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The associated variety of a finite-dimensional simple annihilator is the origin

Example

Let g be a finite-dimensional complex semisimple Lie algebra and let L(λ) be a finite-dimensional simple U(g)-module, with λ dominant integral. Then I(λ)=Ann⁡U(g)L(λ) has finite codimension in U(g), and its associated variety is the origin:

V(I(λ))={0}⊆g∗. Indeed, with d=dim⁡CL(λ), Cayley-Hamilton shows that for every y∈g the power yd is the symbol of an element of I(λ), so gr⁡I(λ) contains the d-th powers of all of g. Over C, the polarization identity shows that these pure d-th powers span Sd(g): for y1,…,yd∈g, d! y1⋯yd=∑J⊆{1,…,d}(−1)d−∣J∣(∑j∈Jyj)d. Since gr⁡I(λ) is an ideal in S(g), it contains every Sk(g) for k≥d. Its zero set is therefore {0}; equivalently, every point ξ in it satisfies ξ(y)d=0 for all y∈g. The finite-dimensionality of L(λ) is essential here. Infinite-dimensional (Verma-type) annihilators behave differently: their associated varieties need not be {0}, and on this page no positive-dimensionality statement about them is asserted.

Facts & Assumptions

Given: A finite-dimensional complex semisimple Lie algebra g, a dominant integral weight λ, and the finite-dimensional simple module L(λ) with d:=dim⁡CL(λ)≥1.

[F1]

I(λ) is the kernel of the unital algebra homomorphism U(g)→End⁡C(L(λ)) given by the action, so it is a proper two-sided ideal and U(g)/I(λ) embeds into the finite-dimensional algebra End⁡C(L(λ)) (The annihilator of a module over an enveloping algebra, Finite-dimensional simple modules are classified by dominant highest weights).

[F2]

Cayley-Hamilton: for every endomorphism T of the finite-dimensional space L(λ), the characteristic polynomial χT satisfies χT(T)=0; it is monic of degree d (Cayley-Hamilton: every finite-dimensional endomorphism satisfies its characteristic polynomial, χT(T)=0, Polynomial evaluation at an endomorphism: p(T)=∑kakTk).

[F3]

The symbol of an element p(y)=yd+(terms of degree <d) of the PBW filtration is yd∈Sd(g); symbols multiply, and V(I) is the zero locus of gr⁡I in g∗ with g∗ 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

technique · direct
1.1F1given

The action map U(g)→End⁡C(L(λ)) has kernel I(λ) by definition, so U(g)/I(λ) is isomorphic to a subalgebra of the finite-dimensional algebra End⁡C(L(λ)); hence I(λ) has finite codimension in U(g). It is proper because 1 acts as the identity on the nonzero module L(λ).

1.2F2F3algebra

Fix y∈g and let χy be the characteristic polynomial of the operator by which y acts on L(λ); by [F2] it is monic of degree d and χy(y) acts as 0 on L(λ), that is, χy(y)∈I(λ). Writing χy(y)=yd+(terms of PBW degree <d), its symbol in the associated graded is yd by [F3]; hence yd∈(gr⁡I(λ))d⊆Sd(g).

2.1step 1.2F1F3algebra∎

Let ξ∈V(I(λ)). By step 1.2 and the definition of the associated variety, 0=ξ(yd)=ξ(y)d for every y∈g, so ξ(y)=0 for every y∈g and hence ξ=0; thus V(I(λ))⊆{0}. Conversely, I(λ) is a proper ideal, so I(λ)∩C⋅1=0 and gr⁡I(λ) has no nonzero degree-zero element; every f∈gr⁡I(λ) therefore has zero constant term and satisfies f(0)=0, so 0∈V(I(λ)). Hence V(I(λ))={0}.

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