Abstract
Let k be an algebraically closed field. Let B be the Borel subgroup of GLn(k) consisting of nonsingular upper triangular matrices. Let b = Lie B be the Lie algebra of upper triangular n × n matrices and u the Lie subalgebra of b consisting of strictly upper triangular matrices. We classify all Lie ideals n of b, satisfying u' ⫅ n ⫅ u, such that B acts (by conjugation) on n with a dense orbit. Further, in case B does not act with a dense orbit, we give the minimal codimension of a B-orbit in n. This can be viewed as a first step towards the difficult open problem of classifying of all ideals n ⫅ u such that B acts on n with a dense orbit. The proofs of our main results require a translation into the representation theory of a certain quasi-hereditary algebra At,1. In this setting we find the minimal dimension of Ext1At,1(M,M) for a δ-good At,1-module of certain fixed δ-dimension vectors.
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Goodwin, S., Hille, L. Prehomogeneous spaces for Borel subgroups of general linear groups. Transformation Groups 12, 475–498 (2007). https://doi.org/10.1007/s00031-006-0052-1
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DOI: https://doi.org/10.1007/s00031-006-0052-1