An Elementary Proof of a Theorem of Hardy and Ramanujan

TL;DR

This paper provides a shorter, elementary proof of Hardy and Ramanujan's theorem on the asymptotic behavior of the count of integers with ordered prime exponents, simplifying the original complex analytic approach.

Contribution

It introduces a simple combinatorial and inductive method to prove the Hardy-Ramanujan theorem, replacing the original analytic techniques with elementary arguments.

Findings

Elementary proof of Hardy-Ramanujan theorem achieved

Recurrence relation for $Q(n)$ established

Asymptotic formula for $Q(n)$ confirmed

Abstract

Let $Q(n)$ denote the number of integers $1 \leq q \leq n$ whose prime factorization $q= \prod^{t}_{i=1}p^{a_i}_i$ satisfies $a_1\geq a_2\geq \ldots \geq a_t$. Hardy and Ramanujan proved that $$ \log Q(n) \sim \frac{2\pi}{\sqrt{3}} \sqrt{\frac{\log(n)}{\log\log(n)}}\;. $$ Before proving the above precise asymptotic formula, they studied in great detail what can be obtained concerning $Q(n)$ using purely elementary methods, and were only able to obtain much cruder lower and upper bounds using such methods. In this paper we show that it is in fact possible to obtain a purely elementary (and much shorter) proof of the Hardy--Ramanujan Theorem. Towards this goal, we first give a simple combinatorial argument, showing that $Q(n)$ satisfies a (pseudo) recurrence relation. This enables us to replace almost all the hard analytic part of the original proof with a short inductive argument.