Show that $\left(\mathrm{D} ; \mathrm{d}: \frac{1}{2} ; \frac{1}{2}\right)$ is the only perfect Bayesian equilibrium of the above game.
a) First show that ( $\mathrm{D} ; \mathrm{d}: \frac{1}{2} ; \frac{1}{2}$ ) is a perfect Bayesian equilibrium of the game.
b) Show that $\mathrm{Q}$ strictly dominates $\mathrm{A}$ (and $\mathrm{q}$ strictly dominates a). Consequently, A and a can never appear in a perfect Bayesian equilibrium strategy.
c) Show that once neither player ever attacks (play $\mathrm{A}$ or a), D strictly dominates $\mathrm{Q}$ (and d strictly dominates $\mathrm{q}$ ). Thus only the given belief-strategy pairing can be a perfect Bayesian equilibrium.