Pop polynomial formulas for weak, Tamari, Cambrian and root-poset lattices

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Let WW be a finite Coxeter group. Write Weak(W)\mathrm{Weak}(W) for its right weak order, Tamari(W)\mathrm{Tamari}(W) for a Cambrian lattice associated to a Coxeter element obtained from a linear orientation of a path Dynkin diagram, Cambbi(W)\mathrm{Camb}_\mathrm{bi}(W) for a Cambrian lattice associated to a bipartite Coxeter element, and J(ΦW+)J(\Phi^+_W) for the distributive lattice of order ideals of the positive root poset of type WW. Let Pop(L;q)\mathsf{Pop}(L;q) denote the pop polynomial of a lattice LL. The pop-polynomial conjectures. The following equalities hold:

[qn−1]Pop(Weak(Bn);q)=3n−2n−1.[q^{n-1}]\mathsf{Pop}(\mathrm{Weak}(B_n);q)=3^n-2n-1. Pop(Tamari(An);q)=∑k=0n1k+1(2kk)(n2k)qn−k.\mathsf{Pop}(\mathrm{Tamari}(A_n);q)=\sum_{k=0}^n\frac{1}{k+1}\binom{2k}{k}\binom{n}{2k}q^{n-k}. Pop(Tamari(Bn);q)=∑k=0⌊(n+1)/2⌋(n−1k)(n+1−kk)qn−k.\mathsf{Pop}(\mathrm{Tamari}(B_n);q)=\sum_{k=0}^{\lfloor (n+1)/2\rfloor}\binom{n-1}{k}\binom{n+1-k}{k}q^{n-k}. Pop(Cambbi(An);q)=∑k=1⌊(n+3)/2⌋k(−1)k−1n−k+3∑j=0n−k+3(jn−j+3)(n−k+3j)qj−2.\mathsf{Pop}(\mathrm{Camb}_\mathrm{bi}(A_n);q)=\sum_{k=1}^{\lfloor (n+3)/2\rfloor}\frac{k(-1)^{k-1}}{n-k+3}\sum_{j=0}^{n-k+3}\binom{j}{n-j+3}\binom{n-k+3}{j}q^{j-2}. Pop(J(ΦAn+);q)=∑k=0n1k+1∑j=0n−k+1(k+1j−1)(k+1j)(n−j+1n−k−j+1)qk+1.\mathsf{Pop}(J(\Phi^+_{A_n});q)=\sum_{k=0}^n\frac{1}{k+1}\sum_{j=0}^{n-k+1}\binom{k+1}{j-1}\binom{k+1}{j}\binom{n-j+1}{n-k-j+1}q^{k+1}. Pop(J(ΦBn+);q)=(−q)n+∑k=0n∑j=1k(k+1j)(n−k−1j−1)(n−jn−k).\mathsf{Pop}(J(\Phi^+_{B_n});q)=(-q)^n+\sum_{k=0}^n\sum_{j=1}^{k}\binom{k+1}{j}\binom{n-k-1}{j-1}\binom{n-j}{n-k}.

These formulas conjecturally extend enumerative results for pop-stack sorting on weak orders, Cambrian lattices and order-ideal lattices; the displayed sequences agree with the cited OEIS data, but the source supplies no resolution of the conjectures.

References

Primary source

Colin Defant and Nathan Williams, “Semidistrim Lattices”, arXiv:2111.08122 (2021).

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