)>0} then this is true by definition of the gain function. Now assume that Gain i ( σ ∗ , a ) = 0 {\displaystyle {\text{Gain}}_{i}(\sigma ^{*},a)=0} . By our previous statements we have that
σ i ∗ ( a ) = ( 1 C − 1 ) Gain i ( σ ∗ , a ) = 0 {\displaystyle \sigma _{i}^{*}(a)=\left({\frac {1}{C-1}}\right){\text{Gain}}_{i}(\sigma ^{*},a)=0}
and so the left term is zero, giving us that the entire expression is 0 {\displaystyle 0} as needed.
So we finally have that
0 = u i ( σ i ∗ , σ − i ∗ ) − u i ( σ i ∗ , σ − i ∗ ) = ( ∑ a ∈ A i σ i ∗ ( a ) u i ( a i , σ − i ∗ ) ) − u i ( σ i ∗ , σ − i ∗ ) = ∑ a ∈ A i σ i ∗ ( a ) ( u i ( a i , σ − i ∗ ) − u i ( σ i ∗ , σ − i ∗ ) ) = ∑ a ∈ A i σ i ∗ ( a ) Gain i ( σ ∗ , a ) by the previous statements = ∑ a ∈ A i ( C − 1 ) σ i ∗ ( a ) 2 > 0 {\displaystyle {\begin{aligned}0&=u_{i}(\sigma _{i}^{*},\sigma _{-i}^{*})-u_{i}(\sigma _{i}^{*},\sigma _{-i}^{*})\\&=\left(\sum _{a\in A_{i}}\sigma _{i}^{*}(a)u_{i}(a_{i},\sigma _{-i}^{*})\right)-u_{i}(\sigma _{i}^{*},\sigma _{-i}^{*})\\&=\sum _{a\in A_{i}}\sigma _{i}^{*}(a)(u_{i}(a_{i},\sigma _{-i}^{*})-u_{i}(\sigma _{i}^{*},\sigma _{-i}^{*}))\\&=\sum _{a\in A_{i}}\sigma _{i}^{*}(a){\text{Gain}}_{i}(\sigma ^{*},a)&&{\text{ by the previous statements }}\\&=\sum _{a\in A_{i}}\left(C-1\right)\sigma _{i}^{*}(a)^{2}>0\end{aligned}}}
where the last inequality follows since σ i ∗ {\displaystyle \sigma _{i}^{*}} is a non-zero vector. But this is a clear contradiction, so all the gains must indeed be zero. Therefore, σ ∗ {\displaystyle \sigma ^{*}} is a Nash equilibrium for G {\displaystyle G} as needed.
Computing Nash equilibria[edit]
If a player A has a dominant strategy s A {\displaystyle s_{A}} then there exists a Nash equilibrium in which A plays s A {\displaystyle s_{A}} . In the case of two players A and B, there exists a Nash equilibrium in which A plays s A {\displaystyle s_{A}} and B plays a best response to s A {\displaystyle s_{A}} . If s A {\displaystyle s_{A}} is a strictly dominant strategy, A plays s A {\displaystyle s_{A}} in all Nash equilibria. If both A and B have strictly dominant strategies, there exists a unique Nash equilibrium in which each plays their strictly dominant strategy.
In games with mixed-strategy Nash equilibria, the probability of a player choosing any particular strategy can be computed by assigning a variable to each strategy that represents a fixed probability for choosing that strategy. In order for a player to be willing to randomize, their expected payoff for each strategy should be the same. In addition, the sum of the probabilities for each strategy of a particular player should be 1. This creates a system of equations from which the probabilities of choosing each strategy can be derived.[11]
Examples[edit]
Matching pennies
Player B
Player A
Player B plays H
Player B plays T
Player A plays H
−1, +1 +1, −1
Player A plays T
+1, −1 −1, +1
In the matching pennies game, player A loses a point to B if A and B play the same strategy and wins a point from B if they play different strategies. To compute the mixed-strategy Nash equilibrium, assign A the probability p of playing H and (1−p) of playing T, and assign B the probability q of playing H and (1−q) of playing T.
E[payoff for A playing H] = (−1)q + (+1)(1−q) = 1−2q
E[payoff for A playing T] = (+1)q + (−1)(1−q) = 2q−1
E[payoff for A playing H] = E[payoff for A playing T] ⇒ 1−2q = 2q−1 ⇒ q = 1/2
E[payoff for B playing H] = (+1)p + (−1)(1−p) = 2p−1
E[payoff for B playing T] = (−1)p + (+1)(1−p) = 1−2p
E[payoff for B playing H] = E[payoff for B playing T] ⇒ 2p−1 = 1−2p ⇒ p = 1/2
Thus a mixed-strategy Nash equilibrium, in this game, is for each player to randomly choose H or T with p = 1/2 and q = 1/2.
See also[edit]
Adjusted winner procedure
Complementarity theory
Conflict resolution research
Cooperation
Equilibrium selection
Evolutionarily stable strategy
Glossary of game theory
Hotelling's law
Mexican standoff
Minimax theorem
Mutual assured destruction
Extended Mathematical Programming for Equilibrium Problems
Optimum contract and par contract
Self-confirming equilibrium
Solution concept
Stackelberg competition
Wardrop's principle
Notes[edit]
Jump up ^ Osborne, Martin J., and Ariel Rubinstein. A Course in Game Theory. Cambridge, MA: MIT, 1994. Print.:14
Jump up ^ Schelling, Thomas, The Strategy of Conflict, copyright 1960, 1980, Harvard University Press, ISBN 0-674-84031-3.
Jump up ^ De Fraja, G.; Oliveira, T.; Zanchi, L. (2010). "Must Try Harder: Evaluating the Role of Effort in Educational Attainment". Review of Economics and Statistics. 92 (3): 577. doi:10.1162/REST_a_00013.
Jump up ^ Ward, H. (1996). "Game Theory and the Politics of Global Warming: The State of Play and Beyond". Political Studies. 44 (5): 850. doi:10.1111/j.1467-9248.1996.tb00338.x. ,
Jump up ^ "Risks and benefits of catching pretty good yield in multispecies mixed fisheries". ICES Journal of Marine Science. 2017. doi:10.1093/icesjms/fsx062. ,
Jump up ^ "Marketing Lessons from Dr. Nash - Andrew Frank". Retrieved 2015-08-30.
Jump up ^ Chiappori, P. -A.; Levitt, S.; Groseclose, T. (2002). "Testing Mixed-Strategy Equilibria when Players Are Heterogeneous: The Case of Penalty Kicks in Soccer" (PDF). American Economic Review. 92 (4): 1138. doi:10.1257/00028280260344678.
Jump up ^ Cournot A. (1838) Researches on the Mathematical Principles of the Theory of Wealth
Jump up ^ J. Von Neumann, O. Morgenstern, Theory of Games and Economic Behavior, copyright 1944, 1953, Princeton University Press
Jump up ^ Carmona, Guilherme; Podczeck, Konrad (2009). "On the Existence of Pure Strategy Nash Equilibria in Large Games" (PDF). Journal of Economic Theory. 144 (3): 1300–1319. doi:10.1016/j.jet.2008.11.009. SSRN 882466  .
^ Jump up to: a b von Ahn, Luis. "Preliminaries of Game Theory" (PDF). Retrieved 2008-11-07.
Jump up ^ MIT OpenCourseWare. 6.254: Game Theory with Engineering Applications, Spring 2010. Lecture 6: Continuous and Discontinuous Games.
^ Jump up to: a b B. D. Bernheim; B. Peleg; M. D. Whinston (1987), "Coalition-Proof Equilibria I. Concepts", Journal of Economic Theory, 42 (1): 1–12, doi:10.1016/0022-0531(87)90099-8.
Jump up ^ Aumann, R. (1959). "Acceptable points in general cooperative n-person games". Contributions to the Theory of Games. IV. Princeton, N.J.: Princeton University Press. ISBN 1-4008-8216-8.
Jump up ^ D. Moreno; J. Wooders (1996), "Coalition-Proof Equilibrium", Games and Economic Behavior, 17 (1): 80–112, doi:10.1006/game.1996.0095.
Jump up ^ T. L. Turocy, B. Von Stengel, Game Theory, copyright 2001, Texas A&M University, London School of Economics, pages 141-144. Nash proved that a perfect NE exists for this type of finite extensive form game[citation needed] – it can be represented as a strategy complying with his original conditions for a game with a NE. Such games may not have unique NE, but at least one of the many equilibrium strategies would be played by hypothetical players having perfect knowledge of all 10150 game trees[citation needed].
Jump up ^ J. C. Cox, M. Walker, Learning to Play Cournot Duoploy Strategies, copyright 1997, Texas A&M University, University of Arizona, pages 141-144
Jump up ^ Fudenburg, Drew; Tirole, Jean (1991). Game Theory. MIT Press. ISBN 0-262-06141-4.
References[edit]
Game theory textbooks[edit]
Dixit, Avinash, Susan Skeath and David Reiley. Games of Strategy. W.W. Norton & Company. (Third edition in 2009)
Dutta, Prajit K. (1999), Strategies and games: theory and practice, MIT Press, ISBN 978-0-262-04169-0 . Suitable for undergraduate and business students.
Fudenberg, Drew and Jean Tirole (1991) Game Theory MIT Press.
Leyton-Brown, Kevin; Shoham, Yoav (2008), Essentials of Game Theory: A Concise, Multidisciplinary Introduction, San Rafael, CA: Morgan & Claypool Publishers, ISBN 978-1-59829-593-1 . An 88-page mathematical introduction; see Chapter 2. Free online at many universities.
Morgenstern, Oskar and John von Neumann (1947) The Theory of Games and Economic Behavior Princeton University Press
Myerson, Roger B. (1997), Game theory: analysis of conflict, Harvard University Press, ISBN 978-0-674-34116-6
Papayoanou, Paul (2010), Game Theory for Business: A Primer in Strategic Gaming, Probabilistic Publishing, ISBN 978-0964793873
Rubinstein, Ariel; Osborne, Martin J. (1994), A course in game theory, MIT Press, ISBN 978-0-262-65040-3 . A modern introduction at the graduate level.
Shoham, Yoav; Leyton-Brown, Kevin (2009), Multiagent Systems: Algorithmic, Game-Theoretic, and Logical Foundations, New York: Cambridge University Press, ISBN 978-0-521-89943-7 . A comprehensive reference from a computational perspective; see Chapter 3. Downloadable free online.
Gibbons, Robert (1992), Game Theory for Applied Economists, Princeton University Press (July 13, 1992), ISBN 978-0-691-00395-5 . Lucid and detailed introduction to game theory in an explicitly economic context.
Osborne, Martin (2004), An introduction to game theory, Oxford University, ISBN 978-0-19-512895-6 . Introduction to Nash equilibrium.
Original Nash papers[edit]
Nash, John (1950) "Equilibrium points in n-person games" Proceedings of the National Academy of Sciences 36(1):48-49.
Nash, John (1951) "Non-Cooperative Games" The Annals of Mathematics 54(2):286-295.
Other references[edit]
Mehlmann, A. (2000) The Game's Afoot! Game Theory in Myth and Paradox, American Mathematical Society.
Nasar, Sylvia (1998), A Beautiful Mind, Simon & Schuster.
External links[edit]
Hazewinkel, Michiel, ed. (2001) [1994], "Nash theorem (in game theory)", Encyclopedia of Mathematics, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4
Complete Proof of Existence of Nash Equilibria
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values: Edit section: Alternate proof using the Brouwer fixed-point theorem

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values: {\displaystyle A=A_{1}\times \cdots \times A_{N}}

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values: {\displaystyle \sigma \in \Delta ,a\in A_{i}}

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values: {\displaystyle \exists i\in \{1,\cdots ,N\},}

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values: {\displaystyle C=\sum _{a\in A_{i}}g_{i}(\sigma ^{*},a).}

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values: {\displaystyle {\text{Gain}}_{i}(\sigma ^{*},\cdot )}

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