Let K {\displaystyle {}K} be a field, and let U , V , W {\displaystyle {}U,V,W} denote vector spaces over K {\displaystyle {}K} . Establish a linear mapping
that sends ψ ⊗ φ {\displaystyle {}\psi \otimes \varphi } to φ ∘ ψ {\displaystyle {}\varphi \circ \psi } .