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 $x=x(\tau)$ 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 $\pi$ and other related constants. Finally, we indicate some possibilities to extend the functional equations to a two-variable setting.
Categories: math.NT math.CA math.CO