This is a talk by Stefanie Frick of Freie Universität, Berlin, given at the "What is ...?" seminar at the Freie Universität on Oct 24, 2008,... » More

This is a talk by Stefanie Frick of Freie Universität, Berlin, given at the "What is ...?" seminar at the Freie Universität on Oct 24, 2008, https://www.math.fu-berlin.de/w/Math/WhatIsSeminar .

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