Algebraic geometry

GIT quotient

In algebraic geometry, an affine GIT quotient, or affine geometric invariant theory quotient, of an affine scheme with an action by a group scheme G is the affine scheme , the prime spectrum of the ring of invariants of A, and is denoted by . A GIT quotient is a categorical quotient: any invariant morphism uniquely factors through it. Taking Proj (of a graded ring) instead of , one obtains a projective GIT quotient (which is a quotient of the set of semistable points.) A GIT quotient is a categorical quotient of the locus of semistable points; i.e., "the" quotient of the semistable locus. Since the categorical quotient is unique, if there is a geometric quotient, then the two notions coincide: for example, one has for an algebraic group G over a field k and closed subgroup H. If X is a complex smooth projective variety and if G is a reductive complex Lie group, then the GIT quotient of X by G is homeomorphic to the symplectic quotient of X by a maximal compact subgroup of G (Kempf–Ness theorem). (Wikipedia).

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Now that we know what a quotient group is, let's take a look at an example to cement our understanding of the concepts involved.

From playlist Abstract algebra

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From playlist PreCalculus

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From playlist PreCalculus

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From playlist Determining Function Values

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Related pages

Reductive group | Proj construction | Group-scheme action | Algebraic group | Character variety | Conical surface | Graded ring | Invariant theory | Kempf–Ness theorem | Maximal compact subgroup | Projective variety | Algebraic geometry | Categorical quotient | Du Val singularity | Geometric quotient | Group scheme | Quotient stack | Complex Lie group