6

Definition
$6$ (Six) is:


 * The second composite number, and the first with distinct prime factors:
 * $6 = 2 \times 3$


 * Hence the first positive integer after $1$ which is not the power of a prime number.


 * The first perfect number:
 * $6 = 1 + 2 + 3 = 1 \times 2 \times 3$


 * The area and semiperimeter of the $3-4-5$ triangle:
 * $6 = \dfrac {3 \times 4} 2 = \dfrac {3 + 4 + 5} 2$


 * The third factorial after $1$ and $2$:
 * $6 = 1 \times 2 \times 3$


 * The second primorial after $2$:
 * $6 = 2 \times 3$


 * The second hexagonal number:
 * $6 = 1 + 5$


 * The third triangular number:
 * $6 = 1 + 2 + 3$


 * The third automorphic number after $1$ and $5$:
 * $6^2 = 3 \mathbf 6$

Also see

 * Perfect Number is Sum of Successive Odd Cubes except 6