Expanding zeta derivation statement
// Derived from SciPy's special/cephes/zeta.c
// https://github.com/scipy/scipy/blob/master/scipy/special/cephes/zeta.c
+// Made freely available by Stephen L. Moshier without support or guarantee.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
