404

404 = 2² x 101.

404 is a number that is the product of a prime and the square of a different prime.

The numbers 199989³⁹⁴, 199989⁴⁰⁴ and 199989⁴⁶⁴ begin with the digits 394, 404, and 464 respectively.

404 has a representation as a sum of two squares: 404 = 20² + 2².

404 is the sum of seven positive fifth powers: 404 = 1⁵ + 2⁵ + 2⁵ + 2⁵ + 2⁵ + 2⁵ + 3⁵.


404 is the HTTP status code for "file not found," perhaps the most famous HTTP status code.

Source: Number Gossip