### July 17 – Yellow Pig Day

Posted on July 17, 2014

I love a math holiday, like Pi Day (3/14, because pi = 3.14...) or Mole Day (6:02 on 10/23, because Avogadro's Number is 6.02 * 10^23).

Today is a math holiday! It celebrates the number 17, which is the seventh prime number (hence 7/17). But what is the connection to yellow pigs, you may ask?

In the early 1960s, when holiday creators Michael Spivak and David C. Kelly were students at Princeton University, majoring in math, they began to list interesting properties of the number 17. Somehow, a yellow pig with 17 eyelashes was born – and it's been a thing ever since.

Okay, that still sounds pretty random, doesn't it? But how does any tradition start? How did the birth of a religious figure get associated with a gentleman who lives at the North Pole? How did a feast for Saint Patrick get associated with painting faces green and pouring green coloring into rivers—even though St. Patrick's flag is red and white?

At any rate, for probably completely random reasons (or no reason), Spivak and Kelly invented a yellow pig. Since then, Spivak has written many famous math textbooks – and he has included yellow pigs in them. Kelly runs a summer program for high school students who are “into” math, and he has introduced them to the “cult” of the Yellow Pig. Kelly has a collection of hundreds of yellow pigs, and a few other mathematicians have been inspired to collect them, too.

For more than three decades, mathematicians and math students have celebrated Yellow Pig Day by wearing shirts decorated with yellow pigs, playing a frisbee game called Ultimate, displaying collections of yellow pigs and origami yellow pigs, singing Yellow Pig Day carols (I kid you not), and eating a yellow pig cake. Some people exchange yellow pigs and mathematical knick knacks and other gifts.

Of course, the very best way to celebrate Yellow Pig Day is to learn more about the number 17!

The Number 17

17 is not only a prime number (which means that no two numbers multiplied together equal 17 – other than 1 and 17 itself), but it is the sum of the first four prime numbers:

2 + 3 + 5 + 7 = 17

17 is the only known prime that is equal to the sum of the digits of its cube:

17 * 17 * 17 = 4913
4 + 9 + 1 + 3 = 17

17 is the only prime that is the average of two consecutive Fibonacci numbers:

Fibonacci numbers are the sum of each two previous numbers:
1, 1 - 1 + 1 = 2
1, 1, 2 - 1 + 2 = 3
1, 1, 2, 3 - 2 + 3 = 5
1, 1, 2, 3, 5 – 3 + 5 = 8
and so on...
1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89...

13 + 21 = 34
34 divided by 2 = 17

There are a lot of other special things about the number 17, too...

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