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Coefficients of a relative of cyclotomic polynomials

Number Theory 2012-09-27 v1 Combinatorics

Abstract

Let N=p1p2...pnN=p_1p_2... p_n be a product of nn distinct primes. Define PN(x)P_N(x) to be the polynomial (1xN)1i<jn(1xN/(pipj))/i=1n(1xN/pi)(1-x^N)\prod_{1\leq i<j\leq n}(1-x^{N/(p_ip_j)})/\prod_{i=1}^n (1-x^{N/p_i}). (When n=2n=2, Ppq(x)P_{pq}(x) is the pqpq-th cyclotomic polynomial, and when n=3n=3, Ppqr(x)P_{pqr}(x) is (1x)(1-x) times the pqrpqr-th cyclotomic polynomial.) Let the height of a polynomial be the maximum absolute value of one of its coefficients. It is well known that the height of Φpq(x)\Phi_{pq}(x) is 1, and Gallot and Moree showed that the same is true for Ppqr(x)P_{pqr}(x) when n=3n=3. We show that the coefficients of PN(x)P_N(x) depend mainly on the relative order of sums of residues of the form pj1(modpi)p_j^{-1} \pmod {p_i}. This allows us to explicitly describe the coefficients of PN(x)P_N(x) when n=3n=3 and show that the height of PN(x)P_N(x) is at most 2 when n=4n=4. We also show that for any nn there exist PN(x)P_N(x) with height 1 but that in general the maximum height of PN(x)P_N(x) is a function depending only on nn with growth rate 2n2/2+O(nlogn)2^{n^2/2+O(n\log n)}.

Keywords

Cite

@article{arxiv.1209.6026,
  title  = {Coefficients of a relative of cyclotomic polynomials},
  author = {Ricky Ini Liu},
  journal= {arXiv preprint arXiv:1209.6026},
  year   = {2012}
}

Comments

18 pages, 3 figures