Additive functions

A function f from positive integers to real numbers is defined to be additive if for relatively prime numbers m and n,

f(mn) = f(m) + f(n).

The function f is called completely addititive if the above holds for all positive integers m and n, i.e. we drop the requirement that m and n are relatively prime.

Example: total prime factors

One example of an additive function is the function Ω(n) defined to be the number of prime factors of n, counted with multiplicity. For example, Ω(12) = 3 because 12 = 2 × 2 × 3. The numbers 10 and 63 are relatively prime, and

Ω(630) = 5 = Ω(10) + Ω(63).

Example: distinct prime factors

Another example of an additive function is ω(n) defined to be the number of distinct prime factors of n, i.e. not counting with multiplicity. So, for example, ω(12) = 2.

This function is additive but not completely additive because, for example,

ω(20) = 2 ≠ ω(2) + ω(10)  = 3

A theorem of Erdős

Here is a remarkable theorem due to Paul Erdős [1]. Suppose f is an additive function such that

f(n + 1) − f(n)

converges to zero as n goes to infinity. Then

f(n) = c log(n)

for some constant c. And since a multiple of a logarithm is a logarithm to a different base, we can restate the conclusion by simply saying f is a logarithm.

Logarithms are completely additive functions, so even though we only assumed f was additive, this combined with the limit condition proves that in fact f is completely additive.

Related posts

[1] Paul Erdős, “On the distribution function of additive functions,” Ann. of Math., Vol. 47 (1946), pp. 1–20.

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