function [G]=calcg(df,t,w)
% MATLAB HELP:
%
% function [G]=calcg(df,t,w)
%
% computes the coupling matrix G
%
% inputs
%  df, the time derivative of the isolation filter
%  t,  the time grid to compute on
%  w,  the frequencies
% outputs
%  G, the coupling matrix
%
%  see codes.ps for details
%
% aaron birch may 21 2002
%
% this code calculates the
% integral of df*(cos(w(i)*t)-cos(w(j)*t))/(w(i)^2-w(j)^2)
% assume t evenly spaced

dt=t(2)-t(1);
N=length(w);
G=zeros(N,N);
for (i=1:N)
  for (j=1:N)
    if (w(i)~=w(j))
      G(i,j)=sum(df.*(cos(w(i)*t)-cos(w(j)*t))/(w(i)^2-w(j)^2));
    else 
      %
      % special case of self-coupling
      %
      G(i,j)=-1*sum(df.*(t.*sin(w(i)*t)/(2*w(i)))); 
    end;
  end;
end;
G=G*dt;