Binomial power-utility terminal wealth (source code)

= Binomial power-utility terminal wealth
{title2=$X^*=R^nw_0Z^{-\eta}/\mathbb E Z^{1-\eta}$}

In a complete $n$-period <binomial market> with riskless gross return $R$, pricing density $Z$ and <utility function> $U(x)=\gamma x^{1/\gamma}$, put $\eta=\gamma/(\gamma-1)$. The optimal terminal payoff is $X^*=R^nw_0Z^{-\eta}/\mathbb E[Z^{1-\eta}]$. Its marginal utility is proportional to $Z$; the supporting-line inequality for the strictly concave <utility function> proves optimality using the pricing <budget constraint>.