Often the unital associative algebra is the algebra of endomorphisms of a vector space V over a field k , that is, A = End ( V ) . With respect to a basis { e i } of V , the components of the matrices R ∈ ∈ End ( V ) ⊗ ⊗ End ( V ) ≅ ≅ End ( V ⊗ ⊗ V ) are written R i j k l , which is the component associated to the map e i ⊗ ⊗ e j ↦ ↦ e k ⊗ ⊗ e l . Omitting parameter dependence, the component of the Yang–Baxter equation associated to the map e a ⊗ ⊗ e b ⊗ ⊗ e c ↦ ↦ e d ⊗ ⊗ e e ⊗ ⊗ e f reads