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In mathematics, and specifically in number theory, a 'divisor function' is an arithmetical function related to the divisors of an integer. When referred as ''the'' divisor function, it counts the ''number of divisors of an integer''. It appears in a number of remarkable identities, including relationships on the Riemann zeta function and the Eisenstein series of modular forms. Divisor functions were studied by Ramanujan, who gave a number of important congruences and identities.
A related function is the divisor summatory function, which, as the name implies, is a sum over the divisor function.

Contents

Definition

Example

Properties

Series expansion

Series relations

Approximate growth rate

See also

References

Definition

The 'sum of positive divisors function' σ_''x''(''n'') is defined as the sum of the ''x''^th powers of the positive divisors of n, or
:$sigma\_\{x\}(n)=sum\_\{d|n\}\; d^x,!\; .$
The notations ''d''(''n'') and $au(n)$ (the tau function) are also used to denote σ₀(''n''), or the number of divisors of ''n''. When ''x'' is 1, the function is called the 'sigma function' or 'sum-of-divisors function', and the subscript is often omitted, so σ(''n'') is equivalent to σ₁(''n''). The 'aliquot sum' of ''n'' is the sum of the proper divisors (that is, the divisors excluding ''n'' itself), and equals σ₁(''n'') - ''n''; the aliquot sequence of ''n'' is formed by repeatedly applying the aliquot sum function.

Example

For example, σ₀(12) is the number of the divisors of 12:

{|
|-
| $sigma\_\{0\}(12)$
| $=\; 1^0\; +\; 2^0\; +\; 3^0\; +\; 4^0\; +\; 6^0\; +\; 12^0$
|-
|
| $=\; 1\; +\; 1\; +\; 1\; +\; 1\; +\; 1\; +\; 1\; =\; 6.$
|-
|}

while σ₁(12) is the sum of all the divisors:

{|
| $sigma\_\{1\}(12)$
| $=\; 1^1\; +\; 2^1\; +\; 3^1\; +\; 4^1\; +\; 6^1\; +\; 12^1$
|-
|
| $=\; 1\; +\; 2\; +\; 3\; +\; 4\; +\; 6\; +\; 12\; =\; 28.$
|-
|}

Properties

For a prime number ''p'',
:$d(p)\; =\; 2$
:$d(p^n)\; =\; n+1$
:$sigma(p)\; =\; p+1$
because by definition, the factors of a prime number are 1 and itself. Clearly, 1 < ''d''(n) < ''n'' and σ(''n'') > ''n'' for all ''n'' > 2.
The divisor function is multiplicative, but not completely multiplicative. The consequence of this is that, if we write
:$n\; =\; prod\_\{i=1\}^\{r\}p\_\{i\}^\{a\_\{i\}\}$
where ''r'' is the number of distinct prime factors of ''n'', ''p_i'' is the ''i''^th prime factor, and ''a_i'' is the maximum power of ''p_i'' by which ''n'' is divisible, then we have
:
which is equivalent to the useful formula:
:$$
sigma_x(n) = prod_{i=1}^{r} sum_{j=0}^{a_{i}} p_{i}^{j x} =
prod_{i=1}^{r} (1 + p_{i}^x + p_{i}^{2x} + ... + p_{i}^{a_i x})

An equation for calculating $au(n)$ is
:$$
au(n)=prod_{i=1}^{r} (a_i+1)

For example, if ''n'' is 24, there are two prime factors (''p₁'' is 2; ''p₂'' is 3); noting that 24 is the product of 2³×3¹, ''a₁'' is 3 and ''a₂'' is 1. Thus we can calculate $au(24)$ as so:

{|
|-
| $au(24)$
| $=\; prod\_\{i=1\}^\{2\}\; (a\_i+1)$
|-
|
| $=\; (3\; +\; 1)(1\; +\; 1)\; =\; 4\; imes\; 2\; =\; 8.$
|-
|}

The eight divisors counted by this formula are 1, 2, 4, 8, 3, 6, 12, and 24.
We also note $s(n)\; =\; sigma(n)\; -\; n$. Here $s(n)$ denotes the sum of the proper divisors of n, i.e. the divisors of n excluding n itself.
This function is the one used to recognize perfect numbers which are the ''n'' for which $s(n)\; =\; n$. If s(n) > n then n is an abundant number and if s(n) < n then n is a deficient number.
As an example, for two distinct primes ''p'' and ''q'', let
:$n\; =\; pq.$
Then
:$phi(n)\; =\; (p-1)(q-1)\; =\; n\; +\; 1\; -\; (p+q),$
:$sigma(n)\; =\; (p+1)(q+1)\; =\; n\; +\; 1\; +\; (p+q).$
In 1984, Roger Heath-Brown proved that
:''d''(''n'') = ''d''(''n'' + 1)
will occur infinitely often.

Series expansion

The divisor function can be written as a finite trigonometric series
:$sigma\_x(n)=sum\_\{mu=1\}^\{n\}\; mu^\{x-1\}sum\_\{$
u=1}^{mu}cosrac{2pi
u n}{mu}
without an explicit reference to the divisors of $n$,
see Teilersumme.

Series relations

Two Dirichlet series involving the divisor function are:
:
and
:
A Lambert series involving the divisor function is:
:
for arbitrary complex |''q''| ≤ 1 and ''a''. This summation also appears as the Fourier series of the Eisenstein series and the invariants of the Weierstrass elliptic functions.

Approximate growth rate

The behaviour of the sigma function is irregular.
The growth rate of the sigma function can be expressed by:
:$$
limsup_{n
ightarrowinfty}rac{sigma(n)}{n log log n}=e^gamma .

where lim sup is the limit superior and $gamma$ is Euler's constant. This result is 'Gronwall's theorem', published in 1913.
In 1984 Guy Robin proved that
:

is true if and only if the Riemann hypothesis is true (this is ''Robin's theorem''). The largest known value that violates the inequality is ''n''=5,040. If the Riemann hypothesis is true, there are no greater exceptions. If the hypothesis is false then there are an infinite number of values of ''n'' that violate the inequality.
A related bound was given by Jeffrey Lagarias in 2002, who proved that the Riemann hypothesis is equivalent to the statement that
:$sigma(n)\; le\; H\_n\; +\; ln(H\_n)e^\{H\_n\}$
for every natural number ''n'', where $H\_n$ is the ''n''th harmonic number.

See also

★ Euler's totient function (Euler's phi function)

★ Riemann zeta function

References

★ Tom M. Apostol, ''Introduction to Analytic Number Theory'', (1976) Springer-Verlag, New York. ISBN 0-387-90163-9

★ Eric Bach and Jeffrey Shallit, ''Algorithmic Number Theory'', volume 1, 1996, MIT Press. ISBN 0-262-02405-5, see page 234 in section 8.8.

★ Robin, G. "Grandes Valeurs de la fonction somme des diviseurs et hypothèse de Riemann." J. Math. Pures Appl. 63, 187-213, 1984. Original publication of Robin's theorem.

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