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November 12, 2016

GIT-quotient of affine variety

Let G be a reductive group, X a G-variety, L a line G-bundle on X. Then the GIT-quotient is defined as
X //_L G:=\operatorname{Proj} \left( \bigoplus_{n \geqslant 0} H^0(X, L^{\otimes n})^G \right).
If X is affine and L=\mathcal O_X(\chi) for a character \chi: G \to \mathbb G_m, then the GIT-quotient may be considered as
  • first taking an affine quotient \operatorname{Spec} (\mathbb k[X]^{G_\chi}) by G_\chi:=\operatorname{Ker}(\chi: G \to \mathbb G_m),
  • then taking a quotient by \mathbb G_m=G / G_\chi via replacing \operatorname{Spec} to \operatorname{Proj} of non-negative gradings.
For the details on the construction see
Mukai An introduction to invariants and moduli, remark 6.14.

The first step is fairly stupid, but a proof that GIT-quotient is very general is given in
Mumford Geometric invariant theory, converse 1.13 of chapter 1.4.

From MO:253379.

Tangent space to moduli space of sheaves

Let X be a projective variety. Suppose that the moduli space \mathcal M of nice sheaves with fixed rank and Hilbert polynomial exists. Then
T_{[E]}\mathcal M=\mathrm{Ext}^1(E, E).

For a partial proof see
Hartshorne Lectures on Deformation Theory, theorem 2.6

From MO:253578.

Castelnuovo-Mumford regularity

Let X be a projective variety. Then a coherent sheaf F is called m-regular by Castelnuovo-Mumford if
H^i(F(m-i))=0.
It looks strange that one does not simply require
H^i(F(m))=0.
But then m-regularity would not imply (m+1)-regularity. For example, \Omega^1_{\mathcal P^n} with n>1 is (-1)-regular, but not 0-regular.

For properties of Castelnuovo-Mumford regularity, see
Mumford Lectures on Curves on an Algebraic Surface, lecture 14.

From MSE:2008517.