Find all triplets $(a, b, c)$ of positive integers, such that $a+bc, b+ac, c+ab$ are primes and all divide $(a^2+1)(b^2+1)(c^2+1)$.
Problem
Source: ARO Regional stage 2024 10.9
Tags: number theory
Tintarn
02.02.2024 15:41
First of all, if these three primes are distinct, their product would divide $(a^2+1)(b^+1)(c^2+1)$ and hence in particular
\[(bc+a)(ca+b)(ab+c) \le (a^2+1)(b^2+1)(c^2+1)\]which, after expanding, is equivalent to
\[(abc-1)(a^2+b^2+c^2+1) \le 0\]which is clearly wrong. Hence, two of the primes have to be the same, say $a+bc=b+ac$ i.e. $(a-b)(c-1)=0$. So either $a=b$ or $c=1$.
If $a=b$, then $a+bc=a(c+1)$ is prime, i.e. $a=b=1$, but then $c+1$ is prime and divides $4(c^2+1)$, hence divides $8$, hence $c=1$ as well and we get the solution $(1,1,1)$.
Else, if all three of $a,b,c$ are distinct, we get that $ab+1$ and $a+b$ are primes $\ge 3$ dividing $2(a^2+1)(b^2+1)$, hence dividing $(a^2+1)(b^2+1)$.
But if w.l.o.g. $a<b$, then $ab+1>a^2+1$ and hence $ab+1 \mid b^2+1$ and hence $ab+1 \mid b^2-ab=b(b-a)$, but both factors are positive and smaller than $ab+1$, contradiction.
Hence the only solution is $(1,1,1)$.
Sourorange
15.02.2024 16:41
WLOG, $a\le b\le c$ In fact, in most cases $c^2+1$ is smaller than any one of the $a+bc, b+ac, c+ab$. So we only need to compare $(a^2+1)(b^2+1)$ with $(a+bc)( b+ac)(c+ab)$, which is quite obvious.
Inspector_Maygray
06.05.2024 20:55
Assume all primes different. Then product of primes divides the expression given. But this is impossible because when you multiply out, you get a number dividing into a smaller number. Clearly wrong! Hence, at least two of the primes are equal. It is then easy case work to derive solutions such as a=b=c=1