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arXiv · 1604.01106

Crouching AGM, Hidden Modularity

Shaun Cooper·Jesús Guillera·Armin Straub·Wadim Zudilin
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Abstract

Special arithmetic series $f(x)=\sum_{n=0}^{\infty}c_nx^n$, whose coefficients $c_n$ are normally given as certain binomial sums, satisfy "self-replicating" functional identities. For example, the equation $$\frac1{(1+4z)^2}f\biggl(\frac{z}{(1+4z)^3}\biggr)=\frac1{(1+2z)^2}f\biggl(\frac{z^2}{(1+2z)^3}\biggr)$$ generates a modular form $f(x)$ of weight 2 and level 7, when a related modular parametrization $x=x(τ)$ is properly chosen. In this note we investigate the potential of describing modular forms by such self-replicating equations as well as applications of the equations that do not make use of the modularity. In particular, we outline a new recipe of generating AGM-type algorithms for computing $π$ and other related constants. Finally, we indicate some possibilities to extend the functional equations to a two-variable setting.

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Shaun Cooper, Jesús Guillera, Armin Straub, Wadim Zudilin. 2016-04-05. Crouching AGM, Hidden Modularity. https://doi.org/10.1142/9789813228887_0009

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