Foreword: MASS and REU at Penn State University This book is part of a collection published jointly by the Amer- ican Mathematical Society and the MASS (Mathematics Advanced Study Semesters) program as a part of the Student Mathematical Library series. The books in the collection are based on lecture notes for advanced undergraduate topics courses taught at the MASS and/or Penn State summer REU (Research Experiences for Under- graduates). Each book presents a self-contained exposition of a non- standard mathematical topic, often related to current research areas, accessible to undergraduate students familiar with an equivalent of two years of standard college mathematics and suitable as a text for an upper division undergraduate course. Started in 1996, MASS is a semester-long program for advanced undergraduate students from across the USA. The program’s curricu- lum amounts to sixteen credit hours. It includes three core courses from the general areas of algebra/number theory, geometry/topology and analysis/dynamical systems, custom designed every year an in- terdisciplinary seminar and a special colloquium. In addition, ev- ery participant completes three research projects, one for each core course. The participants are fully immersed into mathematics, and xi
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