A formally smooth scheme is a scheme with the property that every map into it admits an infinitesimal lift :
for any “test space”, i.e. an affine scheme with underlying reduced scheme (obtained from by removing all infinitesimal directions, compare formal scheme) every map lifts to a map .
Similarly, one can talk about formally smooth algebraic spaces, algebraic stacks and so on.
A scheme over some ground field , given by a morphism , is formally smooth if that morphism is a formally smooth morphism, as described there.
If the scheme is regarded as an absolute (as opposed to relative) scheme, then the morphism is required to be a formally smooth morphism for to be formally smooth.
(Nowadays people will also start to consider schemes over the field with one element…).
There is an interpretation of formal smoothness of (set-valued) functors as an epipresheaf condition with respect to Sasha Rosenberg’s generalization of Grothendieck topologies, so-called Q-category formalism, namely the Q-category of nilpotent (infinitesimal) thickenings. For more on this see formally smooth morphism.
Discussion of formally smooth schemes in the general context of formally smooth morphisms is at
Maxim Kontsevich, Alexander Rosenberg, Noncommutative spaces, preprint MPI-2004-35 (ps)
T. Brzeziński, Notes on formal smoothness, in: Modules and Comodules (series Trends in Mathematics). T Brzeziński, JL Gomez Pardo, I Shestakov, PF Smith (eds), Birkhäuser, Basel, 2008, pp. 113-124 (doi, arXiv:0710.5527)
Last revised on February 8, 2013 at 12:08:29. See the history of this page for a list of all contributions to it.