Distinct positive integers $A$ and $B$ are given$.$ Prove that there exist infinitely many positive integers that can be represented both as $x_{1}^2+Ay_{1}^2$ for some positive coprime integers $x_{1}$ and $y_{1},$ and as $x_{2}^2+By_{2}^2$ for some positive coprime integers $x_{2}$ and $y_{2}.$ (Golovanov A.S.)
Problem
Source: SRMC 2022 P2
Tags: number theory
24.05.2022 05:55
Is it y2 or y2^2?
26.05.2022 21:49
Justanaccount wrote: Is it y2 or y2^2? In which part !? (Everything that I wrote is the correct form of problem.)
26.05.2022 21:54
You edited the problem after @2above wrote the message though.
28.05.2022 06:50
Let's nail this. Suppose $B>A$ and fix $y_1=y_2=4p$, where $p>2$ is a prime. Notice that $x_1^2 + Ay_1^2 = x_2^2 + By_2^2$ implies $(x_1-x_2)(x_1+x_2) = 16p^2(B-A)$. Taking $x_1-x_2=2$ and $x_1+x_2 = 8p^2(B-A)$, we see that \[ (x_1,y_1,x_2,y_2) = \bigl(4p^2(B-A)+1,4p,4p^2(B-A)-1,4p\bigr) \]is a quadruple such that $x_1^2+Ay_1^2$ and $x_2^2+By_2^2$ both realize the same number with $(x_1,y_1)=(x_2,y_2)=1$. As $p$ is arbitrary, the proof is complete.
01.07.2023 01:18
It has a bash solution, I remembered one of my friends solve it with that bash solution, if you are interested to see that sol, tell me.