15

2

Maple has a function `InverseTotient( c )`

, which returns all those natural numbers $n$ whose Euler totient function $\phi( n ) = c$. Is there an equivalent
inverse of `EulerPhi[ ]`

in Mathematica? If not, what would be a good method for
computing the inverse totient, faster than just trying all possibilities?

3

The hidden inverse totient function revealed by @MichaelE2 was a welcome surprise! (+1). For more information please see the CNT.m package in the Computational Number Theory book by Bressoud and Wagon. Also see the notebook by Maxim Rytin.

– KennyColnago – 2017-11-29T17:10:07.963