Hey guys, maybe you can help me with the following problem. I have to calculate the commutator relations in position representation: a) [V,ρ] b) [p,ρ] c) [p^2,ρ] Note that <q'|ρ|q>=ρ(q',q) is a matrix element of the density operator I already solved the first one. You just have to apply the potential operator on a matix element of the density operator. [V,ρ]=<q'|Vρ|q>-<q'|ρV|q>=...=(V(q')-V(q))*ρ(q',q) The rest however is more tricky as the momentum operator is not diagonal in this domain. [p,ρ]=<q'|pρ|q>-<q'|ρp|q>=...? I got the hint that I should try an integration over an auxiliary variable which should lead to something like <q'|ρ|q''>~δ(q'-q'') (Delta functions) A Fourier transformation is NOT necessary as far as I know. The result of c should be something like: -(d^2/dq'^2-d^2/dq^2)*ρ(q',q) Thanks a lot for your help!!!!