Geodesic
The freeness of ideal subarrangements of Weyl arrangements
Journal of the European Mathematical Society, Tome 18 (2016) no. 6, pp. 1339-1348

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A Weyl arrangement is the arrangement defined by the root system of a finite Weyl group. When a set of positive roots is an ideal in the root poset, we call the corresponding arrangement an ideal subarrangement. Our main theorem asserts that any ideal subarrangement is a free arrangement and that its exponents are given by the dual partition of the height distribution, which was conjectured by Sommers–Tymoczko. In particular, when an ideal subarrangement is equal to the entireWeyl arrangement, our main theorem yields the celebrated formula by Shapiro, Steinberg, Kostant, and Macdonald. The proof of the main theorem is classification-free. It heavily depends on the theory of free arrangements and thus greatly differs from the earlier proofs of the formula
DOI : 10.4171/jems/615
Classification : 32-XX, 05-XX, 17-XX
Keywords: Arrangement of hyperplanes, root system,Weyl arrangement, free arrangement, ideals, dual partition theorem 
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     title = {The freeness of ideal subarrangements of {Weyl} arrangements},
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Takuro Abe; Mohamed Barakat; Michael Cuntz; Torsten Hoge; Hiroaki Terao. The freeness of ideal subarrangements of Weyl arrangements. Journal of the European Mathematical Society, Tome 18 (2016) no. 6, pp. 1339-1348. doi: 10.4171/jems/615

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