28

28 = 2² x 7.

28 is the second perfect number. It is the only even perfect number of the form x³ + 1.

28 is the sum of the first five prime numbers: 2 + 3 + 5 + 7 + 11 = 28.

28 is the seventh triangular number.

28 is the smallest number with three representations as a sum of four squares: 28 = 1² + 1² + 1² + 5² = 1² + 3² + 3² + 3² = 2² + 2² + 2² + 4².

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

28 is 44 in base 6.


A standard domino set contains 28 dominoes.

Source: Number Gossip