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Divisor function σ 0 (n) up to n = 250 Sigma function σ 1 (n) up to n = 250 Sum of the squares of divisors, σ 2 (n), up to n = 250 Sum of cubes of divisors, σ 3 (n) up to n = 250. In mathematics, and specifically in number theory, a divisor function is an arithmetic function related to the divisors of an integer.
An example of an arithmetic function is the divisor function whose value at a positive integer n is equal to the number of divisors of n. Arithmetic functions are often extremely irregular (see table ), but some of them have series expansions in terms of Ramanujan's sum .
A divisor of an integer n is an integer m, for which n/m is again an integer (which is necessarily also a divisor of n). For example, 3 is a divisor of 21, since 21/7 = 3 (and therefore 7 is also a divisor of 21). If m is a divisor of n, then so is −m. The tables below only list positive divisors.
Divisor function d(n) up to n = 250 Prime-power factors In number theory , a superior highly composite number is a natural number which, in a particular rigorous sense, has many divisors . Particularly, it is defined by a ratio between the number of divisors an integer has and that integer raised to some positive power.
σ k (n) is the divisor function (i.e. the sum of the k-th powers of the divisors of n, including 1 and n). σ 0 (n), the number of divisors of n, is usually written d(n) and σ 1 (n), the sum of the divisors of n, is usually written σ(n). If s > 0,
A positive divisor of that is different from is called a proper divisor or an aliquot part of (for example, the proper divisors of 6 are 1, 2, and 3). A number that does not evenly divide but leaves a remainder is sometimes called an aliquant part of . An integer > whose only proper divisor is 1 is called a prime number. Equivalently, a prime ...
The harmonic mean H(n) of the divisors of any number n can be expressed as the formula = () where σ i (n) is the sum of i th powers of the divisors of n: σ 0 is the number of divisors, and σ 1 is the sum of divisors ().
where () =, is the multiplicative identity for Dirichlet convolutions, = is the identity function for powers, denotes the characteristic function for the squares, () which counts the number of distinct prime factors of (see prime omega function), is Jordan's totient function, and () = is the divisor function (see Dirichlet convolutions).