2. Show that $\sum_s u^s(p)\bar u^s(p)=\gamma\cdot p +m$.
4. Among the following interaction lagrangians $g\Phi(x)\bar\psi(x)\psi(x)$, $g(\partial_\mu A^\mu)^4$, $g \Phi^3$, which ones are renormalisable in spacetimes of dimension 4 or 6 ?