CAS: sometimes doesn't detect Zeta series (Bug?)

[parisse]

There is indeed a bug in the normal code, called by the hypergeometric recognition function, and also a bug in this one. They are now fixed in source and in Xcas.
I do not plan to add recognition of sum(1/n^k,n,...,inf) to Zeta(k) now. Unlike for rational fractions where there is an algorithm (partial fraction expansion), it would only work for this serie, it's too much work for almost no benefit (especially since there is no closed form of Zeta for rationals in giac).