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

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 cSortedRec(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.

Sources & referencesView supporting material

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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