dEdX_student.m

% This software is made available under the Creative Commons
% Attribution-Noncommercial License.
% (http://creativecommons.org/licenses/by-nc/3.0/)
% copyright 2010 Jascha Sohl-Dickstein

function dE = dEdX_student( X, theta )%x, phi, nu )
    
        X = [X;ones(1,size(X,2))];

%    ff = phi*x;
%    bb = 1./(1 + (ff).^2 / nu);
%    dE = (nu+1)/2 * 2 * (phi' * (ff.*bb))/nu;
    
            % product of student-t experts
        ndims = size(X, 1);
        nbatch = size(X, 2);
        nexperts = prod(size(theta)) / (ndims+1);
        W = reshape( theta, [nexperts, ndims+1] );
        alpha = W(:,ndims+1);
        W = W(:, 1:ndims);

        ff = W * X;
        ff2 = ff.^2;
        off2 = 1 + ff2;
        %        lt = diag(alpha) * (ff./off2);        
        lt = bsxfun(@times, ff./off2, alpha);        
        dE = 2 * W' * lt;

        dE = dE(1:end-1,:);

        %        E = E_POE( theta, X ); %%% temporary for diagnostic purposes
        %figure(4);
        %plot(sort(E));