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    <title>Topology Seminar</title>
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  <body> <b><span style="text-decoration: underline;">Orateur:</span> Gerrit
      Herrmann (University of Regensburg)<br>
      <span style="text-decoration: underline;">Titre:</span> Thurston norm and L^2-Betti numbers. </b><br>
    <span style="font-weight: bold;"><span style="text-decoration: underline;">Résumé:</span>
    </span>In my talk I will define the Thurston norm of a closed irreducible
    3-manifold M. This is a semi-norm on the second homology H_2(M;Z). Morally
    speaking it measures the complexity of a class by embedded surfaces
    representing this class. I will then characterize embeddings of surfaces
    which release the Thurston norm by certain L^2-Betti numbers.<br>
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