English

On the variance of the digits of $1/p$

Number Theory 2026-05-21 v3

Abstract

Let p>3p>3 be a prime and b2b\ge 2 an integer such that pp does not divide bb. Then 1/p1/p has a periodic digit expansion with respect to the basis bb. The length qq of the period is the (multiplicative) order of bb mod pp. In the case q=p1q=p-1 a formula for the variance of the digits of a period was given previously. This formula involves a Dedekind sum. We determine the variance in the case q=(p1)/2q=(p-1)/2. If p3p\equiv 3 mod 4 a Dedekind sum and the class number of Q(p)\mathbb Q(\sqrt{-p}) occur in the respective formula. If p1p\equiv 1 mod 4, the formula may be much more complex since it involves linear combinations of (possibly many) products of two Bernoulli numbers attached to odd characters.

Keywords

Cite

@article{arxiv.2601.08416,
  title  = {On the variance of the digits of $1/p$},
  author = {Kurt Girstmair},
  journal= {arXiv preprint arXiv:2601.08416},
  year   = {2026}
}