For incoming four-momenta, the momentum-space Feynman rules in the chosen Minkowski metric convention are:

Each internal line contributes the scalar propagator $i / (p^{2} - m^{2} + i 0)$ .
Each quartic interaction vertex contributes $- i λ_{0}$ and $(2 π)^{D} δ^{D} (\sum p)$ for momentum conservation.
Each independent loop momentum is integrated with $d^{D} k / (2 π)^{D}$ ; after eliminating redundant Dirac delta functions, one overall conservation factor remains.
Each graph is divided by its symmetry factor $S$ , with external labels held fixed.

There is one connected one-loop one-particle irreducible two-point graph: a single quartic vertex with two external lines and one tadpole diagram. Its symmetry factor is

S = 2

from interchange of the two loop half-edges.

There are three connected one-loop one-particle irreducible four-point graphs: two quartic vertices joined by two parallel internal lines, with external partitions