52

This number is a composite.

The number of 8-digit primes (on a calculator) that remain prime if viewed upside down, in a mirror, or upside down in a mirror. [Russo]

Mersenne wrongly thought 2^257-1 is prime. In fact, 52 is the smallest base b for which (b^257-1)/b-1 is actually prime.

52! + 52 + 1 is prime. Note that 52 is the (5-2)rd untouchable number. [Mojdeh]

The element with atomic number 52 (Tellurium) is the only one whose atomic weight's integer part is a Mersenne prime. [Wu]

52 = π(5!*2!). [Firoozbakht]