function eigplot(A,n)
% This program visualizes eigenvectors for 2x2 matrices. 
% A is a 2x2 matrix. n is optional and determines the number
% of vectors drawn.
% The program works best for positive definite matrices. Negative 
% eigenvalues less than -1 give particularly cluttered images.
%
% As a reference see the CMJ vol.26,no4, Sept. 1995, p.316 (S. Schonefeld).
%
% Typical examples use the matrices:
%
% A=[13 9;3 7]/8; (fairly generic)
% A=[cos(pi/4) -sin(pi/4);sin(pi/4) cos(pi/4)];  %% (nonreal eigenvalues)
% A=[2 -2; -1 1]; (one zero eigenvalue)
% A=[2-sqrt(3) -3; 1 2+sqrt(3)]/2; (double eigenvalue)
%
% All rights reseved: Matthias Kawski, October 1995
%                     http://math.asu.edu/~kawski/
% Most recent update: Aug 2004. Corrected placement of aspect ratio command.
if nargin == 0
   A=[13 9;3 7]/8;
   end;
if nargin < 2 
   n=48;
   end;
phi=2*pi/n;
z=zeros(1,n);
x=cos(phi*[0:n-1]);
y=sin(phi*[0:n-1]);
v=A(1,:)*[x;y];
w=A(2,:)*[x;y];
hold on
[V,D]=eig(A);
m=1.2*max([1,max(abs(v)),max(abs(w))]);
myfig=figure
hold on
text(-0.8*m,0.9*m,num2str(D));
text( 0.2*m,0.9*m,num2str(V));
plot([z;x],[z;y],'b-',[x;x+v],[y;y+w],'r-')
axis([-m,m,-m,m]);
axis equal
figure(myfig);
disp('Hit enter to close the plot window')
pause
close(myfig)