where p is the number of interaction fields (or, equivalently, the number of interacting particles) and n is the development order (or the number of vertices of interaction). For example, if v = g y 4 ⇒ ⇒ : v i ( y 1 ) : = : ϕ ϕ i ( y 1 ) ϕ ϕ i ( y 1 ) ϕ ϕ i ( y 1 ) ϕ ϕ i ( y 1 ) :