# Elliptic hypergeometric series

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In mathematics, an elliptic hypergeometric series is a series Σcn such that the ratio cn/cn−1 is an elliptic function of n, analogous to generalized hypergeometric series where the ratio is a rational function of n, and basic hypergeometric series where the ratio is a periodic function of the complex number n. They were introduced by Frenkel & Turaev (1997) in their study of elliptic 6-j symbols.

For surveys of elliptic hypergeometric series see Gasper & Rahman (2004) or Spiridonov (2008).

## Definitions

The q-Pochhammer symbol is defined by

${\displaystyle \displaystyle (a;q)_{n}=\prod _{k=0}^{n-1}(1-aq^{k})=(1-a)(1-aq)(1-aq^{2})\cdots (1-aq^{n-1}).}$
${\displaystyle \displaystyle (a_{1},a_{2},\ldots ,a_{m};q)_{n}=(a_{1};q)_{n}(a_{2};q)_{n}\ldots (a_{m};q)_{n}.}$

The modified Jacobi theta function with argument x and nome p is defined by

${\displaystyle \displaystyle \theta (x;p)=(x,p/x;p)_{\infty }}$
${\displaystyle \displaystyle \theta (x_{1},...,x_{m};p)=\theta (x_{1};p)...\theta (x_{m};p)}$

The elliptic shifted factorial is defined by

${\displaystyle \displaystyle (a;q,p)_{n}=\theta (a;p)\theta (aq;p)...\theta (aq^{n-1};p)}$
${\displaystyle \displaystyle (a_{1},...,a_{m};q,p)_{n}=(a_{1};q,p)_{n}\cdots (a_{m};q,p)_{n}}$

The theta hypergeometric series r+1Er is defined by

${\displaystyle \displaystyle {}_{r+1}E_{r}(a_{1},...a_{r+1};b_{1},...,b_{r};q,p;z)=\sum _{n=0}^{\infty }{\frac {(a_{1},...,a_{r+1};q;p)_{n}}{(q,b_{1},...,b_{r};q,p)_{n}}}z^{n}}$

The very well poised theta hypergeometric series r+1Vr is defined by

${\displaystyle \displaystyle {}_{r+1}V_{r}(a_{1};a_{6},a_{7},...a_{r+1};q,p;z)=\sum _{n=0}^{\infty }{\frac {\theta (a_{1}q^{2n};p)}{\theta (a_{1};p)}}{\frac {(a_{1},a_{6},a_{7},...,a_{r+1};q;p)_{n}}{(q,a_{1}q/a_{6},a_{1}q/a_{7},...,a_{1}q/a_{r+1};q,p)_{n}}}(qz)^{n}}$

The bilateral theta hypergeometric series rGr is defined by

${\displaystyle \displaystyle {}_{r}G_{r}(a_{1},...a_{r};b_{1},...,b_{r};q,p;z)=\sum _{n=-\infty }^{\infty }{\frac {(a_{1},...,a_{r};q;p)_{n}}{(b_{1},...,b_{r};q,p)_{n}}}z^{n}}$

## Definitions of additive elliptic hypergeometric series

The elliptic numbers are defined by

${\displaystyle [a;\sigma ,\tau ]={\frac {\theta _{1}(\pi \sigma a,e^{\pi i\tau })}{\theta _{1}(\pi \sigma ,e^{\pi i\tau })}}}$

where the Jacobi theta function is defined by

${\displaystyle \theta _{1}(x,q)=\sum _{n=-\infty }^{\infty }(-1)^{n}q^{(n+1/2)^{2}}e^{(2n+1)ix}}$

The additive elliptic shifted factorials are defined by

• ${\displaystyle [a;\sigma ,\tau ]_{n}=[a;\sigma ,\tau ][a+1;\sigma ,\tau ]...[a+n-1;\sigma ,\tau ]}$
• ${\displaystyle [a_{1},...,a_{m};\sigma ,\tau ]=[a_{1};\sigma ,\tau ]...[a_{m};\sigma ,\tau ]}$

The additive theta hypergeometric series r+1er is defined by

${\displaystyle \displaystyle {}_{r+1}e_{r}(a_{1},...a_{r+1};b_{1},...,b_{r};\sigma ,\tau ;z)=\sum _{n=0}^{\infty }{\frac {[a_{1},...,a_{r+1};\sigma ;\tau ]_{n}}{[1,b_{1},...,b_{r};\sigma ,\tau ]_{n}}}z^{n}}$

The additive very well poised theta hypergeometric series r+1vr is defined by

${\displaystyle \displaystyle {}_{r+1}v_{r}(a_{1};a_{6},...a_{r+1};\sigma ,\tau ;z)=\sum _{n=0}^{\infty }{\frac {[a_{1}+2n;\sigma ,\tau ]}{[a_{1};\sigma ,\tau ]}}{\frac {[a_{1},a_{6},...,a_{r+1};\sigma ,\tau ]_{n}}{[1,1+a_{1}-a_{6},...,1+a_{1}-a_{r+1};\sigma ,\tau ]_{n}}}z^{n}}$

## References

• Frenkel, Igor B.; Turaev, Vladimir G. (1997), "Elliptic solutions of the Yang-Baxter equation and modular hypergeometric functions", The Arnold-Gelfand mathematical seminars, Boston, MA: Birkhäuser Boston, pp. 171–204, ISBN 978-0-8176-3883-2, MR 1429892
• Gasper, George; Rahman, Mizan (2004), Basic hypergeometric series, Encyclopedia of Mathematics and its Applications, 96 (2nd ed.), Cambridge University Press, ISBN 978-0-521-83357-8, MR 2128719
• Spiridonov, V. P. (2002), "Theta hypergeometric series", Asymptotic combinatorics with application to mathematical physics (St. Petersburg, 2001), NATO Sci. Ser. II Math. Phys. Chem., 77, Dordrecht: Kluwer Acad. Publ., pp. 307–327, arXiv:, MR 2000728
• Spiridonov, V. P. (2003), "Theta hypergeometric integrals", Rossiĭskaya Akademiya Nauk. Algebra i Analiz, 15 (6): 161–215, arXiv:, doi:10.1090/S1061-0022-04-00839-8, MR 2044635
• Spiridonov, V. P. (2008), "Essays on the theory of elliptic hypergeometric functions", Rossiĭskaya Akademiya Nauk. Moskovskoe Matematicheskoe Obshchestvo. Uspekhi Matematicheskikh Nauk, 63 (3): 3–72, doi:10.1070/RM2008v063n03ABEH004533, MR 2479997
• Warnaar, S. Ole (2002), "Summation and transformation formulas for elliptic hypergeometric series", Constructive Approximation. an International Journal for Approximations and Expansions, 18 (4): 479–502, arXiv:, doi:10.1007/s00365-002-0501-6, MR 1920282