Tanya wrote numbers in forms $n^7-1$ for $n=2,3,...$ and noticed that for $n=8$ she got number divisible by $337$. For what minimal $n$ did she get number divisible by $2022$?
Problem
Source: Moscow mathematic olympiad 2022, Day 2 P11.5
Tags: number theory
30.09.2022 03:31
(redacted)
30.09.2022 04:17
DavyDuf wrote: Fake solve (?) From the problem we know that for $n \equiv 8 mod 337 \ \ \rightarrow \ \ n^7 \equiv 1 \mod 337$ $n^7 \equiv 1 \mod 3 \ \ \rightarrow \ \ n \equiv 1 \mod 3$ $n^7 \equiv 1 \mod 2 \ \ \rightarrow \ \ n \equiv 1 \mod 2$ Quick CRT, you will get the smallest number of $n$ which is 1356 A quick bruteforce search shows that the minimal value of $n$ is $79$.
30.09.2022 05:31
$8^7\equiv 1\pmod{337}\implies (8^k)^7\equiv 1\pmod{337}$ By brute force, we have $$8^1,8^2,8^3,8^4,8^5,8^6,8^7\equiv 8,64,175,52,79,295,1\pmod{337}.$$These are the roots of $n^7-1\equiv 0\pmod{337}$. We also need $n\equiv 1\pmod {6}$ for $6$ to divide $n^7-1$ so $\boxed{79}$ is the smallest number. (since $1$ isn’t allowed.)
30.09.2022 07:05
Quidditch wrote: $8^7\equiv 1\pmod{337}\implies (8^k)^7\equiv 1\pmod{337}$ By brute force, we have $$8^1,8^2,8^3,8^4,8^5,8^6,8^7\equiv 8,64,175,52,79,295,1\pmod{337}.$$These are the roots of $n^7-1\equiv 0\pmod{337}$. We also need $n\equiv 1\pmod {6}$ for $6$ to divide $n^7-1$ so $\boxed{79}$ is the smallest number. (since $1$ isn’t allowed.) Could there exist other residues not of the form $8^k$?
30.09.2022 07:35
TYT wrote: Quidditch wrote: $8^7\equiv 1\pmod{337}\implies (8^k)^7\equiv 1\pmod{337}$ By brute force, we have $$8^1,8^2,8^3,8^4,8^5,8^6,8^7\equiv 8,64,175,52,79,295,1\pmod{337}.$$These are the roots of $n^7-1\equiv 0\pmod{337}$. We also need $n\equiv 1\pmod {6}$ for $6$ to divide $n^7-1$ so $\boxed{79}$ is the smallest number. (since $1$ isn’t allowed.) Could there exist other residues not of the form $8^k$? There should be at most $7$ roots of the equation $n^7-1\equiv 0\pmod{337}$ so there is no other root.