Triangle Feynman diagram
= Triangle Feynman diagram
A triangle <Feynman diagram> has a loop of three propagators joining three vertices. In <six-dimensional cubic scalar field theory> it gives a one-loop three-point proper vertex. Attaching a cubic tree vertex makes a four-point connected diagram, but the connecting propagator is a bridge, so the latter is a <one-particle-reducible Feynman diagram>.