feynlag.vertices.extract.vertex_multiplicity¶ feynlag.vertices.extract.vertex_multiplicity(field_tuple)[source]¶ Product of k! over the multiplicities of identical fields. This is the combinatorial factor relating a monomial coefficient to the Feynman vertex: for L ⊃ c · φ₁^{k₁} … φₙ^{kₙ} the vertex is i · c · ∏ kᵢ!.