Proofs of Chappelon and Alfons\'ın Conjectures On Square Frobenius Numbers and its Relationship to Simultaneous Pell's Equations

Abstract

Recently, Chappelon and Alfons\'ın defined the square Frobenius number of coprime numbers $m$ and $n$ to be the largest perfect square that cannot be expressed in the form $mx+ny$ for nonnegative integers $x$ and $y$. When $m$ and $n$ differ by $1$ or $2$, they found simple expressions if neither $m$ nor $n$ is a perfect square. If either $m$ or $n$ is a perfect square, they formulated some interesting conjectures which have an unexpected close connection with a known recursive sequence, related to the denominators of Farey fraction approximations to $\sqrt{2}$. In this note, we prove these conjectures. Our methods involve solving Pell's equations $x^2-2y^2=1$ and $x^2-2y^2=-1$. Finally, to complete our proofs of these conjectures, we eliminate several cases using a bunch of results related to solutions of simultaneous Pell's equations.