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8 votes

Hecke algebra and $H^*(G/B)$

The early work of Borel showed in effect how to interpret the cohomology algebra of a flag variety as the coinvariant algebra associated to the Weyl group, which affords the regular representation of …
Jim Humphreys's user avatar
5 votes

Automorphism group of flag manifolds?

While the question and answers have explained different aspects of the story for complex groups and homogeneous spaces, it might be worthwhile to place the question in a broader context. The complex …
Jim Humphreys's user avatar
4 votes

Computing relative Lie algebra cohomology (as appears in Borel-Weil-Bott theorem)

The theorems of Borel-Weil and then Bott, along with Kostant's translation of the ideas into the language of Lie algebra cohomology, do much to illuminate classical representation theory (Cartan-Weyl) …
Jim Humphreys's user avatar
4 votes

What is the significance that the Springer resolution is a moment map?

The short answer might be that this viewpoint provides an attractive alternative way to construct the Springer resolution as a special case in a broader geometric framework, following ideas of Kostant …
Jim Humphreys's user avatar
9 votes
Accepted

The anticanonical bundle on a flag variety is ample

It's worth pointing out that the criterion for ampleness applies uniformly for reductive groups and flag varieties in any characteristic. From the algebraic group viewpoint, the only subtle point is …
Jim Humphreys's user avatar
65 votes
6 answers
9k views

Origin of terms "flag", "flag manifold", "flag variety"?

These terms have become common in Lie theory and related algebraic geometry and combinatorics, as seen in many questions posted on MO, but it's unclear to me where they first came into use. Probably …
Jim Humphreys's user avatar
39 votes
Accepted

Rep Theory Consequences of Bott--Weil--Borel

It's always a good idea to ask (as students typically do) why one is studying a particular subject or theorem. Here are some of my views, from the algebraic side of representation theory: 1) The or …
Jim Humphreys's user avatar