The number of solutions of the Erd\H{o}s-Straus Equation and sums of $k$ unit fractions

Abstract

We prove new upper bounds for the number of representations of an arbitrary rational number as a sum of three unit fractions. In particular, for fixed $m$ there are at most $\mathcal{O}_{\epsilon}(n^{3/5+\epsilon})$ solutions of $\frac{m}{n}=\frac{1}{a_1}+\frac{1}{a_2}+\frac{1}{a_3}$. This improves upon a result of Browning and Elsholtz (2011) and extends a result of Elsholtz and Tao (2013) who proved this when $m=4$ and $n$ is a prime. Moreover there exists an algorithm finding all solutions in expected running time $\mathcal{O}_{\epsilon}\left(n^{\epsilon}\left(\frac{n^3}{m^2}\right)^{1/5}\right)$, for any $\epsilon >0$. We also improve a bound on the maximum number of representations of a rational number as a sum of $k$ unit fractions. Furthermore, we also improve lower bounds. In particular we prove that for given $m\in \mathbb{N}$ in every reduced residue class $e \bmod f$ there exist infinitely many primes $p$ such that the number of solutions of the equation $\frac{m}{p}=\frac{1}{a_1}+\frac{1}{a_2}+\frac{1}{a_3}$ is $\gg_{f,m} \exp\left(\left(\frac{5\log 2}{12 \mathrm{lcm}(m,f)}+o_{f,m}(1)\right)\frac{\log p}{\log \log p}\right)$. Previously the best known lower bound of this type was of order $(\log p)^{0.549}$.