generating function

[Anood]

Find a generating function for the number of solutions to a^2 +b^2 +c^2 +d^2=r,where a,b,c,and d are nonnegative integers.

actually i don't how to solve this problem i know how to find generating functions for the easy stuff but not for the question above!

[Epsilon=One]

Find a generating function for the number of solutions to a^2 +b^2 +c^2 +d^2=r,where a,b,c,and d are nonnegative integers.See: Tangent Circles (http://www.physicsmathforums.com/showthread.php?t=158) and Tini Circle Groups (http://www.physicsmathforums.com/showthread.php?t=159); then, ask questions.

[Anood]

i read it but stll i don't know how to get a generating function for the equation.