Solution
= Solution
A correction with three external legs can be made from one cubic and one quartic vertex by contracting four of their seven fields into internal lines. Consequently the first correction to $\alpha_0$ involving $\lambda_0$ is
$$
\boxed{O(\alpha_0\lambda_0)}.
$$
A purely quartic interaction cannot generate an odd interaction because its $\mathbb Z_2$ symmetry forbids odd powers of $\phi$.