550

550 = 2 x 5 x 5 x 11.

550 is a pentagonal pyramidal number (A002411).

505 is 202101 in base 3. It is 1414 in base 7.

550 is the number of primes less than 4000 (A028505 and A038812).

550 is a divisor of 43⁴ - 1.

Source: What's Special About This Number?