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The associated variety of a finite-dimensional simple annihilator is the origin
Example
Let be a finite-dimensional complex semisimple Lie algebra and let be a finite-dimensional simple -module, with dominant integral. Then has finite codimension in , and its associated variety is the origin:
Indeed, with , Cayley-Hamilton shows that for every the power is the symbol of an element of , so contains the -th powers of all of . Over , the polarization identity shows that these pure -th powers span : for , Since is an ideal in , it contains every for . Its zero set is therefore ; equivalently, every point in it satisfies for all . The finite-dimensionality of is essential here. Infinite-dimensional (Verma-type) annihilators behave differently: their associated varieties need not be , and on this page no positive-dimensionality statement about them is asserted.
Facts & Assumptions
Given: A finite-dimensional complex semisimple Lie algebra , a dominant integral weight , and the finite-dimensional simple module with .
is the kernel of the unital algebra homomorphism given by the action, so it is a proper two-sided ideal and embeds into the finite-dimensional algebra (The annihilator of a module over an enveloping algebra, Finite-dimensional simple modules are classified by dominant highest weights).
Cayley-Hamilton: for every endomorphism of the finite-dimensional space , the characteristic polynomial satisfies ; it is monic of degree (Cayley-Hamilton: every finite-dimensional endomorphism satisfies its characteristic polynomial, , Polynomial evaluation at an endomorphism: ).
The symbol of an element of the PBW filtration is ; symbols multiply, and is the zero locus of in with in duality with the degree-one symbols (The associated graded variety of a two-sided ideal, The PBW filtration by tensor degree on the enveloping algebra, PBW gives an ordered monomial basis for the enveloping algebra, Classical affine zero loci form the Zariski closed sets).
Verification
The action map has kernel by definition, so is isomorphic to a subalgebra of the finite-dimensional algebra ; hence has finite codimension in . It is proper because acts as the identity on the nonzero module .
Fix and let be the characteristic polynomial of the operator by which acts on ; by [F2] it is monic of degree and acts as on , that is, . Writing , its symbol in the associated graded is by [F3]; hence .
Let . By step 1.2 and the definition of the associated variety, for every , so for every and hence ; thus . Conversely, is a proper ideal, so and has no nonzero degree-zero element; every therefore has zero constant term and satisfies , so . Hence .
Depends on
- The associated graded variety of a two-sided ideal
- The annihilator of a module over an enveloping algebra
- Finite-dimensional simple modules are classified by dominant highest weights
- The PBW filtration by tensor degree on the enveloping algebra
- PBW gives an ordered monomial basis for the enveloping algebra
- Classical affine zero loci form the Zariski closed sets
- Cayley-Hamilton: every finite-dimensional endomorphism satisfies its characteristic polynomial, $\chi_T(T)=0$
- Polynomial evaluation at an endomorphism: $p(T)=\sum_k a_kT^k$
Used by
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Sources
- D. Barbasch, Cells in Weyl groups and primitive ideals (AIM workshop notes, 2006) (standard reference, not scraped)
- D. A. Vogan, The orbit method and primitive ideals for semisimple Lie algebras (CMS Conf. Proc. 1986) (standard reference, not scraped)