Any number that reduces to 1 when you take the sum of the square of its digits and continually reiterate until it yields 1 is a happy number, any number that doesn't isn't, a happy prime is a number that's both happy and prime, now type it in! I don't know, talk about dumbing down. Don't they teach recreational mathematics anymore?

The Doctor, explaining happy primes

from Doctor Who by Chris Chibnall

submitted by Camilla
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