Why doesn't inverse iteration always converge towards the eigenvector with the smallest eigenvalue?

900 Views Asked by At

Here's my reasoning. Power iteration converges towards the eigenvector with the largest eigenvalue. Inverse iteration is power iteration using the matrix $(A-\mu I)^{-1}$. The eigenvalues of this matrix are $(\lambda_i - \mu)^{-1}$ with the $\lambda_i$ being the eigenvalues of $A$, so the largest eigenvalue of $(A-\mu I)^{-1}$ is going to correspond to the smallest eigenvalue of $A$. So inverse iteration converges towards the eigenvector with the smallest eigenvalue.

But inverse iteration is supposed to converge towards the eigenvector with the eigenvalue closest to $\mu$. So i made a mistake somewhere. What am i getting wrong ?

1

There are 1 best solutions below

0
On BEST ANSWER

If $B = A - \mu I$, iterating $B$ converges (under appropriate conditions) to an eigenvector for the largest eigenvalue (in magnitude) of $B$. Iterating $B^{-1}$ should similarly converge to an eigenvector for the largest eigenvalue of $B^{-1}$. Now the eigenvalues of $B^{-1} = (A - \mu I)^{-1}$ are $(\lambda - \mu)^{-1}$ where $\lambda$ are the eigenvalues of $A$. The absolute value of this is largest when $\lambda$ is closest to $\mu$.