That's awesome  thanks very much. I had a suspicion that was the case but have only ever worked at such a miniscule order I wasn't sure if it was the case. Thanks mate!
Quote by Avodyne
An npoint vertex function is a sum of oneparticleirreducible diagrams with external propagators removed. (There is also a nonperturbative definition in terms of Legendre transforms of the functional integral with a source, but I don't remember it precisely enough to quote it.)
For 22 scattering, you need the 4point vertex function. But when you compute it, at high enough orders, you will have subdiagrams that involve npoint vertices for arbitrarily high n. And these have to be renormalized, so you will need the corresponding parameters.
