English

On $q$-analogues of some series for $\pi$ and $\pi^2$

Combinatorics 2019-02-15 v5 Number Theory

Abstract

We obtain a new qq-analogue of the classical Leibniz series k=0(1)k/(2k+1)=π/4\sum_{k=0}^\infty(-1)^k/(2k+1)=\pi/4, namely \begin{equation*} \sum_{k=0}^\infty\frac{(-1)^kq^{k(k+3)/2}}{1-q^{2k+1}}=\frac{(q^2;q^2)_{\infty}(q^8;q^8)_{\infty}}{(q;q^2)_{\infty}(q^4;q^8)_{\infty}}, \end{equation*} where qq is a complex number with q<1|q|<1. We also show that the Zeilberger-type series k=1(3k1)16k/(k(2kk))3=π2/2\sum_{k=1}^\infty(3k-1)16^k/(k\binom{2k}k)^3=\pi^2/2 has two qq-analogues with q<1|q|<1, one of which is n=0qn(n+1)/21q3n+21q(q;q)n3(q;q)n(q3;q2)n3=(1q)2(q2;q2)4(q;q2)4.\sum_{n=0}^\infty q^{n(n+1)/2} \frac {1-q^{3n+2}} {1-q} \cdot\frac{(q;q)_n^3 (-q;q)_n}{(q^3;q^2)_{n}^3} = (1-q)^2 \frac{(q^2;q^2)^4_\infty}{(q;q^2)^4_\infty}.

Keywords

Cite

@article{arxiv.1802.01506,
  title  = {On $q$-analogues of some series for $\pi$ and $\pi^2$},
  author = {Qing-Hu Hou and Christian Krattenthaler and Zhi-Wei Sun},
  journal= {arXiv preprint arXiv:1802.01506},
  year   = {2019}
}

Comments

11 pages