--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/toolbox/gersh.m	Thu May 07 18:36:24 2015 +0200
@@ -0,0 +1,38 @@
+function  [G, e] = gersh(A, noplot)
+%GERSH    Gershgorin disks.
+%         GERSH(A) draws the Gershgorin disks for the matrix A.
+%         The eigenvalues are plotted as crosses `x'.
+%         Alternative usage: [G, E] = GERSH(A, 1) suppresses the plot
+%         and returns the data in G, with A's eigenvalues in E.
+%
+%         Try GERSH(LESP(N)) and GERSH(SMOKE(N,1)).
+
+if diff(size(A)), error('Matrix must be square.'), end
+
+n = max(size(A));
+m = 40;
+G = zeros(m,n);
+
+d = diag(A);
+r = sum( abs( A.'-diag(d) ) )';
+e = eig(A);
+
+radvec = exp(i * seqa(0,2*pi,m)');
+
+for j=1:n
+    G(:,j) = d(j)*ones(size(radvec)) + r(j)*radvec;
+end
+
+if nargin < 2
+
+   ax = cpltaxes(G(:));
+   for j=1:n
+       plot(real(G(:,j)), imag(G(:,j)),'-c5')      % Plot the disks.
+       hold on
+   end
+   plot(real(e), imag(e), 'xg')    % Plot the eigenvalues too.
+   axis(ax);
+   axis('square');
+   hold off
+
+end