Keith's parity conjecture for reciprocals of false theta functions

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Let Ψ(a,b)\Psi(a,b) be the false theta function

Ψ(a,b):=∑n=0∞an(n+1)/2bn(n−1)/2−∑n=−∞−1an(n+1)/2bn(n−1)/2.\Psi(a,b):=\sum_{n=0}^\infty a^{n(n+1)/2}b^{n(n-1)/2}-\sum_{n=-\infty}^{-1}a^{n(n+1)/2}b^{n(n-1)/2}.

For integers r,sr,s, define cr,s(n)c_{r,s}(n) by

∑n=0∞cr,s(n)qn=1Ψ(−qr,qs).\sum_{n=0}^\infty c_{r,s}(n)q^n=\frac{1}{\Psi(-q^r,q^s)}.

Keith's conjecture. For n≥0n\geq 0,

c13,1(32n+23)≡0(mod2),c_{13,1}(32n+23)\equiv 0 \pmod 2, c13,1(64n+63)≡0(mod2),c_{13,1}(64n+63)\equiv 0 \pmod 2,

and

c13,1(72n+j)≡0(mod2),c_{13,1}(72n+j)\equiv 0 \pmod 2,

where j∈{15,21,39,69}j\in\{15,21,39,69\}. This conjecture predicts infinite families of parity congruences for coefficients of reciprocals of false theta functions; the supplied source gives no resolution, so its status remains open.

References

Primary source

Jing Jin, Huan Xu and Olivia X. M. Yao, “Parity results for the reciprocals of false theta functions”, arXiv:2512.02324 (2025).

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