A statistic-preserving CTI–ITC bijection conjecture for sorted recurrent configurations

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Let Sn,dS_{n,d} be the complete split graph, and let SortedRec(Sn,d)\mathsf{SortedRec}(S_{n,d}) be the set of sorted recurrent configurations on Sn,dS_{n,d}. For a configuration cc, write height(c)\mathsf{height}(c) for its height, and write wtoppleCTI(c)\mathsf{wtopple}_{CTI}(c) and wtoppleITC(c)\mathsf{wtopple}_{ITC}(c) for the CTI and ITC weighted-toppling statistics. Statistic-preserving bijection conjecture. There exists a bijection

Ψ:SortedRec(Sn,d)↦SortedRec(Sn,d)\Psi:\mathsf{SortedRec}(S_{n,d})\mapsto\mathsf{SortedRec}(S_{n,d})

such that, for every c∈SortedRec(Sn,d)c\in\mathsf{SortedRec}(S_{n,d}),

(height(c),wtoppleCTI(c))=(height(Ψ(c)),wtoppleITC(Ψ(c))).(\mathsf{height}(c),\mathsf{wtopple}_{CTI}(c))=(\mathsf{height}(\Psi(c)),\mathsf{wtopple}_{ITC}(\Psi(c))).

Such a bijection would explain the correspondence between the CTI and ITC polynomials directly in the sandpile model; the paper states that the authors expect such an explanation, but no bijection is currently known.

References

Primary source

Henri Derycke, Mark Dukes and Yvan Le Borgne, “The sandpile model on the complete split graph: q,t-Schröder polynomials, sawtooth polyominoes, and a cycle lemma”, arXiv:2402.15372 (2025).

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