perform_tensorial_transform(x,A)

function y = perform_tensorial_transform(x,A)

% tensorial_transform - compute tensorial multiplication
%   using a fast algorithm.
%
%   y = tensorial_transform(x,A);
%
%   A is a pxp matrix, x is of size s*p, 
%   compute (A**s)*x where A**s is the s-fold
%   tensorial product (Kronecker) of A.
%
%   Copyright (c) 2003 Gabriel Peyr


s = length(A); m = length(x); m0 = m/s;
ifm==1)
    y = x; 
    return; 
end;
B = zeros(m0,s); % rsulats temporaires
for j=1:s
    sel = ((j-1)*m0+1):(j*m0);
    B(:,j) = decompose_tensoriel(x(sel),A);
end
y = zeros(m,1);
for i=1:s
    sel = ((i-1)*m0+1):(i*m0);
    for j=1:s
        y(sel) = y(sel) + A(i,j)*B(:,j);    
    end
end