= One-loop proper vertices of massless phi-fourth theory
{title2=$\widehat\tau_4^{(1)}=\frac{\lambda^2}{2}(f_s+f_t+f_u)$}
The <interaction vertex> $-i\lambda$ and <scalar propagator> $-i/(p^2-i\epsilon)$ give one four-point <bubble diagram> for each of three <momentum> channels, each with <Feynman-diagram symmetry factor> $1/2$. The two-point <tadpole diagram> is zero as a scaleless integral in <dimensional regularization>. If full proper vertices include the free quadratic kernel, $\widehat\tau_2^{(0)}=-p^2$; the interaction self-energy convention instead starts at zero.
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