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 (the Lagarias arithmetic derivative), such as Ihara's arithmetic derivative and Buium's arithmetic derivatives.
- for any prime .
- for any (Leibniz rule).
Extensions beyond natural numbers
Edward J. Barbeau 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 of primes are allowed to be arbitrary rational numbers, allowing expressions like to be computed. 
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.
The arithmetic derivative has also been extended to the ring of integers modulo n.
The Leibniz rule implies that (take ) and (take ).
The power rule is also valid for the arithmetic derivative. For any integers p and n ≥ 0:
This allows one to compute the derivative from the prime factorisation of an integer, :
The logarithmic derivative is a totally additive function:
Inequalities and bounds
E. J. Barbeau examined bounds of the arithmetic derivative. He found that
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 .
Dahl, Olsson and Loiko found the arithmetic derivative of natural numbers is bounded by
where is the least prime in and equality holds when is a power of .
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
for any δ > 0, where
Relevance to number theory
Victor Ufnarovski 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 .
- In this article we use Oliver Heaviside's notation for the arithmetic derivative of . There are various other notations possible, such as ; a full discussion is available here for general differential operators, of which the arithmetic derivative can be considered one. Heaviside's notation is used here because it highlights the fact that the arithmetic derivative is a function over the integers and yields itself better notation-wise to function iteration for second and higher-order arithmetic derivatives.
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