12

Number
$12$ (twelve) is:


 * $2^2 \times 3$


 * Divisible by both the sum and product of its digits:
 * $12 = \left({1 + 2}\right) \times 4 = \left({1 \times 2}\right) \times 6$


 * The number of edges of the regular octahedron and its dual, the cube


 * The smallest abundant number:
 * $1 + 2 + 3 + 4 + 6 + 12 = 16 > 12$


 * The $5$th superabundant number, and the smallest which is also abundant:
 * $1, 2, 4, 6, 12, \ldots$


 * The $2$nd semiperfect number after $6$:
 * $12 = 2 + 4 + 6$


 * The $3$rd pentagonal number after $1, 5$:
 * $12 = 1 + 4 + 7 = \dfrac {3 \left({3 \times 3 - 1}\right)} 2$


 * The base of the duodecimal number system.


 * The square of the reverse of $12$ equals the reverse of the square of $12$:
 * $12^2 = 144$
 * $21^2 = 441$


 * $12 = 3 \times 4$, and $56 = 7 \times 8$

Also see

 * 12 Pentominoes
 * Product of Proper Divisors of 12
 * 12 times Sigma of 12 equals 14 times Sigma of 14
 * 8 Mutually Non-Attacking Queens on Chessboard
 * 12 Knights to Attack or Occupy All Squares on Chessboard
 * Twelve Factorial plus One is divisible by 13 Squared