On algorithms to find p-ordering

TL;DR

This paper presents new algorithms for efficiently finding p-orderings of subsets modulo prime powers, improving computational complexity and explicitly describing root set structures for small exponents.

Contribution

It introduces two algorithms for computing p-orderings with different efficiencies and characterizes root set structures for prime powers with small exponents.

Findings

First algorithm runs in O(nk log p) time for set size n.

Second algorithm improves efficiency using succinct representation, running in O(d^2k log p + nk log p + nd).

Explicit descriptions of root sets for p^2, p^3, and p^4 are provided.

Abstract

The concept of p-ordering for a prime p was introduced by Manjul Bhargava (in his PhD thesis) to develop a generalized factorial function over an arbitrary subset of integers. This notion of p-ordering provides a representation of polynomials modulo prime powers, and has been used to prove properties of roots sets modulo prime powers. We focus on the complexity of finding a p-ordering given a prime p, an exponent k and a subset of integers modulo p^k. Our first algorithm gives a p-ordering for set of size n in time O(nk\log p), where set is considered modulo p^k. The subsets modulo p^k can be represented succinctly using the notion of representative roots (Panayi, PhD Thesis, 1995; Dwivedi et.al, ISSAC, 2019); a natural question would be, can we find a p-ordering more efficiently given this succinct representation. Our second algorithm achieves precisely that, we give a p-ordering in time O(d^2k\log p + nk \log p + nd), where d is the size of the succinct representation and n is the required length of the p-ordering. Another contribution that we make is to compute the structure of roots sets for prime powers p^k, when k is small. The number of root sets have been given in the previous work (Dearden and Metzger, Eur. J. Comb., 1997; Maulick, J. Comb. Theory, Ser. A, 2001), we explicitly describe all the root sets for p^2, p^3 and p^4.