## A cubic counterpart of Jacobi’s identity and the AGM.(English)Zbl 0725.33014

The authors’ introduction: “We produce exact cubic analogues of Jacobi’s celebrated theta function identity and of the arithmetic-geometric mean iteration of Gauss and Legendre. The iteration in question is $a_{n+1}:=\frac{a_ n+2b_ n}{3}\text{ and } b_{n+1}:=^ 3\sqrt{b_ n(\frac{a^ 2_ n+a_ nb_ n+b^ 2_ n}{3})}.$ The limit of this iteration is identified in terms of the hypergeometric function $${}_ 2F(1/3,2/3;1;\cdot)$$, which supports a particularly simple cubic transformation.

### MSC:

 33E05 Elliptic functions and integrals 33C05 Classical hypergeometric functions, $${}_2F_1$$ 39B12 Iteration theory, iterative and composite equations 11F27 Theta series; Weil representation; theta correspondences
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