Mock Jacobi property of the generating function for planar hexagons

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

f5(z;τ):=∑n∈Z4∩Dsgn⁡(n1+α1−n3−α3)q3n1n2−32n32−32n42ζ13n2ζ23n1ζ3−3n3ζ4−3n4,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∣,∣α4∣≤min⁡(∣α1∣,∣α2∣)\lvert \alpha_3\rvert,\lvert \alpha_4\rvert\leq\min(\lvert \alpha_1\rvert,\lvert \alpha_2\rvert), and

D:=−α+{x∈R4:∣x3∣,∣x4∣≤min⁡(∣x1∣,∣x2∣) and x1x2,x1x3,x1x4≥0}.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.

References

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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