Q:

Define a function interactively with sum

I have to define a function which has 1e4 terms added up together.
f(x1, x2) = (sum i, j, k, l from 1 to 10)
    (C(i) * C(j) * C(k) * C(l) * exp( (alpha(i)+alpha(j)) * x1 + (alpha(k)+alpha(l)) * x2 )
all C and alpha are known and f is a function of x1 and x2.
Is there any easy way(without for loop) to input the function in to mathematica and draw the contour of f depending on x1 and x2?
Thanks.

A:

f[x1_, x2_] := \!\(
\*UnderoverscriptBox[\(\[Sum]\), \(i = 1\), \(10\)]\(
\*UnderoverscriptBox[\(\[Sum]\), \(j = 1\), \(10\)]\(
\*UnderoverscriptBox[\(\[Sum]\), \(k = 1\), \(10\)]\(
\*UnderoverscriptBox[\(\[Sum]\), \(l = 1\), \(10\)]c[[i]] c[[j]] c[[k]] c[[l]] 
\*SuperscriptBox[\(E\), \(\((\[Alpha][[
           i]] + \[Alpha][[j]] x1)\) + \((\[Alpha][[
           k]] + \[Alpha][[l]] x2)\)\)]\)\)\)\)

ContourPlot[f[x1, x2], {x1, -10, 10}, {x2, -10, 10}]

