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Degree of Parabolic Quantum Groups - Dipartimento di Matematica ...

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2. Algebras with trace 22<br />

Remark 2.2.8. (1) If R is an order in a central simple algebra <strong>of</strong> degree<br />

d then T equals the center <strong>of</strong> R, furthermore since the central simple<br />

algebra splits in a d <strong>di</strong>mensional matrix algebra which one may consider<br />

as a generic irreducible representation, it is easily seen that the open<br />

set ΩR is non empty.<br />

(2) If T is a finitely generated module over a subalgebra Z0, we can consider<br />

the finite surjective morphism<br />

τ : Spec(T) → Spec(Z0).<br />

Then by the properness <strong>of</strong> τ we get that the set<br />

Ω 0 R := a ∈ Spec(Z0) : τ −1 <br />

(a) ⊂ ΩR<br />

is a Zariski open dense subset <strong>of</strong> Spec(Z0).<br />

We will use this remark in the theory <strong>of</strong> quantum groups where there is<br />

a natural subalgebra Z0 which appears in the picture.

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