Monday, January 4, 2016

Model theory and algebraic geometry, 5 — Algebraic differential equations from coverings

In this final post of this series, I return to elimination of imaginaries in DCF and explain the main theorem from Tom Scanlon's paper Algebraic differential equations from covering maps.

The last ingredient to be discussed is jet spaces.

Differential algebra is seldom used explicitly in algebraic geometry. However, differential techniques have furnished a crucial tool for the study of the Mordell conjecture over function fields (beginning with the proof of this conjecture by Grauert and Manin), and its generalizations in higher dimension (theorem of Bogomolov on surfaces satisfying $c_1^2>3c_2$), or for holomorphic curve (conjecture of Green-Griffiths). They are often reformulated within the language of jet bundles.

Let us assume that $X$ is a smooth variety over a field $k$. Its tangent bundle $T(X)$ is a vector bundle over $X$ whose fiber at a (geometric) point $x$ is the tangent space $T_x(X)$ of $X$ at $x$. By construction, every morphism $f\colon Y\to X$ of algebraic varieties induces a tangent morphism $Tf\colon T(Y)\to T(X)$: it maps a tangent vector $v\in T_y(Y)$ at a (geometric) point $y\in Y$ to the tangent vector $T_yf(v)\int T_{f(y)}(X)$ at $f(y)$. This can be rephrased in the language of differential algebra as follows: for every differential field $(K,\partial)$ whose field of constants contains $k$, one has a derivative map $\nabla_1\colon X(K)\to T(X)(K)$. Here is the relation, where we assume that $K$ is the field of functions of a variety $Y$. A derivation $\partial$ on $K$ can be viewed as a vector field $V$ on $Y$, possibly not defined everywhere; replacing $Y$ by a dense open subset if needed, we assume that it is defined everywhere. Now, a point $x\in X(K)$ can be identified with a rational map $f\colon Y\dashrightarrow X$, defined on an open subset $U$ of $Y$. Then, we simply consider the morphism from $U$ to $T(X)$ given by $p\mapsto T_pf (V_p)$. At the level of function fields, this is our point $\nabla_1(x)\in T(X)(K)$.

If one wants to look at higher derivatives, the construction of the tangent bundle can be iterated and gives rise to jet bundles which are varieties $J_m(X)$, defined for all integers $m\geq 0$, such that $J_0(X)=X$,  $J_1(X)=T(X)$, and for $m\geq 1$, $J_m(X)$ is a vector bundle over $J_{m-1}X$ modelled on the $m$th symmetric product of $\Omega^1_X$.  For every differential field $(K,\partial)$ whose field of constants contains $k$, there is a canonical $m$th derivative map $\nabla_m\colon X(K) \to J_m(X) (K)$.

The construction of the jet bundles can be given so that the following three requirements are satisfied:
• If $X=\mathbf A^1$ is the affine line, then $J_m(X)$ is an affine space of dimension $m+1$, and $\nabla_m$ is just given by $\nabla_m (x) = (x,\partial(x),\dots,\partial^m(x))$ for $x\in X(K)=K$;
• Products: $J_m(X\times Y)=J_m(X)\times_k J_m(Y)$;
• Open immersions: if $U$ is an open subset of $X$, then $J_m(U)$ is an open subset of $X$ given by the preimage of $U$ under the projection $J_m(X)\to J_{m-1}(X)\to \dots\to J_0(X)=X$.
• When $X$ is an algebraic group, with origin $e$, then $J_m(X)$ is canonically isomorphic to the product of $X$ by the affine space $J_m(X)_e$ of $m$-jets at $e$.
We now describe Scanlon's application.

Let $G$ be a complex algebraic group acting on a complex algebraic variety $X$; let $S\colon X\to Z$ be the corresponding generalized Schwarzian map. Here, $Z$ is a complex algebraic variety, but $S$ is a differential map of some order $m$. In other words, there exists a constructible algebraic map $\tilde S\colon J_m(X)\to Z$ such that $S(x)=\tilde S(\nabla_m(x))$ for every differential field $(K,\partial)$ and every point $x\in X(K)$.

Let $U$ be an open subset of $X(\mathbf C)$, for the complex topology, and let $\Gamma$ be a Zariski dense subgroup of $G(\mathbf C)$ which stabilizes $U$. We assume that there exists a complex algebraic variety $Y$ and a biholomorphic map $p\colon \Gamma\backslash U \to Y(\mathbf C)$.

Locally, every open holomorphic map $\phi\colon\Omega\to Y(\mathbf C)$ can be lifted to a holomorphic map $\tilde\phi\colon \Omega\to U$. Two liftings differ locally by the action of an element of $\Gamma$, so that the composition $S\circ\tilde\phi$ does not depend on the choice of the lifting, by definition of the generalized Schwarzian map $S$. This gives a well-defined differential-analytic map $T\colon Y\to Z$. Let $m$ be the maximal order of derivatives appearing in a formula defining $T$. Then one may write $T\circ\phi =\tilde T\circ \nabla_m\tilde\phi$, where $\tilde T$ is a constructible analytic map from $J_m(Y)$ to $Z$.

Theorem (Scanlon). — Assume that there exists a fundamental domain $\mathfrak F\subset U$ such that the map $p|_{\mathfrak F}\colon \mathfrak F\to Y(\mathbf C)$ is definable in an o-minimal structure. Then $T$ is differential-algebraic: there exists a constructible map $\tilde T\colon J_m(Y)\to Z$ such that $T\circ \phi=\tilde T \circ J_m(\phi)$ for every $\phi$ as above.

For the proof, observe that the map $\tilde T$ is definable in an o-minimal structure, because it comes, by quotient of a definable map from the preimage in $J_m(U)$ of $\mathfrak F$, and o-minimal structures allow elimination of imaginaries. By the theorem of Peterzil and Starchenko, it is constructible algebraic.