  It is known by a formula of Hasse-Sondow that the Riemann zeta function is
given, for any $ s=\sigma+it \in \mathbb{C}$, by $ \sum_{n=0}^{\infty}
\widetilde{A}(n,s)$ where $$ \widetilde{A}(n,s):=\frac{1}{2^{n+1}(1-2^{1-s})}
\sum_{k=0}^n \binom{n}{k}\frac{(-1)^k}{(k+1)^s} .$$ We prove the following
approximate functional equation for the Hasse-Sondow presentation: For $ \vert
t \vert = \pi xy $ and $ 2y \neq (2N-1)\pi $ then $$ \zeta(s)= \sum_{n \leq x }
\widetilde{A}(n,s)+\frac{\chi(s)}{1-2^{s-1}} \left (\sum_{k \leq y}
(2k-1)^{s-1} \right ) +O \left (e^{-\omega(x,y) t} \right ), $$ where $ 0
<\omega(x,y)$ is a certain transcendental number determined by $ x$ and $ y$. A
central feature of our new approximate functional equation is that its error
term is of exponential rate of decay. The proof is based on a study, via saddle
point techniques, of the asymptotic properties of the function $$
\widetilde{A}(u,s):= \frac{1}{2^{u+1} (1-2^{1-s}) \Gamma(s)} \int_{0}^{\infty}
\left ( e^{-w} \left ( 1- e^{-w} \right)^u \right ) w^{s-1} dw,$$ and integrals
related to it.
