Keith's parity conjecture for reciprocals of false theta functions

From papers

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

Ψ(a,b):=n=0an(n+1)/2bn(n1)/2n=1an(n+1)/2bn(n1)/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=0cr,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 n0n\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.

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Sources & referencesView supporting material

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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