  function Mar3()
% For GS, the iteration matrix M = -inv(D+L)*U.
% If A = [a b ; c d] then M = [0 -b/a; 0 (b*c)/(a*d)].
% The spectral radius of this matrix = abs(largest eig) =
% abs((b*c)/((a*d)).
% So we must have |b*c|<|a*d|. 