6 (six/ˈsɪks/) is the natural number following 5 and preceding 7.
The SI prefix for 10006 is exa (E), and for its reciprocal atto- (a).
6 is the smallest positive integer which is neither a square number nor a prime number. Six is the second smallest composite number; its proper divisors are 1, 2 and 3.
Since six equals the sum of its proper divisors, six is the smallest perfect number, Granville number, and -perfect number.
As a perfect number: 6 is related to the Mersenne prime 3, since 21(22 - 1) = 6. (The next perfect number is 28.) 6 is the only even perfect number that is not the sum of successive odd cubes. As a perfect number, 6 is the root of the 6-aliquot tree, and is itself the aliquot sum of only one number; the square number, 25.