Explicit formula conjecture for truncated Ehrhart-like polynomials

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Let Φ\Phi be a root system with positive roots Φ+\Phi^+, weight lattice PP, and root lattice QQ. Let Lλtr(k)L^{\mathrm{tr}}_{\lambda}(\mathbf{k}) count the weights whose truncated interval-firing stabilization under a good parameter k∈N[Φ]W\mathbf{k}\in\mathbb{N}[\Phi]^W is λ\lambda. For a linearly independent subset X⊆Φ+X\subseteq\Phi^+, write rVolQ(X)\mathrm{rVol}_Q(X) for its relative volume and kX\mathbf{k}^{X} for the corresponding monomial.

Truncated Ehrhart-like polynomial conjecture. For every λ∈P\lambda\in P and every good k∈N[Φ]W\mathbf{k}\in\mathbb{N}[\Phi]^W,

Lλtr(k)=∑XrVolQ(X) kX,L^{\mathrm{tr}}_{\lambda}(\mathbf{k})=\sum_X \mathrm{rVol}_Q(X)\,\mathbf{k}^{X},

where the sum runs over all X⊆Φ+X\subseteq\Phi^+ such that XX is linearly independent and

⟨λ,α∨⟩∈{0,1}\langle\lambda,\alpha^\vee\rangle\in\{0,1\}

for every α∈Φ+∩Span⁡R(X)\alpha\in\Phi^+\cap\operatorname{Span}_{\mathbb{R}}(X).

The formula is motivated by the established formula for the symmetric Ehrhart-like polynomials and by the decomposition of symmetric fibers into truncated fibers. Its validity for all good parameters and weights remains open.

References

Primary source

Sam Hopkins and Alexander Postnikov, “A positive formula for the Ehrhart-like polynomials from root system chip-firing”, arXiv:1803.08472 (2019).

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