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Since there are only finitely many residues mod $n$, there is some function $H(n) \le n$ such that for all integers $n>1$, $r$, and $e>H(n)$, if $r$ is an $e$-th power mod $n$ then there is some $p \le H(n)$ such that $r$ is a $p$-th power mod $n$. (As usual, $p$ denotes a prime number.)

Is $H_0$ such a function, where $H_0(n)=p$ is defined by $$ (p-1)\# \le n \le p\# $$ with the primorial $x\#=\prod_{p\le x}p=\exp(\vartheta(x))$?

For example, 10 is not a square, cube, or fifth or seventh power mod 36, so this would say that it is not an $e$-th power for any $e>7$.

Are any such residues coprime to their modulus? (I assume not.)

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  • $\begingroup$ I do not understand the question: every $e$-th power is an $e$-th power modulo every integer. Do you ask for some converse? $\endgroup$ Commented Dec 20, 2022 at 14:45
  • $\begingroup$ @ChristopheLeuridan I'm only looking at powers mod $n$ in this question. For integer powers there's no $n$ for the $H(n)$, and the analogue with fixed $H$ fails for integer powers because, e.g., $2^p$ for $p>H$ is a $p-th$ power but not any lower power. $\endgroup$
    – Charles
    Commented Dec 20, 2022 at 14:52

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So your question is as follows: let $n$ be a positive integer, $q$ the smallest prime such that $q\sharp > n$, $e>q$ be an integer and $r$ be an $e$-th power mod $n$. Then is $r$ a $p$-th power for some $p\leq q$?

In general, no. Take a large prime $q$ and $n$ be the greatest power of $2$ less than $q\sharp$: then $n \geq e^{(1+o(1))q}$ by the PNT, so we can find (PNT again) some prime $s>q$ such that $2^s < n$. Take $r=2^s$ and $e=s$, assume that $r$ is a $p$-th power for some $p \leq q$. But this implies that the $2$-adic valuation of $r$ is divisible by $p$, which is impossible.


Now, let’s state a simple condition under which the statement holds (albeit for mostly trivial reasons).

Note that $q\sharp= e^{\theta(q)} \in [c^{-1},c]e^q$ where $c=e^{0.007q/\ln{q}}$ (by eg this): so if $n \geq 6$, we see that $n \leq q\sharp < 3^q$.

Our assumption is that we also have $v_2(n) \leq q$.

If $e$ has a prime divisor $p \leq q$, then $p$ works. Otherwise, replacing $e$ with one of its prime divisors, we can assume that $e$ is prime.

As $\varphi(n) < n \leq q\sharp$, there is some $p\leq q$ not dividing $\varphi(n)$, so that taking the $p$-th power in $(\mathbb{Z}/d\mathbb{Z})^{\times}$ is an isomorphism for all $d |n$.

Moreover, if $s$ is a prime power dividing $n$ and coprime to $n/s$, such that $r$ and $s$ are not coprime, then the valuation of $s$ is at most $q$ (by the assumption), so the only noninvertible $e$-th power mod $s$ is zero (every noninvertible element to the $e$-th power is zero). Thus $r$ is zero mod $s$ (hence a $p$-th power).

It follows by CRT that $s$ is a $p$-th power mod $n$, QED.

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