Additive function
 |
This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. (February 2013) (Learn how and when to remove this template message)
|
In number theory, an additive function is an arithmetic function f(n) of the positive integer n such that whenever a and b are coprime, the function of the product is the sum of the functions:[1]
- f(ab) = f(a) + f(b).
Contents
Completely additive[edit]
An additive function f(n) is said to be completely additive if f(ab) = f(a) + f(b) holds for all positive integers a and b, even when they are not co-prime. Totally additive is also used in this sense by analogy with totally multiplicative functions. If f is a completely additive function then f(1) = 0.
Every completely additive function is additive, but not vice versa.
Examples[edit]
Example of arithmetic functions which are completely additive are:
- The restriction of the logarithmic function to N.
- The multiplicity of a prime factor p in n, that is the largest exponent m for which pm divides n.
- a0(n) - the sum of primes dividing n counting multiplicity, sometimes called sopfr(n), the potency of n or the integer logarithm of n (sequence A001414 in the OEIS). For example:
-
- a0(4) = 2 + 2 = 4
- a0(20) = a0(22 · 5) = 2 + 2+ 5 = 9
- a0(27) = 3 + 3 + 3 = 9
- a0(144) = a0(24 · 32) = a0(24) + a0(32) = 8 + 6 = 14
- a0(2,000) = a0(24 · 53) = a0(24) + a0(53) = 8 + 15 = 23
- a0(2,003) = 2003
- a0(54,032,858,972,279) = 1240658
- a0(54,032,858,972,302) = 1780417
- a0(20,802,650,704,327,415) = 1240681
- The function Ω(n), defined as the total number of prime factors of n, counting multiple factors multiple times, sometimes called the "Big Omega function" (sequence A001222 in the OEIS). For example;
-
- Ω(1) = 0, since 1 has no prime factors
- Ω(4) = 2
- Ω(16) = Ω(2·2·2·2) = 4
- Ω(20) = Ω(2·2·5) = 3
- Ω(27) = Ω(3·3·3) = 3
- Ω(144) = Ω(24 · 32) = Ω(24) + Ω(32) = 4 + 2 = 6
- Ω(2,000) = Ω(24 · 53) = Ω(24) + Ω(53) = 4 + 3 = 7
- Ω(2,001) = 3
- Ω(2,002) = 4
- Ω(2,003) = 1
- Ω(54,032,858,972,279) = 3
- Ω(54,032,858,972,302) = 6
- Ω(20,802,650,704,327,415) = 7
Example of arithmetic functions which are additive but not completely additive are:
- ω(n), defined as the total number of different prime factors of n (sequence A001221 in the OEIS). For example:
-
- ω(4) = 1
- ω(16) = ω(24) = 1
- ω(20) = ω(22 · 5) = 2
- ω(27) = ω(33) = 1
- ω(144) = ω(24 · 32) = ω(24) + ω(32) = 1 + 1 = 2
- ω(2,000) = ω(24 · 53) = ω(24) + ω(53) = 1 + 1 = 2
- ω(2,001) = 3
- ω(2,002) = 4
- ω(2,003) = 1
- ω(54,032,858,972,279) = 3
- ω(54,032,858,972,302) = 5
- ω(20,802,650,704,327,415) = 5
- a1(n) - the sum of the distinct primes dividing n, sometimes called sopf(n) (sequence A008472 in the OEIS). For example:
-
- a1(1) = 0
- a1(4) = 2
- a1(20) = 2 + 5 = 7
- a1(27) = 3
- a1(144) = a1(24 · 32) = a1(24) + a1(32) = 2 + 3 = 5
- a1(2,000) = a1(24 · 53) = a1(24) + a1(53) = 2 + 5 = 7
- a1(2,001) = 55
- a1(2,002) = 33
- a1(2,003) = 2003
- a1(54,032,858,972,279) = 1238665
- a1(54,032,858,972,302) = 1780410
- a1(20,802,650,704,327,415) = 1238677
Multiplicative functions[edit]
From any additive function f(n) it is easy to create a related multiplicative function g(n) i.e. with the property that whenever a and b are coprime we have:
- g(ab) = g(a) × g(b).
One such example is g(n) = 2f(n).
See also[edit]
References[edit]
Further reading[edit]
- Janko Bračič, Kolobar aritmetičnih funkcij (Ring of arithmetical functions), (Obzornik mat, fiz. 49 (2002) 4, pp. 97–108) (MSC (2000) 11A25)
