% m6_lanczos.m - play with Lanczos iteration

   A = diag(sparse(1:999));
   e = lanczos(A,20); e(end-3:end)
   e = lanczos(A,100); e(end-3:end)
   e = lanczos(A,200); e(end-3:end)
   eigs(A)				% Matlab's version of ARPACK:
					% robust implicitly restarted
					% Lanczos/Arnoldi algorithm
   A = sprandsym(10000,.001);
   A(500,500) = 10;
   A(700,700) = 8;
   spy(A)
   e = lanczos(A,20); e(end-3:end)
   e = lanczos(A,40); e(end-3:end)
   e = lanczos(A,60); e(end-3:end)
   e = lanczos(A,80); e(end-3:end)
 
   eigs(A)
   eigs(A,4)
   eigs(A,4,'LA')