function intf = guasslengendre(f,a,b)
% function intf = guasslengendre(f,a,b)
% Approximate the integral of f from a to b using a 6-point Gauss-Legendre rule.
% f is either the name of a function file, or an inline function.

 nodes   = [-0.9324695142031520; -0.6612093864662645; -0.2386191860831969; 
             0.2386191860831969;  0.6612093864662645;  0.9324695142031520];
 weights = [ 0.1713244923791703;  0.3607615730481386;  0.4679139345726910;
             0.4679139345726910;  0.3607615730481386;  0.1713244923791703];

% change of variables from [-1,1] to [a,b]
 ab_nodes   = a + (b-a)*(nodes+1)/2;
 ab_weights = weights*(b-a)/2;

% apply Guass-Legendre rule
 intf = sum(ab_weights.*feval(f,ab_nodes));