687

687 = 3 x 229.

687 is a number that cannot be written as a sum of three squares.

687 is 2001 in base 7 and 575 in base 11. It is 22233 in base 4 and 223 in base 18.

687 primes less than 10⁴ do not contain the digit 9.

In the Harry Potter books and movies, the Potter family vault number at Gringotts Bank is 687.

Source: Positive Integers