The macroscopic entropy and the attractor equations for extremal black hole solutions follow from a variational principle based on an entropy function. We review this variational principle for static extremal black holes in four space-time dimensions and we discuss an example in the context of N = 2 supergravity. (C) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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