Find all triples $(x, y, z)$ of nonnegative integers such that $$ x^5+x^4+1=3^y7^z $$
Problem
Source: Baltic Way 2022, Problem 19
Tags: number theory
Kimchiks926
12.11.2022 20:23
This problem was proposed by me.
Note that:
\begin{align*}
x^5+x^4+1=x^5+x^4+x^3+1-x^3=\\
x^3(x^2+x+1)+(1-x)(1+x+x^2) =\\
=(x^3-x+1)(x^2+x+1)
\end{align*}Consider the following cases:
$x \equiv 1 \pmod 3$, then $x^2+x+1 \equiv 1+1+1 \equiv 0 \pmod 3$ and $x^3-x+1 \equiv 1-1+1 \equiv 1 \pmod 3$
$x \equiv 2 \pmod 3$, then $x^2+x+1 \equiv 2^2+2+1 \equiv 1 \pmod 3$ and $x^3-x+1 \equiv 2^3-2+1 \equiv 1 \pmod 3$
$x \equiv 0 \pmod 3$, then $x^2+x+1 \equiv 0^2+0+1 \equiv 1 \pmod 3$ and $x^3-x+1 \equiv 0-0+1 \equiv 1 \pmod 3$
This means that $3$ does not divide $x^3-x+1$, and $x^2+x+1$ is divisible by $3$ if and only if $x \equiv 1 \pmod 3$. Suppose that $x=3k+1$, then:
$$ (3k+1)^2+(3k+1)+1 =9k^2+6k+1+3k+1+1=9k^2+9k+3 $$This means $x^2+x+1$ is not divisible by $9$. Therefore, $y=0$ or $y=1$.
Now suppose that $d=\gcd(x^3-x+1,x^2+x+1)$. Then:
$$ d \mid x(x^2+x+1) -(x^3-x+1)=x^2+2x-1 $$Observe that in that case:
$$ d \mid x^2+2x-1 -x^2-x-1 =x-2 $$This means that $x \equiv 2 \pmod d \implies 0 \equiv 2^2+2+1 \equiv 7 \pmod d$. We conclude that $d=7$ or $d=1$.
This means that one of the brackets is divisible by at most $7$ (but not by $49$). This gives us the following possibilities:
$x^3-x+1 =1 $, then $x=0$ or $x=1$. In the former case, we get that $y=z=0$, whereas in the latter case $y=1$, $z=0$.
$x^3-x+1 =7 \implies x^3-x-6=0$. The only integer root is $x=2$, which gives $y=0, z=2$.
$x^2+x+1=1$, then $x=0$. This case has been considered before.
$x^2+x+1=3$, then $x=1$. This case has been considered before.
$x^2+x+1=7$, then $x=2$. This case has been considered before.
$x^2+x+1=21$, then $x=4$. On the other hand, then $x^3-x+1 =61$ - not divisible by either $3$ or $7$.
Thus, there are only $3$ possible triplets $(x,y,z)$, which are $(0,0,0)$, $(1,1,0)$ and $(2,0,2)$.
grupyorum
12.11.2022 20:27
Get that \[ x^5+x^4+1=(x^2+x+1)(x^3-x+1). \]Notice that $x^3\equiv x\pmod{3}$, thus $3\nmid x^3-x+1$. Moreover, $9\nmid x^2+x+1$ as otherwise $9\mid (2x+1)^2+3$, but $(-3/9)=-1$. Hence $y\le 1$. Observe also that if $d=(x^2+x+1,x^3-x+1)$ then $x^3\equiv 1\pmod{d}$, yielding $x^3-x+1\equiv 2-x\pmod{d}$. Thus $x\equiv 2\pmod{d}$, yielding $d=7$ to be only possibility. Equipped with these, suppose first $y=0$. Check $x\le 2$ manually. As $x^3-x+1>x^2+x+1$ for $x>2$, we have that for $x>2$, $x^2+x+1=7$ is the only possibility, which is already inspected. The case $y=1$ is analogous.