English

Multiple reciprocal sums and multiple reciprocal star sums of polynomials are almost never integers

Number Theory 2018-07-03 v2

Abstract

Let nn and kk be integers such that 1kn1\le k\le n and f(x)f(x) be a nonzero polynomial of integer coefficients such that f(m)0f(m)\ne 0 for any positive integer mm. For any kk-tuple s=(s1,...,sk)\vec{s}=(s_1, ..., s_k) of positive integers, we define Hk,f(s,n):=1i1<<iknj=1k1f(ij)sjH_{k,f}(\vec{s}, n):=\sum\limits_{1\leq i_{1}<\cdots<i_{k}\le n} \prod\limits_{j=1}^{k}\frac{1}{f(i_{j})^{s_j}} and Hk,f(s,n):=1i1iknj=1k1f(ij)sj.H_{k,f}^*(\vec{s}, n):=\sum\limits_{1\leq i_{1}\leq \cdots\leq i_{k}\leq n} \prod\limits_{j=1}^{k}\frac{1}{f(i_{j})^{s_j}}. If all sjs_j are 1, then let Hk,f(s,n):=Hk,f(n)H_{k,f}(\vec{s}, n):=H_{k,f}(n) and Hk,f(s,n):=Hk,f(n)H_{k,f}^*(\vec{s}, n):=H_{k,f}^*(n). Hong and Wang refined the results of Erd\"{o}s and Niven, and of Chen and Tang by showing that Hk,f(n)H_{k,f}(n) is not an integer if n4n\geq 4 and f(x)=ax+bf(x)=ax+b with aa and bb being positive integers. Meanwhile, Luo, Hong, Qian and Wang established the similar result when f(x)f(x) is of nonnegative integer coefficients and of degree no less than two. For any kk-tuple s=(s1,...,sk)\vec{s}=(s_1, ..., s_k) of positive integers, Pilehrood, Pilehrood and Tauraso proved that Hk,f(s,n)H_{k,f}(\vec{s},n) and Hk,f(s,n)H_{k,f}^*(\vec{s},n) are nearly never integers if f(x)=xf(x)=x. In this paper, we show that if f(x)f(x) is a nonzero polynomial of nonnegative integer coefficients such that either degf(x)2\deg f(x)\ge 2 or f(x)f(x) is linear and sj2s_j\ge 2 for all integers jj with 1jk1\le j\le k, then Hk,f(s,n)H_{k,f}(\vec{s}, n) and Hk,f(s,n)H_{k,f}^*(\vec{s}, n) are not integers except for the case f(x)=xmf(x)=x^{m} with m1m\geq1 being an integer and n=k=1n=k=1, in which case, both of Hk,f(s,n)H_{k,f}(\vec{s}, n) and Hk,f(s,n)H_{k,f}^*(\vec{s}, n) are integers. Furthermore, we prove that if f(x)=2x1f(x)=2x-1, then both Hk,f(s,n)H_{k,f}(\vec{s}, n) and Hk,f(s,n)H_{k,f}^*(\vec{s}, n) are not integers except when n=1n=1, in which case Hk,f(s,n)H_{k,f}(\vec{s}, n) and Hk,f(s,n)H_{k,f}^*(\vec{s}, n) are integers. The method of the proofs is analytic and pp-adic.

Keywords

Cite

@article{arxiv.1703.07263,
  title  = {Multiple reciprocal sums and multiple reciprocal star sums of polynomials are almost never integers},
  author = {Shaofang Hong and Liping Yang and Qiuyu Yin and Min Qiu},
  journal= {arXiv preprint arXiv:1703.07263},
  year   = {2018}
}

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20 pages