i use qgauss for performing double integral, but the result of integration of domain:
(0,2*P_MPI) for external variable
(0,1) and internal variable
I evaluate this for 10 points, i try 20 points or more point.
I see the solution is very difference, i prefers the solution using 10-12 points,

i take the point from:
http://mymathlib.webtrellis.net/integration.html,

i see matlab use adaptive simpson quadrature for dblquad.

there is a good code on c language for performing double integrals using a function (similar to qgauss)?