The price of a bond at time $T$, measured in terms of its yield, is $G\left(y_T\right)$. Assume geometric Brownian motion for the forward bond yield $y$ in a world that is defined by a numeraire equal to a bond maturing at time $T$. Suppose that the growth rate of the forward bond yield is $\alpha$ and its volatility $\sigma_y$.
(a) Use ItĂ´'s lemma to calculate the process for the forward bond price in terms of $\alpha$, $\sigma_y, y$, and $G(y)$.
(b) The forward bond price should follow a martingale in the world considered. Use this fact to calculate an expression for $\alpha$.
(c) Show that the expression for $\alpha$ is, to a first approximation, consistent with equation (30.1).