Determine all quadruplets ($x, y, z, t$) of positive integers, such that $12^x + 13^y - 14^z = 2013^t$.
Problem
Source: Balkan MO Shortlist 2013 N3 BMO
Tags: Diophantine equation, diophantine, number theory
kaede
09.03.2020 20:57
Suppose that $12^{x} +13^{y} -14^{z} =2013^{t}$ for $( x,y,z,t) \in \mathbb{N}^{4}$.
For any $m\in \mathbb{N}$, let $E_{m}$ be the equation $12^{x} +13^{y} -14^{z} \equiv 2013^{t}\pmod m$.
From $E_{3}$ , we have $1-2^{z} \equiv 0\pmod 3$, which implies $z\equiv 0\pmod 2$.
From $E_{13}$ , we have $( -1)^{x} -1\equiv ( -2)^{t}\pmod{13}$, which implise $x\equiv 1\pmod 2$ and $t\equiv 1\pmod{12}$.
First, we will show that $x=1$.
Assume,for the sake of contradiction, that $x\geq 2$.
If $z\geq 4$, then from $E_{16}$ and $t\equiv 1\pmod 4$, we have $13^{y} \equiv 13\pmod{16}$, which implise $y\equiv 1\pmod 4$.
From $y\equiv 1\pmod 4$, $t\equiv 1\pmod 4$, and $E_{5}$, $2^{x} \equiv ( -1)^{z}\pmod 5$, which contradicts $x\equiv 1\pmod 2$.
If $z=2$ and $t=1$, then $12^{x} +13^{y} =2209$, which is impossible.
Assume that $z=2$ and $t\geq 2$.
From $E_{9}$, we have $4^{y} +2\equiv 0\pmod 9$, which implise $y\equiv 2\pmod 3$.
From $y\equiv 2\pmod 3$, $2013\equiv 0\pmod{61}$ and $E_{61}$, we have $12^{x} \equiv 27\pmod{61}$, impossible.
So we must have $x=1$ and suppose henceforth that $x=1$.
Assume that $z\geq 4$.
From $E_{16}$ and $t\equiv 1\pmod 4$, we have $13^{y} \equiv 1\pmod{16}$, which implise $y\equiv 0\pmod 4$.
From $t\equiv 1\pmod 4$, $y\equiv 0\pmod 4$, and $E_{5}$, we have $-14^{z} \equiv 0\pmod 5$, impossible.
So we must have $z=2$ and suppose henceforth that $z=2$.
Assume that $t\geq 2$.
From $E_{9}$, we have $4^{y} \equiv 4\pmod 9$, which implise $y\equiv 1\pmod 3$.
Since $y\equiv 1\pmod 3$, from $E_{61}$, we have $12\equiv 0\pmod{61}$, impossible.
So we must have $t=1$ and suppose henceforth $t=1$.
Finally, we have $13^{y} =2197$, which implise $y=3$.
Hence the only solution for the given equation is $\boxed{( x,y,z,t) =( 1,3,2,1)}$.
NTFEGAC
12.03.2021 22:38
see also here: https://www.youtube.com/watch?v=yIoNphetObU&ab_channel=EpicNumberTheory