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Combinatorial and algebro-geometric cohomology classes on the moduli spaces of curves. (English) Zbl 0886.14007
Let $$\overline {\mathcal M}_{g,n}$$ denote the moduli space of $$n$$-pointed stable curves of genus $$g$$. There are combinatorical cycles on $$\overline {\mathcal M}_{g,n}$$ which are defined using Strebel’s theory of quadratic differentials. On the other hand there are algebro-geometric cohomology classes of $$\overline {\mathcal M}_{g,n}$$, the so-called Mumford-Morita-Miller classes, defined via the relative dualizing sheaf of the universal curve over $$\overline {\mathcal M}_{g,n}$$. It was first conjectured by Witten that the classes of the combinatorical cycles can be expressed in terms of the Mumford-Morita-Miller classes. The intersection numbers of the combinatorical cycles are best organized as coefficients of an infinite series $$F$$ in infinitely many $$s$$- and $$t$$-variables. P. Di Francesco, C. Itzykon, and J.-B. Zuber proved in Commun. Math. Phys. 151, No. 1, 193-219 (1993; Zbl 0831.14010) that the $$s$$-derivatives of $$F$$ are linear combinations of the $$t$$-derivatives of $$F$$ evaluated at the same point. The authors conjecture that this result interpreted geometrically should provide the link between the combinatorical and algebro-geometric classes on $$\overline {\mathcal M}_{g,n}$$. The main result of the paper is the proof of this conjecture in codimension 1. Moreover in some cases the authors give an explicit expression for the Di Francesco-Itzykson-Zuber-correspondence. In these cases the correspondence gives an explicit expression for the intersection numbers of the combinatorial cycles in terms of the algebro-geometric classes.
Reviewer: H.Lange (Erlangen)

##### MSC:
 14H10 Families, moduli of curves (algebraic) 14H15 Families, moduli of curves (analytic)
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