# Construction of Hilb and Quot

*Posted on February 16, 2016 by Dima*

## Hilbert polynomial

If \(X\) is a projective variety and \({\mathcal{F}}\) is a sheaf on \(X\) then the function \[
\chi(\mathcal F, n) = \sum (-1)^i H^i(X, \mathcal F(n))
\] is called the *Hilbert function* of \(\mathcal F\).

**Lemma** (Snapper lemma). It is a polynomial.

*Proof*. Induction on the dimension of support of \({\mathcal{F}}\). Notice that \(\chi\) is additive in extensions, i.e. \(\chi({\mathcal{F}}/{\mathcal{G}}) = \chi({\mathcal{F}}) - \chi({\mathcal{G}})\), as follows from long exact sequence of cohomology.

Recall that for a divisor \(D\) the sheaf \({\mathcal{O}}(D)\) is defined as \[ {\mathcal{O}}(D)(U) := {\{\ f \in \mathcal{K}(U) \ \mid\ (f) + D \geq 0 \ \}} \] where \(\mathcal{K}\) is the constant sheaf of rational functions on \(X\). Let \(H\) be a hyperplane in \({\mathbb{P}}^n\), one has a natural short exact sequence \[ 0 \to {\mathcal{O}}_{{\mathbb{P}}^n}(-H) \to {\mathcal{O}}_{{\mathbb{P}}^n} \to {\mathcal{O}}_H \to 0 \] Assume that \(H\) is chosen so that \(\dim {\operatorname{supp}}{\mathcal{F}}\cap H < \dim {\operatorname{supp}}{\mathcal{F}}\). Tensoring with \({\mathcal{F}}\), taking into account that \({\mathcal{O}}_{{\mathbb{P}}^n}(-H) \cong {\mathcal{O}}_{{\mathbb{P}}^n}(-1)\), and passing to the long exact sequence we get \[ 0 \to Tor_1({\mathcal{F}}, {\mathcal{O}}_H) \to {\mathcal{F}}(-1) \to {\mathcal{F}}\to {\mathcal{F}}\otimes {\mathcal{O}}_H \to 0 \] Note that by looking at long exact cohomolgy sequences one establishes that Hilbert polynomial is an additive invariant. Read more (comments)

# Grauert's criterion of ampleness

*Posted on October 1, 2015 by Dima*

Here’s a writeup of a proof of Grauert’s criterion for ampleness (here is the link to the original paper). Since often proving that a variety is algebraic is not far from proving that it is projective, this criterion can be useful in judging algebraicity of a variety.

We start with some observations on cohomology of invertible sheaves which are powers of the invertible sheaf associated to hyperplane section.

Recall that if \(D\) is a (Cartier) divisor on a variety \(X\) then it gives rise to a sheaf \[
{\cal O}(D) := \{ f \in k(X) \mid (f) + D \geq 0 \}
\] and a section \(s \in H^0(X, {\cal O}(D))\) such that \((s) = D\). Let \(V = H^0(X, L)\). Then a (generally speaking, partial) map \(\iota: X \dashrightarrow \mathbb{P}(V^\vee)\) is defined: \(x \mapsto (f \mapsto f(x))\). The value on the right, before projectivization, depends on trivialization, hence is only well-defined up to a constant, but since we projectivize, the map is well-defined. A line bundle (or a divisor \(D\)) is called *very ample* if \(\iota\) is a closed embedding, *ample* if some multiple of it is ample.

If \(D\) is ample, then \(H:=mD\) is a hyperplane section for some \(m > 0\). Then \(D_H^r\) is the degree of \(X\), hence positive.

**Theorem**. Let \(X=\mathbb{P}^d\)

- \(H^i(X, {\cal O}(n)) = 0\), if \(0 < i < d\) or \(i < 0\)
- \(H^i(X, {\cal O}(n)) \cong H^{n-i}(X, {\cal O}(-d-n-1))^\vee\)

# What I have learned about quantum mechanics

*Posted on September 28, 2015 by Dima*

Being confused for a long time about the main aspects of quantum mechanics and quantisation, I decided now to summarise my current knowledge.

So, dear diary, non-relativistic classical mechanics essentially equals symplectic geometry. To be totally honest, while a symplectic manifold captures all the relevant information about a mechanical system, it only appears as result of a complete understanding of the system. If one wishes to derive the equations of motion from fundamental principles then one proceeds as follows.

Take a manifold (probably a Riemannian manifold) which describes possible positions of elements of your mechanical system. Trajectories are just curves in this manifold. Knowing that your system is in a state \(x_0\) we need to understand how it will evolve in time. So we have a functional called *action* on the space of possible trajectories obtained by integrating something called *Lagrangian density* over the curve. The principle of least action says that the system will evolve according to the path which minimizes the action; the differential equations of motion are called the *Euler-Lagrange equations*. One can prove a theorem that for mechanical systems of a certain kind (? not sure what are the precise requirements here) the trajectories can be described in a particularly nice way as curves parallel with respect to Hamiltonian flow on a symplectic manifold. The translation from Riemannian to symplectic picture is called Legendre transform.

# Artin approximation and moduli

*Posted on May 29, 2015 by Dima*

One is often interested whether a covariant functor \(F\) from the category of (affine) schemes to the category of sets can be represented by a scheme so that \(F(X) = h_S(X) = \mathrm{Hom}(X, S)\). It is natural to assume that all sensible functors are locally of finite presentation, that is, preserve inverse limits.

Passing to the opposite category of rings, it means that the contravariant functor \(F^{\mathrm op}\) from rings to sets turns *direct* limits into inverse limits. This is going to be useful for the following reason: any ring is a direct limit of its finitely generated (over some base) subrings, and so, any object \(\xi \in F^{\mathrm op}(R)\) will be defined over a finitely generated subring \(\xi \in F^{\mathrm op}(R_i)\), for \(F^{\mathrm op}(\varinjlim R_i) \cong \varprojlim F^{\mathrm op}(R_i)\). Example: let \(\mathcal{A}\) be the functor on the category of \(k\)-algebras such that \(\mathcal{A}(R)\) be the set of isomorphism classes of polirized Abelian varieties over \(X=\mathrm{Spec}\,R\). Then a particular \(\xi \in \mathcal{A}^{\mathrm op}(R)\) is defined by some equations with coefficients that belong to a finitely generated over \(k\) algebra \(R_i\).

Let \(R\) be a local ring with the maximal ideal \(\mathfrak m\). An infinitesimal deformation is an element \(\xi \in F(\mathrm{Spec}\,R/(\mathfrak{m}^n)\) for some \(n > 1\). A formal deformation is a sequence of compatible elements \(\xi_i \in F(\mathrm{Spec}\,R/(\mathfrak{m}^n))\). An effective formal deformation is an element \(\xi \in F(\mathrm{Spec}\,\hat R)\), where \(\hat R = \varprojlim R/(\mathfrak{m}^n)\) so in particular an effective formal deformation is a deformation.

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