Friday, May 1, 2009

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

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