by Ricardo Fernández Serrata
Computes the Zeta function at max precision if s >= 2, but if s < 2 then (based on k) it stops incrementing n.
Initial values of sum and n have a positive offset of 1 to skip 1 iteration (less latency, better performance)
s > -2 instead of s >= -1 is needed because inputs like 1.9 and 1.5 required an ETERNITY to return a result. Inputs closer to 1 require more iterations to converge. Inputs less than or equal to 1 never converge.
This implementation doesn't support complex numbers, yet. And it doesn't have analytic continuation.
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