Computing $\pi(N)$: An elementary approach in $\tilde{O}(\sqrt{N})$ time

TL;DR

This paper introduces an elementary, efficient algorithm for counting primes up to N in near square-root time, surpassing previous combinatorial methods and offering space-time trade-offs, with applications to other number-theoretic functions.

Contribution

The authors develop a new elementary algorithm for prime counting with improved time complexity and space trade-offs, avoiding complex analysis and arbitrary precision calculations.

Findings

Achieves rom ilde{O}(\sqrt{N}) ext{ to } ilde{O}( ext{N}^{8/15}) ext{ space-time trade-off}

Improves complexity for computing the Mertens function to ilde{O}(\sqrt{N})

Provides implementation code for practical use

Abstract

We present an efficient and elementary algorithm for computing the number of primes up to $N$ in $\tilde{O}(\sqrt N)$ time, improving upon the existing combinatorial methods that require $\tilde{O}(N ^ {2/3})$ time. Our method has a similar time complexity to the analytical approach to prime counting, while avoiding complex analysis and the use of arbitrary precision complex numbers. While the most time-efficient version of our algorithm requires $\tilde{O}(\sqrt N)$ space, we present a continuous space-time trade-off, showing, e.g., how to reduce the space complexity to $\tilde{O}(\sqrt[3]{N})$ while slightly increasing the time complexity to $\tilde{O}(N^{8/15})$. We apply our techniques to improve the state-of-the-art complexity of elementary algorithms for computing other number-theoretic functions, such as the the Mertens function (in $\tilde{O}(\sqrt N)$ time compared to the known $\tilde{O}(N^{0.6})$), summing Euler's totient function, counting square-free numbers and summing primes. Implementation code is provided.