Explicit formula conjecture for truncated Ehrhart-like polynomials

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 kN[Φ]W\mathbf{k}\in\mathbb{N}[\Phi]^W is λ\lambda. For a linearly independent subset XΦ+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 λP\lambda\in P and every good kN[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 XΦ+X\subseteq\Phi^+ such that XX is linearly independent and

λ,α{0,1}\langle\lambda,\alpha^\vee\rangle\in\{0,1\}

for every αΦ+SpanR(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.

Sources & referencesView supporting material

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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