Let $n$ be a prime number. Show that there is a permutation $a_1,a_2,...,a_n$ of $1,2,...,n$ so that $a_1,a_1a_2,...,a_1a_2...a_n$ leave distinct remainders when divided by $n$
Problem
Source: Singapore Open Round 2, 2016 SMO p3
Tags: number theory, permutation, remainder
kaede
26.03.2020 17:54
Suppose that $n$ is an odd prime number.
Define a finite sequence $( b_{j})$ of length $n-1$ by $b_{2k} =2k-1$, $b_{2k-1} =n+1-2k$
$( k=1,\dotsc ,( n-1) /2)$.
Let $g$ be a primitive root modulo $n$.
For any $i\in \mathbb{N} \ ( 1\leq i\leq n-1)$, we can take $a_{i} \in [ 1,n-1] \cap \mathbb{N}$ uniquely such that $a_{i} \equiv g^{b_{i}}\bmod n$.
Set $a_{n} =n$.
It is not difficult to verify that $( a_{1} ,\dotsc ,a_{n})$ satisfies the condition.
$\blacksquare $
Kamran011
26.03.2020 19:46
I've seen it somewhere else , idc
If $a_i=n$ for some $i<n$ , then $a_1a_2\dots{a_i}$ and $a_1a_2\dots{a_{i+1}}$ both are divisible by $n$, contradiction .
$\Rightarrow{a_n=n}$ . So $a_1a_2\dots{a_{n-1}}=(n-1)!$ and $n$ doesn't divide it by assumption . So $n$ is either a prime number or $n=4$ (well known).
If $n=4$ we can take the permutation $a_1=1 , a_2=3 , a_3=2 , a_4=4$
If $n$ is prime , we consider the permutation $a_1=1 , a_n=n$ and $a_i=1+(i-1)^{-1}$ where $(i-1)^{-1}$ is the inverse of $i-1$ modulo $n$ we can easily check that it satisfies the given condition . $\blacksquare$
Mathasocean
26.03.2020 22:53
parmenides51 wrote: Let $n$ be a prime number. Show that there is a permutation $a_1,a_2,...,a_n$ of $1,2,...,n$ so that $a_1,a_1a_2,...,a_1a_2...a_n$ leave distinct remainders when divided by $n$ Also Netherland BxMO TST 2018