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arXiv · 1608.06810

Short addition sequences for theta functions

Andreas Enge·William Hart·Fredrik Johansson
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Abstract

The main step in numerical evaluation of classical Sl2 (Z) modular forms and elliptic functions is to compute the sum of the first N nonzero terms in the sparse q-series belonging to the Dedekind eta function or the Jacobi theta constants. We construct short addition sequences to perform this task using N + o(N) multiplications. Our constructions rely on the representability of specific quadratic progressions of integers as sums of smaller numbers of the same kind. For example, we show that every generalised pentagonal number c 5 can be written as c = 2a + b where a, b are smaller generalised pentagonal numbers. We also give a baby-step giant-step algorithm that uses O(N/ log r N) multiplications for any r > 0, beating the lower bound of N multiplications required when computing the terms explicitly. These results lead to speed-ups in practice.

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Andreas Enge, William Hart, Fredrik Johansson. 2018-03-08. Short addition sequences for theta functions. https://arxiv.org/abs/1608.06810

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