function  spectrum(a,x)

% Plots eigenvalues of `a'
% `x' must be character string accepted by plot 
% as the symbol/colour item, e.g., x='b*' for blue stars

if nargin==1, x='r+'; end
[n,m]=size(a);
hold on
h=eig(a);
if x==1
  for j=1:n
       plot(real(h(j)),imag(h(j)),'+')   
  end
else
   for j=1:n
      plot(real(h(j)),imag(h(j)),x)
   end
end