Let $p$ and $q$ be different prime numbers. We are given an infinite decreasing arithmetic progression in which each of the numbers $p^{23}, p^{24}, q^{23}$ and $q^{24}$ occurs. Show that the numbers $p$ and $q$ also occur in this progression. Proposed by A. Kuznetsov
Problem
Source: All-Russian MO 2024 10.1
Tags: number theory, number theory proposed
22.04.2024 22:25
Let $r$ be the difference of arithmetic progression. $r|p^{24}-p^{23}\implies r|p-1$ since $r|p\implies r|q$ and $r|p^{24}-q^{24}$ where both cannot hold at the same time. $r|p-1\implies r|p^{23}-1\implies r|p^{24}-p\implies p$ is in aritmetic progression. We can apply same process for $q$. @Below, if we say $r=\frac{a}{b}$ since $r$ is rational, we can apply same with $a$ I think.
23.04.2024 08:56
bin_sherlo wrote: Let $r$ be the difference of arithmetic progression. $r|p^{24}-p^{23}\implies r|p-1$ since $r|p\implies r|q$ and $r|p^{24}-q^{24}$ where both cannot hold at the same time. $r|p-1\implies r|p^{23}-1\implies r|p^{24}-p\implies p$ is in aritmetic progression. We can apply same process for $q$. In fact, $r$ is not necessary integer.
07.06.2024 23:20
Notice that the integer terms within a arithmetic progression form themselves an arithmetic progression. Let the ratio of this progression be $r$, then we must have $$p^{23}\equiv p^{24}\equiv q^{23}\equiv q^{24}\pmod{r}$$Clearly $p$ and $q$ do not divide $r$ so we get $p\equiv q\equiv 1$ and we are done as $$p\equiv p^{23} \equiv p^{24}\equiv q \equiv q^{23} \equiv q^{24}\equiv 1 \pmod{r}$$