typographical errors and inaccuracies in earlier versions of the manuscript.
I am grateful for both their help and their friendship.
This text served as the basis for a graduate topics course taught by
the author during the Spring 2009 semester at the University of Illinois at
Urbana-Champaign. I am grateful to the U of I for allowing me this op-
portunity. Almost concurrently, Carl Pomerance used a preliminary version
of these notes to teach a quarter-long course at Dartmouth College. This
manuscript is better for his numerous insightful suggestions.
Finally, I would like to thank the American Mathematical Society, espe-
cially Ed Dunne, Cristin Zanella, and Luann Cole, for their encouragement
of this project at every stage.