buondì,

segnalo due seminari per mercoledì prossimo, il nostro di geometria è anticipato alle 14.30. a presto, Bruno

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Wednesday, April 15, 2015 Ora Inizio: 14:30 Aula: Sala Seminari (Dip. Matematica) Relatore: Paolo Aceto (Università di Firenze) Titolo: Rational homology cobordisms of plumbed 3-manifolds and arborescent link concordance

Abstract: We investigate rational homology cobordisms of 3-manifolds with non-zero first Betti number. This is motivated by the natural generalization of the slice-ribbon conjecture to multicomponent links.

In particular we consider the problem of which rational homology S^1x S^2's bound rational homology S^1xD^3's. We give a simple procedure to construct rational homology cobordisms between plumbed 3-manifold. We introduce a family F of plumbed 3-manifolds with b_1=1. By adapting an obstruction based on Donaldson's diagonalization theorem we characterize all manifolds in F that bound rational homology S^1xD^3's. For all these manifolds a rational homology cobordism to S^1xS^2 can be constructed via our procedure. The family F is large enough to include all Seifert fibered spaces over the 2-sphere with vanishing Euler invariant.

We also describe applications to arborescent link concordance.

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Wednesday, April 15, 2015 Ora Inizio: 16:00 Aula: Sala Seminari (Dip. Matematica) Relatore: Alberto Abbondandolo (Univ. di Bochum e Univ. di Pisa) Titolo: How long is the shortest geodesic on a sphere?