Arithmetic derivative


In number theory, the Lagarias arithmetic derivative, or number derivative, is a function defined for integers, based on prime factorization, by analogy with the product rule for the derivative of a function that is used in mathematical analysis.
There are many versions of "arithmetic derivatives", including the one discussed in this article, such as Ihara's arithmetic derivative and Buium's arithmetic derivatives.

Definition

For natural numbers the arithmetic derivative is defined as follows:
E. J. Barbeau was most likely the first person to formalize this definition. He also extended it to all integers by proving that uniquely defines the derivative over the integers. Barbeau also further extended it to rational numbers, showing that the familiar quotient rule gives a well-defined derivative on :
Victor Ufnarovski and Bo Åhlander expanded it to certain irrationals. In these extensions, the formula above still applies, but the exponents are allowed to be arbitrary rational numbers.
The arithmetic derivative can also be extended to any unique factorization domain, such as the Gaussian integers and the Eisenstein integers, and its associated field of fractions. If the UFD is a polynomial ring, then the arithmetic derivative is the same as the derivation over said polynomial ring. For example, the regular derivative is the arithmetic derivative for the rings of univariate real and complex polynomial and rational functions, which can be proven using the fundamental theorem of algebra.

Elementary properties

The Leibniz rule implies that and .
The power rule is also valid for the arithmetic derivative. For any integers and :
This allows one to compute the derivative from the prime factorisation of an integer, :
where, a prime omega function, is the number of distinct prime factors in, and is the p-adic valuation of.
For example:
or
The sequence of number derivatives for begins :

Related functions

The logarithmic derivative is a totally additive function:

Inequalities and bounds

E. J. Barbeau examined bounds of the arithmetic derivative. He found that the arithmetic derivative of natural numbers is bounded by
where is the least prime in and
where, a prime omega function, is the number of prime factors in.
In both bounds above, equality always occurs when is a perfect power of 2, that is for some.
Alexander Loiko, Jonas Olsson and Niklas Dahl found that it is impossible to find similar bounds for the arithmetic derivative extended to rational numbers by proving that between any two rational numbers there are other rationals with arbitrary large or small derivatives.

Order of the average

We have
and
for any δ > 0, where

Relevance to number theory

and Bo Åhlander have detailed the function's connection to famous number-theoretic conjectures like the twin prime conjecture, the prime triples conjecture, and Goldbach's conjecture. For example, Goldbach's conjecture would imply, for each the existence of an so that. The twin prime conjecture would imply that there are infinitely many for which.