function [vU2,dU2] = eigsort(U,desI)
%  E = EIGSORT(X,desI) is a vector containing the eigenvalues of a square
%    matrix X, where E is sorted descendingly if desI=1 or ascendingly otherwise
%
%  [V,D] = EIG(X) produces a diagonal matrix D of eigenvalues and a
%    full matrix V whose columns are the corresponding eigenvectors so
%    that X*V = V*D.
%
% Written 9/6/98 by Tao Zha

[vU,dU]=eig(U);
dUvec = diag(dU);   % If dU is SPD, use "abs" in front of diag.  This is because,
           % even though dU is supposed to be positive, the numerical solution
			  % for a near-zero eigenvalue can be negative.
			  % *******************
			  %  Second thought: "abs" is not necessary, I think.  If negative, it
			  %      implies zero already -- thus the smallest number
[dUvec2,dUI2] = sort(dUvec);     % ascending
if desI==1      % if descending is required
	dUvec2 = flipud(dUvec2);
	dUI2 = flipud(dUI2);
end

if nargout==1
	vU2 = dUvec2;
	dU2 = NaN;
else
	vU2 = vU(:,dUI2);
	dU2 = diag(dUvec2);    % put it back to matrix form
end