# Is there a “motivic Gromov-Witten invariant”?

I recently attended an interesting seminar, where the concept of motivic Donaldson-Thomas invariants was explained (0909.5088).

Very roughly, the DT invariant is a generating function $\sum q^k e(M_k)$ of a numerical invariant $e(\cdot)$ of a sequence of moduli spaces $M_k$. The motivic DT invariant is obtained by considering $\sum q^k [M_k]$ where $[M_k]$ is the image in $K(Var)$. This contains more info than the ordinary DT invariant.

Can this idea be applied for, say, the GW invariant of Calabi-Yau 3-folds, to get a finer invariant?

(Sorry for my vague question.)

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Let me give an answer from a slightly different point of view.

Let $M_k$ be a moduli space as in your question; say it's a (compact) moduli space of sheaves on some (compact) Calabi-Yau threefold. In general, $M_k$ is going to be very singular. However, it carries a so-called perfect deformation-obstruction theory of dimension zero. This gives a virtual fundamental class on $M_k$, and the technical definition of the numerical invariant $e(M_k)$ is that it is the degree of this virtual fundamental class.

In the case of sheaves on CY3s, the deformation-obstruction theory has a duality property: it is a symmetric obstruction theory. In this case, according to a result of Kai Behrend, $e(M_k)$ can also be expressed as an Euler characteristic, albeit a weighted one:
$e(M_k)=\chi(M_k, \nu_{M_k})$, where $\nu_{M_k}$ is the Behrend function of the singular space $M_k$. In other words, one computes an Euler characteristic, but weighted with a numerical measure of how bad the singularities are.

On can hope that this Euler characteristic definition can now be turned into something motivic. What one needs is a way to attach a motivic weight to points of $M_k$. In some specific moduli problems, such as for Hilbert schemes of points where at least locally the moduli space can be expressed as a critical locus of a function on a smooth variety, this can be done using the tool of the motivic vanishing cycle; indeed, this is what our work does in the paper you cite. The general theory of how one attaches motivic weights is discussed in a (partially conjectural) paper of Kontsevich and Soibelman.

The issue with Gromov-Witten theory on a CY3 is that the deformation-obstruction theory in that case, while it is of dimension zero, is not fully symmetric. It is symmetric on the open part corresponding to stable maps which are immersions from a smooth curve, but (as an expert assures me) not on the whole moduli space.

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I don't know anything about motivic Donaldson-Thomas invariants, but it is possible to define the notion of GW invariants on the level of motives. You should check out Behrend and Manin, "Stacks of stable maps and Gromov-Witten invariants" sections 8 and 9, as well as Toën, "On motives for Deligne-Mumford stacks". Namely, instead of considering GW invariants as a collection of maps $$I^X_{g,n,\beta} : H^\ast (X^n) \to H^\ast(\overline M_{g,n})$$ you can define them as morphisms between the Chow motives associated to $X^n$ and $\overline M_{g,n}$. The notion of a motive associated to a DM-stack is explained in Toën's article. In the case of $\overline M_{g,n}$ it is easy to say explicitly what this means: there is a finite cover $f \colon M \to \overline M_{g,n}$ by a smooth projective scheme M, and we take as the motive associated to $\overline M_{g,n}$ the motive associated to the scheme M and the projector $\frac{1}{\deg f} f^\ast f_\ast$. The correspondence inducing the morphism is given by the pushforward of the virtual fundamental class of $\overline M_{g,n}(X,\beta)$ to $A^\ast(X^n \times \overline M_{g,n})$ along the product of the evaluation maps and the forgetful map.

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See this great talk by Manin: dailymotion.com/video/… –  Peter Arndt Apr 4 '11 at 21:48