33

This number is a composite.

The number of primes and the number of semiprimes up to 33 are equal. [Gupta]

The smallest run of 3 consecutive integers that are semi-primes (the product of two primes) begins with 33. [Wells]

The smallest palindromic number n such that inserting a digit '1' between adjacent digits of n is prime. [Russo]

First exponent to yield 3 generalized cullen primes: 33*4^33+1, 33*14^33+1 and 33*34^33+1 [Chun Wa]

The smallest triadic semiprime. [Capelle]

The largest known number (coincidentally semiprime) that is not the sum of two semiprimes. [Capelle]

The largest integer n such that π(n) = n/3. [Bajpai]

The smallest odd number n such that n+x! is not a prime, for any number x. [Loungrides]

The sum of the digits of the first (3+3) primes with a digit-3 equals 33. Note that π(33) = π(3*π(33)). [Worrom]