In algebraic geometry and synthetic differential geometry, a Grothendieck connection is a way of viewing connections in terms of descent data from infinitesimal neighbourhoods of the diagonal.

Introduction and motivation

The Grothendieck connection is a generalization of the Gauss–Manin connection constructed in a manner analogous to that in which the Ehresmann connection generalizes the Koszul connection. The construction itself must satisfy a requirement of geometric invariance, which may be regarded as the analog of covariance for a wider class of structures including the schemes of algebraic geometry. Thus the connection in a certain sense must live in a natural sheaf on a Grothendieck topology. In this section, we discuss how to describe an Ehresmann connection in sheaf-theoretic terms as a Grothendieck connection.

Let M be a manifold and \pi : E \to M a surjective submersion, so that E is a manifold fibred over M. Let J^1(M, E) be the first-order jet bundle of sections of E. This may be regarded as a bundle over M or a bundle over the total space of E. With the latter interpretation, an Ehresmann connection is a section of the bundle (over E) J^1(M, E) \to E. The problem is thus to obtain an intrinsic description of the sheaf of sections of this vector bundle.

Grothendieck's solution is to consider the diagonal embedding \Delta : M \to M \times M. The sheaf I of ideals of \Delta in M \times M consists of functions on M \times M which vanish along the diagonal. Much of the infinitesimal geometry of M can be realized in terms of I. For instance, \Delta^*\left(I, I^2\right) is the sheaf of sections of the cotangent bundle. One may define a first-order infinitesimal neighborhood M^{(2)} of \Delta in M \times M to be the subscheme corresponding to the sheaf of ideals I^2. (See below for a coordinate description.)

There are a pair of projections p_1, p_2 : M \times M \to M given by projection the respective factors of the Cartesian product, which restrict to give projections p_1, p_2 : M^{(2)} \to M. One may now form the pullback of the fibre space E along one or the other of p_1 or p_2. In general, there is no canonical way to identify p_1^* E and p_2^* E with each other. A Grothendieck connection is a specified isomorphism between these two spaces. One may proceed to define curvature and p-curvature of a connection in the same language.

References

  • Osserman, B., "Connections, curvature, and p-curvature", preprint.
  • Katz, N., "Nilpotent connections and the monodromy theorem", IHES Publ. Math. 39 (1970) 175–232.