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Orthogonal roots, Macdonald representations, and quasiparabolic sets
- Part of
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- Journal:
- Forum of Mathematics, Sigma / Volume 13 / 2025
- Published online by Cambridge University Press:
- 08 July 2025, e110
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Let W be a simply laced Weyl group of finite type and rank n. If W has type
$E_7$, 
$E_8$ or 
$D_n$ for n even, then the root system of W has subsystems of type 
$nA_1$. This gives rise to an irreducible Macdonald representation of W spanned by n-roots, which are products of n orthogonal roots in the symmetric algebra of the reflection representation. We prove that in these cases, the set of all maximal sets of orthogonal positive roots has the structure of a quasiparabolic set in the sense of Rains–Vazirani. The quasiparabolic structure can be described in terms of certain quadruples of orthogonal positive roots which we call crossings, nestings and alignments. This leads to nonnesting and noncrossing bases for the Macdonald representation, as well as some highly structured partially ordered sets. We use the 
$8$-roots in type 
$E_8$ to give a concise description of a graph that is known to be non-isomorphic but quantum isomorphic to the orthogonality graph of the 
$E_8$ root system.
