Solution
= Solution
At order $\lambda^2$, four connected <tree-level Feynman diagrams> contribute: one four-point contact diagram and three diagrams with two cubic vertices joined by a scalar propagator in the $s$, $t$, and $u$ channels.
= Solution
At order $\lambda^2$, four connected <tree-level Feynman diagrams> contribute: one four-point contact diagram and three diagrams with two cubic vertices joined by a scalar propagator in the $s$, $t$, and $u$ channels.