# Noncototient

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In mathematics, a noncototient is a positive integer n that cannot be expressed as the difference between a positive integer m and the number of coprime integers below it. That is, m − φ(m) = n, where φ stands for Euler's totient function, has no solution for m. The cototient of n is defined as n − φ(n), so a noncototient is a number that is never a cototient.

It is conjectured that all noncototients are even. This follows from a modified form of the Goldbach conjecture: if the even number n can be represented as a sum of two distinct primes p and q, then

$pq - \varphi(pq) = pq - (p-1)(q-1) = p+q-1 = n-1. \,$

It is expected that every even number larger than 6 is a sum of distinct primes, so probably no odd number larger than 5 is a noncototient. The remaining odd numbers are covered by the observations $1=2-\phi(2), 3 = 9 - \phi(9)$ and $5 = 25 - \phi(25)$.

The first few noncototients are:

10, 26, 34, 50, 52, 58, 86, 100, 116, 122, 130, 134, 146, 154, 170, 172, 186, 202, 206, 218, 222, 232, 244, 260, 266, 268, 274, 290, 292, 298, 310, 326, 340, 344, 346, 362, 366, 372, 386, 394, 404, 412, 436, 466, 470, 474, 482, 490, 518, 520 (sequence A005278 in OEIS)

Erdős (1913-1996) and Sierpinski (1882-1969) asked whether there exist infinitely many noncototients. This was finally answered in the affirmative by Browkin and Schinzel (1995), who showed every member of the infinite family $2^k \cdot 509203$ is an example. Since then other infinite families, of roughly the same form, have been given by Flammenkamp and Luca (2000).