Lothar Collatz

Lothar Collatz (; 6 July, 1910 – 26 September 1990) was a German mathematician. He was born in Arnsberg, Westphalia. The "3"x" + 1" problem also known as the Collatz conjecture was named after him, and is still not solved. The Collatz–Wielandt formula for the Perron–Frobenius theorem was also named after him. His 1957 paper with Ulrich Sinogowitz, who was killed in the bombing of Darmstadt in World War II, started the field of spectral graph theory.
Biography.
Collatz studied at universities in Germany, including the University of Berlin under Alfred Klose. He received his doctorate in 1935 for writing a paper about linear differential equations. He worked as an assistant for the university. Then, he worked at the Technical University of Karlsruhe in 1935. He habilitated (achieved the highest university degree in European countries) in 1937. He worked as a Privatdozent (sign of position given to special students in Europe) from 1938 to 1943. He worked with Alwin Walther at the Institute for Practical Mathematics. Collatz worked at the University of Hamburg from 1952 until 1978. There, he created the Institute of Applied Mathematics. After leaving his job, he continued to be active.
Honors.
For his work in mathematics, Collatz earned honors, such as:
Death.
He died without knowledge from a heart attack in Varna, Bulgaria, while in a mathematics conference.