% spcnvmat.m
% returns sparse matrix that performs circular convolution
% usage: A = convomat(f)
% A*X will convolve each column of X with the function f

function A = spcnvmat(f);  
     n = length(f);
     I = [];
     J = []; 
     S = []; 
     K = find(~(f==0));
     for i = K
         J = [J 1:n];
         I = [I rem((1:n)+(i-2)*ones(1,n),n)+ones(1,n)];
         S = [S,f(i)*ones(1,n)];
         A = sparse(I,J,S,n,n); 
         end;