# Inverse of the 1-D momentum operator

1. May 8, 2012

### spaghetti3451

1. The problem statement, all variables and given/known data

I have to find the inverse of the 1-D momentum operator.

2. Relevant equations

3. The attempt at a solution

Here's my solution:

Pψ(x) = -iħ dψ/dx
P-1[Pψ(x)] = P-1[-iħ dψ/dx]
[P-1P]ψ(x) = -iħ [P-1 dψ/dx]
Iψ(x) = -iħ [P-1 dψ/dx]
ψ(x) = -iħ [P-1 dψ/dx]

By induction, P-1f(x) = (i/ħ) ∫f(x)dx.

Any mistakes?

On a secondary note, I am not sure what property guarantees the validity of the second sentence: why should two equal expressions remain equal if they are being operated on by the same operator, that is, is an operator allowed to map an element from a vector space to only a single element (and not multiple elements) of the same vector space?

Also, what property of an operator guarantees the validity of the third sentence: does the property of associaitivity hold for operators?

Finally, I am wondering if I had started by calculating the expectation value of P-1 in momentum space and then use the Fourier transform of the wave functions, would I have got the right answer?