mathlib3

9b0d30c0 - feat(number_theory/arithmetic_function): define `is_multiplicative` on `arithmetic_function`s, provides examples (#4312)

feat(number_theory/arithmetic_function): define `is_multiplicative` on `arithmetic_function`s, provides examples (#4312) Provides a few basic examples of important arithmetic functions Defines what it means for an arithmetic function to be multiplicative Co-authored-by: Aaron Anderson <65780815+awainverse@users.noreply.github.com> Co-authored-by: Bryan Gin-ge Chen <bryangingechen@gmail.com>