Acknowledgments We thank Jinpeng An, Benson Farb, Alex Furman, Amos Nevo, Roman Sauer, Ben Schmidt, and, especially, David Fisher for helpful comments on a preliminary version of the manuscript. Long lasting encouragement from Edward Dunne of the AMS played an important role in the completion of this project. The final version of these notes was prepared while D. W. M. was visiting the University of Chicago’s Department of Mathematics. He is grateful to them for their hospitality and financial support during this productive and enjoyable visit. His work was also partially supported by a research grant from the Natural Sciences and Engineering Research Council of Canada. ix
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