# Graber: Rationally Connected Varieties

Introduction

Definition. A field is quasi-algebraically closed ( ) if any polynomial with has a root in .

Any finite field is quasi-algebraically closed, as is any function field of a curve over an algebrically closed field. This implies that any Laurent series ring is .

These generalize to the following three possible definitions:

Definition. A projective variety is rationally connected (RC) if any two general points can be joined by a rational curve .

Definition. is rationally chain connected (RCC) if any two general points can be joined by a chain of rational curves.

Definition. is separably rationally connected (SRC) if is normal and there exists a rational curve such that is ample, i.e. , .

Theorem. If is smooth projective over , then is rationally connected (RC) if and only if is rationally chain connected (RCC) if and only if is separably rationally connected (SRC).

In characteristic zero, a smooth hypersurface of degree in is rationally connected if and only if .

The main result we will consider is the following:

Theorem. [G, Harris, Starr] If , , then any rationally connected variety over for a curve has a rational point.

Remark. In characteristic , the same is true for SRC. (de Jong, Starr)

Theorem. Over , any rationally connected variety has a rational point.

This follows from the function field case. Geometrically, you can find a nonsingular integral model over ; to find a section, it is equivalent to find a reduced component of the central fiber.

Theorem. [Ernault] If is smooth, projective, and geometrically rationally chain connected, then has a rational point.

Proof, a Beginning

We now prove the theorem that an RC variety over has a rational point.

Choose an integral model . First, we reduce to the case . By restriction of scalars, have a map ; the fibres of are products of fibres of , and the product of rationally connected varieties is rationally connected.

Next, choose a curve which dominates . Deform and specialize until it breaks off a section. We have a map of moduli spaces curves in  coverings of  The latter is built out the data of the branch points plus monodromy; fixing the genus of the curve and the degree of the cover, we know that this moduli space is irreducible. It is possible to degenerate any branched cover of to a reducible cover with a section as one component. It is enough to show that this map on moduli spaces is surjective. Now it is just a matter of tracing the monodromy as the branch points move around (at least in characteristic zero). In characteristic , it can also be done (look, for example, at ).

To make all of this more precise, we look at stable maps. Given a smooth projective variety over , and given and , we construct a space consisting of maps such that is a connected nodal curve of arithmetic genus , is a morphism, and , together with a stability criterion.

This space is projective, and given any morphism , one has an induced map from . If , then is a compactification of the space of branched covers. We want to show that is surjective. (We only need the coarse moduli space; in fact, at least in characteristic zero we have a Deligne-Mumford stack.)

When is , the bound is sharp. If is not algebraically closed, then pick a finite extension , and consider ; this has a polynomial of degree with no nontrivial zeros; therefore it is impossible to get a larger class of hypersurfaces.

Is the notion of rational connectivity sharp? In the case of finite fields, we also get: if smooth projective, and either for or for , then has a rational point. (These two are equivalent in characteristic zero.) The analog over function fields is false. In particular, there exists a family of Enriques surfaces over ( is a curve over ) which has no section.

The general statement: Let be a proper morphism of varieties over . Suppose that for all maps , there exists a pullback . It would suffice for there to exist dominating such that the general fibre of is rationally connected. It is a theorem that this is necessary and sufficient (G, Harris, Mazur, Starr).

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