An analogous result applies for the position operator in the momentum basis, ⟨ ⟨ p | x ^ ^ | ψ ψ ⟩ ⟩ = i ℏ ℏ d d p ψ ψ ( p ) , leading to further useful relations, ⟨ ⟨ p | x ^ ^ | p ′ ⟩ ⟩ = i ℏ ℏ d d p δ δ ( p − − p ′ ) , ⟨ ⟨ x | p ^ ^ | x ′ ⟩ ⟩ = − − i ℏ ℏ d d x δ δ ( x − − x ′ ) , where δ stands for Dirac's delta function.