*Doctor Who*episode "42", a sequence of happy primes (313, 331, 367, 379) is used as a code for unlocking a sealed door on a spaceship about to collide with a star. When the Doctor learns that nobody on the spaceship besides himself has heard of happy numbers, he asks, "Don't they teach recreational mathematics anymore?"

A prime number is a number with exactly 2 factors - itself and 1.

A happy number is a number that, when you square each digit and add the squares together, and keep doing that, eventually you get 1.

.....For example, the number 379 is happy:

.....square each digit - 3 squared is 9, 7 squared is 49, 9 squared is 81.

.....add the squares together: 9+49+81 = 139

.....repeat - 1 squared is 1, 3 squared is 9, 9 squared is 81. 1+9+81 = 91

.....repeat - 9 squared is 81, 1 squared is 1. 81+1 = 82

.....repeat - 8 squared is 64, 2 squared is 4. 64+4 = 68

.....repeat - 6 squared is 36, 8 squared is 64. 36+64 = 100

.....repeat - 1 squared is 1, 0 squared is 0, 0 squared is 0. 1+0+0 = 1

.....Eventually you get to 1.

313, 331, 367, 379 are all happy primes.

They add up to 1390, which is a happy number, but is not prime.

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