RDP 9204: The Term Structure of Interest Rates, Real Activity and inflation Equation (A21)

\begin{array}{l} \frac{δ (ϒ - \bar{ϒ})}{δ t} = 0 & \Rightarrow & t^{*} = \frac{\ln (\frac{- θ}{ψ})}{θ+ψ} \end{array}