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Newton-Okounkov polytopes of flag varieties and marked chain-order polytopes. (English) Zbl 1522.14065

The author investigates Newton-Okounkov bodies of the flag variety \(G/B\) for \(G = \mathrm{SL}_{n+1}(\mathbb{C})\) and \(B\) the subgroup of upper triangular matrices. It is already known that several classes of polytopes can be realised as Newton-Okounkov bodies of flag varieties. These are Gelfand-Tsetlin polytopes, Berenstein-Littlemann-Zelevinsky’s string polytopes, Nakashima-Zelevinsky polytopes, Lusztig polytopes, Feigin-Fourier-Littelmann-Vinberg polytopes and polytopes constructed from extended \(g\)-vectors in cluster theory. The main result of the paper is that the class of marked chain-order polytopes, constructed from the Gelfand-Tsetlin posets of type A, can be added to this list: they can be realised as Newton-Okounkov bodies \(\Delta(G/B, \mathcal{L}_{\lambda}, \nu)\) for certain line bundle \(\mathcal{L}_{\lambda}\) and valuation \(\nu\). Moreover, the author uses this result to prove that for a marked chain-order polytope \(\Delta_{\mathcal{C}, \mathcal{O}}\) there is a flat degeneration of \(G/B\) to a normal projective toric variety corresponding to \(\Delta_{\mathcal{C}, \mathcal{O}}\).

MSC:

14M25 Toric varieties, Newton polyhedra, Okounkov bodies
05E10 Combinatorial aspects of representation theory
06A07 Combinatorics of partially ordered sets
14M15 Grassmannians, Schubert varieties, flag manifolds
52B20 Lattice polytopes in convex geometry (including relations with commutative algebra and algebraic geometry)

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