General modularity conjecture for tadpole Nahm sums

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Let χ0(1)\chi_0({\bf 1}) be the Nahm sum associated to the tadpole Cartan matrix TnT_n, with n≥2n\geq 2. For even n=2k≥2n=2k\geq 2, define

R2k,i(τ)=∑n∈Z(−1)nq(k+1)(n−2i−14(k+1))2.R_{2k,i}(\tau)=\sum_{n\in\mathbb{Z}}(-1)^nq^{(k+1)(n-\frac{2i-1}{4(k+1)})^2}.

For odd n=2k−1≥3n=2k-1\geq 3, use the derivative theta functions (∂Θ)i,2k+12(\partial\Theta)_{i,\frac{2k+1}{2}}. Let f(τ)\frak{f}(\tau) and W~D\widetilde{\mathcal{W}}_D denote the functions and modified Wronskian appearing in the source.

General tadpole modularity conjecture. For even n=2k≥2n=2k\geq 2,

qakχ0(1)=f(τ)2kW~D(R2k,1,…,R2k,k)η(τ)k(2k−1),q^{a_k}\chi_0({\bf 1})=\frac{\frak{f}(\tau)^{2k}\widetilde{\mathcal{W}}_D(R_{2k,1},\ldots,R_{2k,k})}{\eta(\tau)^{k(2k-1)}},

where ak=−k(1+4k)48(1+k)a_k=-\frac{k(1+4k)}{48(1+k)}. For odd n=2k−1≥3n=2k-1\geq 3,

qakχ0(1)=f(τ)2k−1W~D((∂Θ)1,2k+12,…,(∂Θ)k−1,2k+12)η(τ)(k−1)(2k−1),q^{a_k}\chi_0({\bf 1})=\frac{\frak{f}(\tau)^{2k-1}\widetilde{\mathcal{W}}_D((\partial\Theta)_{1,\frac{2k+1}{2}},\ldots,(\partial\Theta)_{k-1,\frac{2k+1}{2}})}{\eta(\tau)^{(k-1)(2k-1)}},

where ak=−1+6k−8k296k+48a_k=\frac{-1+6k-8k^2}{96k+48}. More precisely, qakχ0(1)q^{a_k}\chi_0({\bf 1}) is a modular function that is a component of a 3k3k-dimensional vector-valued modular function under Γ(1)\Gamma(1).

The claim is based on numerical evidence and proposes an explicit modular description for all tadpole ranks n≥2n\geq 2. The source does not report a proof or a refutation.

References

Primary source

Antun Milas and Liuquan Wang, “Modularity of Nahm Sums for the Tadpole Diagram”, arXiv:2301.04532 (2023).

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