View source for Derivation of delta e sub p on T=Z

If $t>s$, then
$$\begin{array}{ll}
e_p(t,s) &= \exp \left( \displaystyle\int_{s}^{t} \dfrac{1}{\mu(\tau)} \log(1 + p(\tau)) \Delta \tau \right) \\
&= \exp \left( \displaystyle\sum_{k=s}^{t-1} \log(1+p(k))  \right) \\
&= \displaystyle\prod_{k=s}^{t-1} \left( 1+p(k) \right). \\
\end{array}$$
If $t<s$ then the integral $\displaystyle\int_s^t \dfrac{1}{\mu(\tau)} \log(1+p(\tau))\Delta \tau = -\displaystyle\int_t^s \dfrac{1}{\mu(\tau)}\log(1+p(\tau))\Delta \tau$, and so we see
$$e_p(t,s) = \dfrac{1}{e_p(s,t)}.$$