When I started the book, 17 had the edge. First of all, “At 17” by Janis Ian is probably my favorite song of all time. It came out in 1975, which was my favorite music year of all time. (Perhaps I should have written it in 1975.) But 17 is famous in mathematics for Carl Friedrich Gauss’s famous straightedge-and-compass construction of a regular 17-gon, for the 17 “wallpaper” symmetries of the plane, and for the fact that if you connect 17 suitably spaced dots with a segment of red, blue, or green, you will automatically create a “monochromatic” triangle whose three vertices are among the original 17 dots. And nobody has yet created a solvable Sudoku puzzle with fewer than 17 original entries. How about that?