mathlib3

81843c08 - feat (number_theory/zeta_function): func eqn + link to Bernoulli nos (#19158)

feat (number_theory/zeta_function): func eqn + link to Bernoulli nos (#19158) Functional equation relating `zeta (1 - s)` to `zeta s`, and formula for `zeta (-k)` for nat `k` in terms of Bernoulli numbers.