The Stokes phenomenon associated with the periodic zeta function F(a,s)

The exponentially improved large-$a$ expansion for the Hurwitz zeta function $\zeta(s,a)$ is exploited to examine the expansion of the periodic zeta function $F(a,s)$ in the upper half-plane of the variable $a$. It is shown that a double Stokes phenomenon takes place in the vicinity of the positive imaginary $a$-axis as $|a|\ra\infty$. This is a consequence of the fact that constituent parts of $F(a,s)$ involve two Hurwitz zeta functions resulting in two parallel Stokes lines at unit distance apart. Numerical calculations confirm the theoretical predictions.