Arithmetic Function

In number theory, an arithmetic function, also called an arithmetical or number-theoretic function, is generally any function whose domain is the positive integers and whose range is a subset of the complex numbers, with some authors further requiring that it express some arithmetical property of its input. An example is the divisor function, whose value at a positive integer n is the number of divisors of n. Arithmetic functions are often highly irregular in their behavior, though some admit series expansions in terms of Ramanujan sums. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/