powerm.m

various matlab programs to slove various problems

function [nu1,x1,niter,err]=powerm(A,z0,toll,nmax)
q=z0/norm(z0); 
q2=q; 
err=[]; 
nu1=[]; 
res=toll+1; 
niter=0; 
z=A*q;
while (res >= toll & niter <= nmax)
 q=z/norm(z); 
 z=A*q; 
 lam=q'*z; 
 x1=q;
 z2=q2'*A; 
 q2=z2/norm(z2); 
 q2=q2'; 
 y1=q2; 
 costheta=abs(y1'*x1);
 if (costheta >= 5e-2),
   niter=niter+1; 
   res=norm(z-lam*q)/costheta;
   err=[err; res]; 
   nu1=[nu1; lam];
 else
   disp(' Multiple eigenvalue '); break;
 end
end
return