Conversely, a three-dimensional wave function in momentum space ϕ ϕ ( k ) can be expressed as a weighted sum of orthogonal basis functions ϕ ϕ j ( k ) , ϕ ϕ ( k ) = ∑ ∑ j ψ ψ j ϕ ϕ j ( k ) , or as an integral, ϕ ϕ ( k ) = ∫ ∫ r -space ψ ψ ( r ) ϕ ϕ r ( k ) d 3 r .