English

Practical numbers among the binomial coefficients

Number Theory 2020-12-15 v1

Abstract

A "practical number" is a positive integer nn such that every positive integer less than nn can be written as a sum of distinct divisors of nn. We prove that most of the binomial coefficients are practical numbers. Precisely, letting f(n)f(n) denote the number of binomial coefficients (nk)\binom{n}{k}, with 0kn0 \leq k \leq n, that are not practical numbers, we show that \begin{equation*} f(n) < n^{1 - (\log 2 - \delta)/\log \log n} \end{equation*} for all integers n[3,x]n \in [3, x], but at most Oγ(x1(δγ)/loglogx)O_\gamma(x^{1 - (\delta - \gamma) / \log \log x}) exceptions, for all x3x \geq 3 and 0<γ<δ<log20 < \gamma < \delta < \log 2. Furthermore, we prove that the central binomial coefficient (2nn)\binom{2n}{n} is a practical number for all positive integers nxn \leq x but at most O(x0.88097)O(x^{0.88097}) exceptions. We also pose some questions on this topic.

Keywords

Cite

@article{arxiv.1905.12023,
  title  = {Practical numbers among the binomial coefficients},
  author = {Paolo Leonetti and Carlo Sanna},
  journal= {arXiv preprint arXiv:1905.12023},
  year   = {2020}
}

Comments

10 pages, no figures

R2 v1 2026-06-23T09:29:54.854Z