Sep 24 2009 - 3:30pm
Sep 24 2009 - 4:30pm
Hans Schoutens
NYC College of Technology
CC246
The classical Grothendieck ring over an algebraically closed field
$K$ is the Abelian group on isomorphism classes of varieties modulo the
"scissor" relations $[X]=[Y]+[X-Y]$ for $Y\subset X$, with multiplication
given by the Cartesian product of varieties. When Denef and Loeser in the
late 90s generalized Konsevitch's motivic integration---itself a
generalization of $p$-adic integration with values in the Grothendieck
ring---using certain QE results in valued fields, many model-theorists
(Scanlon, Haskell, Hrushovski, Cluckers, et al.) became very interested in
the Grothendieck ring of an arbitrary first-order theory, the classical case
(allegedly) corresponding to the theory ACF.
For the last decade, I had been contemplating ways of incorporating schemes
(=varieties with nilpotent structure) into a model-theoretic setup.
Geometers, on the other hand, do not know how to treat schemes in a
Grothendieck ring setup. I will simultaneously resolve both conundrums by
restricting the class of formulae (schemic formulae) while working with a
larger theory (the theory of local finite-dimensional $K$-algebras), thus
obtaining the schemic Grothendieck ring (and some of its "infinitary"
variants that are necessary to get a good notion of complement). The main
motivation, however, for this new theory is to generalize the work of
Denef-Loeser on the rationality of the Igusa-zeta series (a sort of abstract
"counting" of approximate solutions). Whereas their proof relies heavily on
resolution of singularities (together with the aforementioned QE results),
and hence does not work in positive characteristic, the present theory is
far more functorial, and preliminary calculations yield rationality in may
instances (e.g., all Du Val singularities in any characteristic).
Although many of these terms might seem frightening (what the heck is a
scheme? a motive?) to a logician, they will sound soon very familiar from
this new perspective, and require only a modest knowledge of first-order
logic.