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Kostant cohomology for sl2
Example
Assume the Axiom of Choice, inherited from Kostant's theorem (The Axiom of Choice). Let , with Cartan subalgebra , positive root , Weyl vector , fundamental weight , and with , so that has dimension and . The Weyl group is with . Kostant's theorem gives with generators and , where has weight . This verifies the sign of the dot action, the -shift and the top degree in the smallest rank, and shows that the degree-one weight is , not .
Verification
Given: The Axiom of Choice; with the usual basis and positive root , the Weyl group , a nonnegative integer , the module of dimension , and .
[L1] , , and the dot action is (Root reflections and the Weyl group action, The Weyl vector rho for a chosen positive system, The special linear Lie algebra sl_2, The root sl_2 triple).
[L2] for and for , since and the Weyl group has the two elements of lengths (Kostant's nilradical cohomology theorem, Kostant cohomology in degrees zero and top, Length and longest Weyl-group element).
[L3] is finite dimensional of dimension , the weight occurs in with multiplicity one, and in degree one the extremal cochain of The extremal weight cochain of a Weyl element is closed and unique is (Finite-dimensional representations of sl_2, Weight and weight space).
, and by [L1] ; since and , , [L2] gives the displayed cohomology.
In degree one, the generator is : its exterior factor has weight and has weight , so the total weight is the dot weight, confirming the -shift; the degree-zero generator is the highest vector .
Counting: , , and for , matching the top degree ; the degree-one weight is , strictly below , so the exterior root shift cannot be dropped.
Depends on
- Kostant's nilradical cohomology theorem
- Kostant cohomology in degrees zero and top
- The extremal weight cochain of a Weyl element is closed and unique
- The special linear Lie algebra sl_2
- The root sl_2 triple
- Finite-dimensional representations of sl_2
- The Weyl vector rho for a chosen positive system
- Integral, dominant, and strictly dominant weights
- Weight and weight space
- Root reflections and the Weyl group action
- Length and longest Weyl-group element
- The Axiom of Choice
Used by
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Sources
- Peter Woit, Lie Algebra Cohomology and the Borel–Weil–Bott Theorem (Math G4344, Spring 2012), printed pp.1–7 (standard reference, not scraped)
- Faisal Al-Faisal, On the Representation Theory of Semisimple Lie Groups (University of Waterloo MMath thesis, 2010), §3.4 printed pp.73–77 (standard reference, not scraped)