(Amherst, Mass.) – For decades, Hampshire College Mathematics Professor David Kelly taught the mathematical and social history of the number 17, which "has fascinated mathematicians for 2,000 years, it has a lot of mathematical properties," he says. His office is filled with 17-related scholarly accounts and ephemera. Countless students have carried on his interest, much as he picked it up from one of his teachers at Princeton. One recent Sunday, Kelly started receiving emails shortly after the Patriots wrapped up their 51-17 win over the Jacksonville Jaguars. He's not that into football, but the score? "Of course 17. And then 51 is 17 times three," he says, laughing.

Hampshire is now marking Kelly’s retirement after four and a half decades of teaching with a lasting tribute. All of the 15mph speed limit signs on campus were replaced by, yup, 17mph signs.

"As he was getting ready to retire last spring, he said he didn't want a party or tribute, he just wanted to see the speed signs changed to 17," says Professor of Public Health Elizabeth Conlisk.  She was one of many voices, together with Hampshire College President Jonathan Lash, who worked to make it happen.

Dating back millennia, the number 17 has had broad applications in mathematics and many other disciplines, Kelly explains. About 17, the seventh prime number, Kelly points out that Roman historian Plutarch records, "The Pythagoreans also have a horror for the number 17, for 17 lies exactly halfway between 16, which is a square, and the number 18, which is the double of a square, these two, 16 and 18, being the only two numbers representing areas for which the perimeter equals the area."

Kelly knows countless facts about 17. There are 17 columns on the long side of the Parthenon in Greece. And, 17 is the smallest number of clues that can be provided for a 9x9 Sudoku puzzle to have a unique solution. (See sidebar “Top 17 Sensational 17 Stats” by David Kelly below.)

Over the years, Kelly's popular classes among his students were Complex Function Theory and Calculus, and he was also widely known for directing the well-respected Hampshire College Summer Studies in Mathematics, for high-ability high school students.

So it’s easy to understand why, for Kelly, it was an exciting and long-awaited sight when he pulled onto campus early one day recently to find the new speed limit signs had been installed overnight.

"It felt very good," he says, "And soon after, someone from admissions told me a prospective student was visiting campus, and when he drove up and saw the 17 mph sign, he said 'I'm going here.' That's just what I wanted. This captures Hampshire's uniqueness in some sense."

Top 17 Sensational 17 Stats by David Kelly

1. The ancient Greeks knew how to construct squares, equilateral triangles, and regular pentagons with straightedges and compasses; in 1796 Carl Friedrich Gauss proved that the euclidean tools sufficed also for the construction of a regular 17-sided polygon.

2. Any sequence of fewer than 17 consecutive positive integers containsat least one number which has no divisor in common with any of the other numbers; the 17-term sequence {2184, 2185, ..., 2200} contains no such number.

3. You can color all the (136) edges joining pairs of 16 points with 3 colors without having 3 of the edges forming a triangle be all the same color; with 17 (or more) points, there must be a monochromatic triangle.

4. A cube can be cut along 7 edges and unfolded to make a cross; a tesseract or 4-dimensional hypercube can be cut along 17 2-dimsional faces and unfolded into a three-dimensional cross.

5. There are 17 muscles in a horse's ear.

6. Roman historian Plutarch records "The Pythagoreans also have a horror for the number 17, for 17 lies exactly halfway between 16, which is a square, and the number 18, which is the double of a square, these two, 16 and 18, being the only two numbers representing areas for which the perimeter equals the area."

7. There are 17 columns on the long side of the Parthenon in Greece.

8. There are 17 mathematically distinct wallpaper patterns. Tilings in the Alhambra, a Moorish castle in Spain, and M.C.Escher's tessellations exhibit all the 17 different combinations of translations, rotations, and reflections.

9. Horses have recently been reported to distinguish among 17 facial expressions.

10. The 17th century was a great century for Japanese Haiku which has 17 syllables (comfortably spoken with a single breath).

11. 17 is the smallest number of clues that can be provided for a 9x9 Sudoku puzzle to have a unique solution.

12. There are 17 ways to write 17 as a sum of primes.

13. 1/17 is the 1st reciprocal of a positive integer whose periodic decimal expansion contains all 10 digits. (1/7 = 0.142857 142857 ... ; 1/11 = .090909...; 1/17 = 0.0588235294117647 0588235294117647...)

14. To calculate a 17% tip, divide the bill by 6 (and round up by 0.33333...%).

15. Morris the Cat died at age 17, the average lifespan of a goldfish.

16. The 17-year locust.

17. There are almost 17 ounces in a pound.