# 자코비 삼중곱(Jacobi triple product)

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## 개요

• 세타함수의 삼중곱

$\sum_{n=-\infty}^\infty z^{n}q^{n^2}= \prod_{m=1}^\infty \left( 1 - q^{2m}\right) \left( 1 + zq^{2m-1}\right) \left( 1 + z^{-1}q^{2m-1}\right)$

• $$z=1$$ 인 경우

$\sum_{n=-\infty}^\infty q^{n^2}= \prod_{m=1}^\infty \left( 1 - q^{2m}\right) \left( 1 + q^{2m-1}\right)^2$

증명

q-초기하급수(q-hypergeometric series)의 다음 등식을 활용 $\prod_{n=0}^{\infty}(1+zq^n)=\sum_{n\geq 0}\frac{q^{n(n-1)/2}}{(1-q)(1-q^2)\cdots(1-q^n)} z^n$ $\prod_{n=0}^{\infty}\frac{1}{1+zq^n}=\sum_{n\geq 0}\frac{(-1)^n}{(1-q)(1-q^2)\cdots(1-q^n)} z^n$

$\prod_{m=0}^\infty \left( 1 + zq^{2m+1}\right)=\sum_{n\geq 0}\frac{q^nz^n}{(1-q^2)(1-q^4)\cdots(1-q^{2n})}$

[Andrews65] 참조 ■

## 또다른 형태

$\sum _{n=-\infty }^{\infty } (-1)^na^nq^{n(n-1)/2}=\prod _{n=1}^{\infty } \left(1-aq^{n-1}\right)\left(1-a^{-1}q^n\right)\left(1-q^n\right)$

$\prod _{n=1}^{\infty } \left(1-x^{2n}\right)\left(1+x^{2n-1}Z\right)\left(1+x^{2n-1}Z^{-1}\text{}\text{}\right)=\sum _{m=-\infty }^{\infty } x^{m^2}Z^m$

## 특별한 경우

$\sum _{m=-\infty }^{\infty } (-1)^mq^{a m^2\pm b m +c}=q^c\prod _{n=1}^{\infty } \left(1-q^{2a n}\right)\left(1-q^{2a n-a+b}\right)\left(1-q^{2a n-a-b}\right)$

$\sum _{m=-\infty }^{\infty } q^{a m^2\pm b m +c}=q^c\prod _{n=1}^{\infty } \left(1-q^{2a n}\right)\left(1+q^{2a n-a+b}\right)\left(1+q^{2a n-a-b}\right)$