English

Certain infinite products in terms of MacMahon type series

Number Theory 2024-07-09 v1

Abstract

Recently, Ono and the third author discovered that the reciprocals of the theta series (q;q)3(q;q)_\infty^3 and (q2;q2)(q;q2)2(q^2;q^2)_\infty(q;q^2)_\infty^2 have infinitely many closed formulas in terms of MacMahon's quasimodular forms Ak(q)A_k(q) and Ck(q)C_k(q). In this article, we use the well-known infinite product identities due to Jacobi, Watson, and Hirschhorn to derive further such closed formulas for reciprocals of other interesting infinite products. Moreover, with these formulas, we approximate these reciprocals to arbitrary order simply using MacMahon's functions and {\it MacMahon type} functions. For example, let Θ6(q):=12nZχ6(n)nqn2124\Theta_{6}(q):=\frac{1}{2}\sum_{n\in\mathbb{Z}} \chi_6(n) n q^{\frac{n^2-1}{24}} be the theta function corresponding to the odd quadratic character modulo 66. Then for any positive integer nn, we have 1Θ6(q)=q3n2+n2k=r1kn(mod2)r2(1)nk2Ak(q)C3nk2(q)+O(qn+1),\frac{1}{\Theta_{6}(q)}= q^{-\frac{3n^2+n}{2}}\sum_{\substack{k=r_1\\ k\equiv n\hspace{-0.2cm}\pmod{2}}}^{r_2}(-1)^{\frac{n-k}{2}}A_{k}(q)C_{\frac{3n-k}{2}}(q)+O(q^{n+1}), where r1:=3n112n+133+1r_1:=\lfloor\frac{3n-1-\sqrt{12n+13}}{3}\rfloor+1 and r2:=3n1+12n+1331r_2:=\lceil\frac{3n-1+\sqrt{12n+13}}{3}\rceil-1.

Keywords

Cite

@article{arxiv.2407.04798,
  title  = {Certain infinite products in terms of MacMahon type series},
  author = {Seokho Jin and Badri Vishal Pandey and Ajit Singh},
  journal= {arXiv preprint arXiv:2407.04798},
  year   = {2024}
}

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16 pages