# Projector Operator

alphaneutrino
An Operator is defined as P|x> = |-x>,
1. Is P^2 a Projector operator?
2. What are the eigen value and eigen function of P?

LAHLH
Try acting again with the operator P

Homework Helper
Be careful, P is not a projector operator (P² is not equal to P); the question asks if P² is a projection operator.
alpha, What is the definition of such an operator?

Staff Emeritus
Gold Member
Alphaneutrino, you should state the definition of "projection operator" that you intend to use, and tell us where you get stuck.

alphaneutrino
Be careful, P is not a projector operator (P² is not equal to P); the question asks if P² is a projection operator.
alpha, What is the definition of such an operator?

Thank you Compuchip!
I am asking about P. Yes, I know that P^2|x> = |x> which is not equal to p|x>. So it is not projection operator. My next confusion is can I write
|-x> = -|x> ?

How can we calculate the eigen value and eigen function of P

sgd37
Thank you Compuchip!
I am asking about P. Yes, I know that P^2|x> = |x> which is not equal to p|x>. So it is not projection operator. My next confusion is can I write
|-x> = -|x> ?

How can we calculate the eigen value and eigen function of P

what you have there is a parity operator and they have eigenvalues $$\pm1$$ and the most generic eigenfunctions I can think of are $$A(e^{kx} \pm e^{-kx}) ; k \in C$$ respectively for the +1 and -1 eigenvalues

Gold Member
|-x>=-|x> only if the states have odd parity in x. Maybe it'd be clearer if you just used say f(-x) and -f(x).

Staff Emeritus
Well, $P^2$ is a projector. It can be shown using the definition of a projector operator on a Hilbert space.