Remarkable relations between the central binomial series, Eulerian polynomials, and poly-Bernoulli numbers

The central binomial series at negative integers are expressed as a linear combination of values of certain two polynomials. We show that one of the polynomials is a special value of the bivariate Eulerian polynomial and the other polynomial is related to the antidiagonal sum of poly-Bernoulli numbers. As an application, we prove Stephan’s observation from 2004.