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.
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.
a surjective submersion, so that
be the first-order jet bundle of sections of
This may be regarded as a bundle over
or a bundle over the total space of
With the latter interpretation, an Ehresmann connection is a section of the bundle (over
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
consists of functions on
which vanish along the diagonal.
Much of the infinitesimal geometry of
is the sheaf of sections of the cotangent bundle.
One may define a first-order infinitesimal neighborhood
to be the subscheme corresponding to the sheaf of ideals
(See below for a coordinate description.)
There are a pair of projections
given by projection the respective factors of the Cartesian product, which restrict to give projections
One may now form the pullback of the fibre space
In general, there is no canonical way to identify
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.