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Quadratic Welschinger invariants

Presented by: 
Marc Levine
Tuesday 14th August 2018 - 09:00 to 10:00
INI Seminar Room 1
This is report on part of a program to give refinements of numerical invariants arising in enumerative geometry to invariants living in the Grothendieck-Witt ring over the base-field. Here we define an invariant in the Grothendieck-Witt ring for ``counting'' rational curves. More precisely, for a del Pezzo surface S over a field k and a positive degree curve class $D$ (with respect to the anti-canonical class $-K_S$), we define a class in the Grothendiek-Witt ring of k, whose rank gives the number of rational curves in the class D containing a given collection of distinct closed points $\mathfrak{p}=\sum_ip_i$ of total degree $-D\cdot K_S-1$. This recovers Welschinger's invariants in case $k=\mathbb{R}$ by applying the signature map. The main result is that this quadratic invariant depends only on the $\mathbb{A}^1$-connected component containing $\mathfrak{p}$ in $Sym^{3d-1}(S)^0(k)$, where $Sym^{3d-1}(S)^0$ is the open subscheme of $Sym^{3d-1}(S)$ parametrizing geometrically reduced 0-cycles.
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University of Cambridge Research Councils UK
    Clay Mathematics Institute London Mathematical Society NM Rothschild and Sons