English: Left: a quasiparticle with momentum k interacts with a quasiparticle with momentum p+q-k hidden below the fermi surface. Three quasiparticles are generated, one with momentum p, one with momentum q and a hole with momentum -p-q+k. Right: The four momenta k, p, q and p+q-k must be close to the fermi surface because of energy conservation, and their pairwise difference must be the momentum transfer p-k. However, for arbitrary k, p, q near the fermi surface, p+q-k in general is not near the fermi surface.