133

This number is a composite.

The reversal of 133 is prime. Note that 133.335 is the Dewey Decimal classification for 'numerology'. If you reverse it, and add: 133.335 + 533.331, you'll discover the beast number, repeated! [Gardner]

133 is the smallest positive integer, n, for which the integer 10n + 5 is divisible by both 3 and 5 and for which none of the integers 10n + 1, 10n + 3, 10n + 7, and 10n + 9 are prime. [Litman]

The sum of the first three squares of semiprimes is itself a semiprime, and this is the first such sum with this property: 133 = 4^2 + 6^2 + 9^2 = 7 * 19. [Post]

1/133 begins with 751 after the leadings zeros. Note that 751 is the 133rd prime. [Izquierdo]

The last 133 digits of 131^132 is prime. [Gupta]

There are 133 corvid species. [McCown]

133 is the personal nemesis of Christian Lawson-Perfect, inventor of "The Is this prime? game."

The smallest composite number formed from a prime (in this case 31) written in base-4 and read as base-10. The sequence begins 133, 221, 323, 1003, 1121, 1211, … . [Honaker]