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17 July is not just another ordinary day; just like a yellow pig is not just another ordinary pig. Today is Yellow Pig Day, the day to take a moment to honour the magical, mathematic pig that has inspired mathematicians for years.

The yellow pig was invented in the early 60’s by two Princeton maths students, Michael Spivak and David C. Kelly, while working on an assignment to identify unique properties of the number 17. After some intense mental gymnastics (and possibly a few pints at the local pub), when they finally ran out of ideas, they thought up the yellow pig, a mythical 17-eyelashed creature (that’s eight lashes on one eye and nine on the other, of course).

OK, so Winnie the Pooh’s friend Piglet is generally considered to be pink, but look closely, and you may notice him turning yellow for just one day of the year…
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Spivak has since written a number of mathematics textbooks, where he regularly includes hidden references to yellow pigs, while David Kelly presents an annual mathematics summer school to high school students, where he introduces them to the “Cult of the Yellow Pig”.  He is rumoured to be the proud owner of an impressive collection of between 289 (17 squared) and 4913 (17 cubed) yellow pigs. When asked about the significance of a yellow pig, he responds, “If you have to ask, you just won’t understand.”

Through Spivak and Kelly’s efforts, yellow pigs have become popular toys among mathematicians. Yellow Pig Day is also celebrated at various (mainly US) University Maths Departments, with the singing of yellow pig carols and the eating of yellow pig cake.

By the way, if you’re unsure about the significance of the number 17, look no further than this list. There’s no doubt the number is as special and magical as the yellow pig itself…

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