In quantum mechanics, the momentum operator is given by p ^ ^ = − − i ℏ ℏ ∂ ∂ ∂ ∂ r (see matrix calculus for the denominator notation) with appropriate domain. The eigenfunctions are ψ ψ k ( r ) = 1 ( 2 π π ) 3 e i k ⋅ ⋅ r and eigenvalues ħ k. So ψ ψ ( r ) = 1 ( 2 π π ) 3 ∫ ∫ k -space ϕ ϕ ( k ) e i k ⋅ ⋅ r d 3 k and we see that the momentum representation is related to the position representation by a Fourier transform.