A note on some polynomial-factorial diophantine equations

Abstract

In 1876 Brocard, and independently in 1913 Ramanujan, asked to find all integer solutions for the equation $n!=x^2-1$. It is conjectured that this equation has only three solutions, but up to now this is an open problem. Overholt observed that a weak form of Szpiro's-conjecture implies that Brocard's equation has finitely many integer solutions. More generally, assuming the ABC-conjecture, Luca showed that equations of the form $n!=P(x)$ where $P(x)\in\mathbb{Z}[x]$ of degree $d\geq 2$ have only finitely many integer solutions with $n>0$. And if $P(x)$ is irreducible, Berend and Harmse proved unconditionally that $P(x)=n!$ has only finitely many integer solutions. In this note we study diophantine equations of the form $g(x_1,...,x_r)=P(x)$ where $P(x)\in\mathbb{Z}[x]$ of degree $d\geq 2$ and $g(x_1,...,x_r)\in \mathbb{Z}[x_1,...,x_r]$ where for $x_i$ one may also plug in $A^{n}$ or the Bhargava factorial $n!_S$. We want to understand when there are finitely many or infinitely many integer solutions. Moreover, we study diophantine equations of the form $g(x_1,...,x_r)=f(x,y)$ where $f(x,y)\in\mathbb{Z}[x,y]$ is a homogeneous polynomial of degree $\geq2$.