Grothendieck ring of "varieties carrying a function" - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-22T05:59:15Z http://mathoverflow.net/feeds/question/108098 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/108098/grothendieck-ring-of-varieties-carrying-a-function Grothendieck ring of "varieties carrying a function" Allen Knutson 2012-09-25T20:45:45Z 2013-03-07T09:25:17Z <p>Fix a base ring $R$, and consider pairs $(X,f)$ where $X$ is a scheme of finite type over $R$ and $f:X\to R$ is an $R$-valued algebraic (not constructible!) function on $X$.</p> <p>I want to consider a Grothendieck $R$-algebra of such pairs, where if $X = Y \coprod Z$, then $[(X,f)] = [(Y,f|_Y)] + [(Z,f|_Z)]$, but also $[(X,f+g)] = [(X,f)] + [(X,g)]$ and $[(X,rf)] = r[(X,f)]$.</p> <blockquote> <p>Surely this is a standard extension of the usual notion of the Grothendieck ring of varieties (which only has $f=1$, and the first sort of relation)? If so, where can I read about it?</p> </blockquote> <p>Maybe I'm misreading the motivic integration survey literature (by K. Smith, and E. Looijenga), but it seems like they're insisting on constructible functions, not algebraic. Ordinarily when a construction like this isn't in the literature, I assume it's because it has too many relations and is $0$, but if $R = {\mathbb Z}$ it seems to me that this ring has many functionals, like $[(X,f)] \mapsto \sum_{x \in X_p} (f(x) \bmod p) \in {\mathbb Z}/p.$ (I don't see an analogue of $[X] \mapsto$ the Euler characteristic $\chi(X_{\mathbb C})$.)</p> <p>EDIT: One problem I see is that $({\mathbb A}^1, f(x)=x)$ is isomorphic under translation to $({\mathbb A}^1, f(x)=x+1)$. So $[({\mathbb A^1}, 1)] = [({\mathbb A}^1, (x+1)-x)] = [({\mathbb A}^1, x+1)] - [({\mathbb A}^1, x)] = 0$. Of course this fits with point-counting $\bmod p$.</p> http://mathoverflow.net/questions/108098/grothendieck-ring-of-varieties-carrying-a-function/123853#123853 Answer by Andrew Stout for Grothendieck ring of "varieties carrying a function" Andrew Stout 2013-03-07T09:19:19Z 2013-03-07T09:25:17Z <p>To answer your question is a bit difficult because anytime you sum elements of something built out of the grothendieck ring of varieties you are doing (or attempting to do) motivic integration. Pairs $\phi = (X, f)$ certainly already exist in the literature (well actually one should consider the tensor product). But, note that you want $f$ to be a function in $n\in \mathbb{Z}$. Thinking of $f$ being algebraic to your base doesn't add anything because that part can be moved over to the 1st factor in the tensor product via the characteristic function on the graph. In fact, I think it is generally accepted that $f$ should be a function <code>$$f : X(K) \to \mathbb{Z}[\mathbb{L}, \mathbb{L}^{-1}, (\mathbb{L}^{i} -1)_{i\in\mathbb{N}}^{-1}]$$</code> where $K$ is a field, and $X(K)$ is definable in the three sorted language of Denef-Pas and where $X$ above lives over $S$ in some suitable sense (factors through some projection $S\times \mathbb{A}^{n}\rightarrow S$).<br> The reason to use the language denef-pas is so that when the summation of a function $\phi(s,n)$ over $n$ exists, then there is in fact a function of the form $I(\phi)$ which can legitimately be called its integral. In other words, the measure exists and generating series will become rational. </p> <p>Perhaps you already know of this article: <a href="http://arxiv.org/abs/math/0410203" rel="nofollow">http://arxiv.org/abs/math/0410203</a> which lays the foundation for both geometric and arithmetic integration and connects it partly to motivic integration over formal schemes. </p> <p>Personally, I think what might be interesting here is trying to develop something non-commutative by taking pairs (X,f) where f is an endomorphism of X. But, to get something non-commutative, you cannot just take an extension of the usual grothendieck ring of varieties -- the multiplicative structure on such pairs needs to correspond with composition of functions. </p>