# Thread: Inverse Matrix - Eigenvalues & Eigenvectors (Proof Needed)

1. ## Inverse Matrix - Eigenvalues & Eigenvectors (Proof Needed)

Hello,

One of the questions I was presented in a tutorial the other day really stumped me and I am unsure as to how to prove it.
The question is:

"Suppose that A is an invertible matrix and that x is an eigenvector for A with eigenvalue 'lambda cannot equal 0'. Show that x is an eigenvector for the inverse matrix of A with eigenvalue 'lambda-1'.

I have been shown a similar question where you had to prove that the matrix A2 had an eigenvalue of lambda2 through the manipulation of the equation Ax = (lambda)x as below:

Ax = (lambda)x
A(Ax) = A(lambda)x
A2x = (lambda)Ax
A2x = (lambda)(lambda)x
A2x = (lambda)2x

Is the solution to this question reached through the same steps, or is there a step I need to do differently?

Any help would be greatly appreciated

2. ## Re: Inverse Matrix - Eigenvalues & Eigenvectors (Proof Needed)

Perhaps you should attempt this question before asking for help. Don't be afraid to try.

$Ax=\lambda x \iff A^{-1}Ax=A^{-1}\lambda x \iff x=\lambda A^{-1}x$

3. ## Re: Inverse Matrix - Eigenvalues & Eigenvectors (Proof Needed)

Let $A$ be a matrix with eigenvalue $\lambda$ and eigenvector $x$.

Then $Ax = \lambda x$

Since $A$ is invertible, we can multiply both sides by $A^{-1}$.

$A^{-1} Ax = A^{-1}\lambda x$
$x = A^{-1} \lambda x$

Solve for $A^{-1} x$.

Thus, $A^{-1} x = \frac{1}{\lambda} x$