Mock Jacobi property of the generating function for planar hexagons

Let f5(z;τ)f_5(\boldsymbol{z};\tau) be the generating function defined by

f5(z;τ):=nZ4Dsgn(n1+α1n3α3)q3n1n232n3232n42ζ13n2ζ23n1ζ33n3ζ43n4,f_5(\boldsymbol{z}; \tau):=\sum_{\boldsymbol{n}\in \mathbb{Z}^4 \cap D }\operatorname{sgn}\left(n_1+\alpha_{1}-n_3-\alpha_{3}\right) q^{ 3 n_1n_2-\frac{3}{2}n_3^2-\frac{3}{2}n_4^2} \zeta_1^{3n_2} \zeta_2^{3n_1} \zeta_3^{-3n_3} \zeta_4^{-3n_4},

where the parameters αk\alpha_k, for k{1,2,3,4}k\in\{1,2,3,4\}, satisfy α3,α4min(α1,α2)\lvert \alpha_3\rvert,\lvert \alpha_4\rvert\leq\min(\lvert \alpha_1\rvert,\lvert \alpha_2\rvert), and

D:=α+{xR4:x3,x4min(x1,x2) and x1x2,x1x3,x1x40}.D:=-\boldsymbol{\alpha}+\left\{\boldsymbol{x}\in\mathbb{R}^4: \lvert x_3\rvert,\lvert x_4\rvert\leq\min(\lvert x_1\rvert,\lvert x_2\rvert)\text{ and }x_1x_2,x_1x_3,x_1x_4\geq 0\right\}.

Here qq and ζj\zeta_j are the variables used in the generating function. Mock Jacobi property conjecture. The generating function f5f_5 has mock Jacobi properties. This conjecture extends the mock modular behavior established for the generating functions of polygons with fewer sides and is motivated by homological mirror symmetry; the precise completion and transformation laws remain to be determined.

Sources & referencesView supporting material

Primary source

Kathrin Bringmann, Jonas Kaszian and Jie Zhou, “Generating functions of planar polygons from homological mirror symmetry of elliptic curves”, arXiv:1904.05058 (2020).

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