# P operator

1. Nov 21, 2008

### najima

1. The problem statement, all variables and given/known data
given that X(operator) and P (operator) operate on functions,and the relation [X,P]=ih/2π,show that if X(operator)=x ,and P (operator) has the representation P=-ih/2π*∂/∂x +f(x)
where f(x) is an arbitrary function of x

2. Relevant equationsquantum mechanic by Liboff

3. The attempt at a solutionI wrote the commutator relation of P and x on an arbitrary function like g(x) ,[x,p]g(x) so XP(g(x))-PX(g(x))=ih/2pi(g(x)) because of X=x so I wrote
xP(g(x))-P(xg(x))=xP(g(x))-xP(g(x))-g(x)Px=-g(x)Px=ih/2pi(g(x)) so I can derive just this part of equation-ih/2π*∂/∂x , what can I do for the part of f(x)?

2. Nov 21, 2008

### gabbagabbahey

If I'm interpreting the question correctly, your not supposed to derive the commutation relation; it's actually a given. You are also told that $X=x$.What you don't know is what $P$ is. You're supposed to show that given $$[X,P]=i \hbar$$ and $X=x$ that $P$ must take the form $$-i \hbar \frac{d}{dx} +f(x)$$....to do this, just operate on a function $g(x)$ by $[x,P]$ while leaving $P$ as an unknown operator....what do you get when you do that?

3. Nov 21, 2008

### najima

thanx now I get it!!

4. Nov 21, 2008

### najima

I think I must derive that P is this form P=-ih/2π*∂/∂x +f(x)