# Diagonal Matrix Proof help

1. Jul 25, 2009

### jon555

Hi I have this question for my Linear Algebra class and I can't seem to figure it out.

Let A and B be n x n matrices such that B = (P^-1)AP and let lambda ne an eigenvalue of A (and hence of B). Prove the following results:
(a) A vector b in R^n is in the eigenspace of A corresponding to labmda if and only if (P^-1)v is in the eigenspace corresponding to lambda.

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My thought process was that since A and B are similar they will will have the same characteristic polynomial, eigenvalues, multiplicity etc. And B is the diagonal matrix of A and the columns of P are a basis for R^n. Also the equation Ax=b is consistent for every b in R^n if A is invertible.

Ive been working on this problem all week and cant seem to get it. I think Im close but I cant seem to make the connection.

Jon

2. Jul 26, 2009

### CompuChip

Probably this is the type of question you're going to slap yourself in the head for, when seeing how straightforward it is.

So you have B = P-1AP. I'll be using c instead of lambda because I'm too lazy to find a HTML lambda. You want to prove that
Av = cv, if and only if B(P-1 v) = c (P-1 v).

So start by the direct implication: suppose that Av = cv. Now calculate B(P-1 v) using what you know about B.

3. Jul 26, 2009

### HallsofIvy

Ahhh! "let lambda be an eigenvalue". I read this as "let lambda not equal an eigenvalue" and couldn't understand what you were saying from here on!