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David Kelly, a mathematics professor at Hampshire College who is getting ready to retire, has long shared his fascination with the number 17 with students. He loves to teach facts about the number, such as: "You can color all the (136) edges joining pairs of 16 points with three colors without having three of the edges forming a triangle be all the same color; with 17 (or more) points, there must be a monochromatic triangle." (For 16 other of his top 17 facts about 17, go here.)

So to honor Kelly upon his retirement, the college is changing all of its speed limit 15 signs to 17.