12

Number
$12$ (twelve) is:


 * $2^2 \times 3$


 * The base of the duodecimal number system


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


 * The number of distinct pentominoes, up to reflection


 * The $1$st power of $12$ after the zeroth $1$:
 * $12 = 12^1$


 * The $1$st of three $2$-digit integers divisible by both the sum and product of its digits:
 * $12 = \paren {1 + 2} \times 4 = \paren {1 \times 2} \times 6$


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


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


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


 * The $3$rd superfactorial after $1$, $2$:
 * $12 = 3\$ = 3! \times 2! \times 1!$


 * The $4$th special highly composite number after $1$, $2$, $6$


 * The $5$th highly composite number after $1$, $2$, $4$, $6$:
 * $\map \tau {12} = 6$


 * The $5$th superabundant number after $1$, $2$, $4$, $6$, and the smallest which is also abundant:
 * $\dfrac {\map \sigma {12} } {12} = \dfrac {28} {12} = 2 \cdotp \dot 3$


 * The $5$th integer $m$ such that $m! - 1$ (its factorial minus $1$) is prime:
 * $3$, $4$, $6$, $7$, $12$


 * The $5$th generalized pentagonal number after $1$, $2$, $5$, $7$:
 * $12 = \dfrac {3 \paren {3 \times 3 - 1} } 2$


 * The $5$th of $35$ integers less than $91$ to which $91$ itself is a Fermat pseudoprime:
 * $3$, $4$, $9$, $10$, $12$, $\ldots$


 * The number of distinct free pentominoes


 * The $6$th even number after $2$, $4$, $6$, $8$, $10$ which cannot be expressed as the sum of $2$ composite odd numbers


 * The $7$th positive integer after $1$, $2$, $3$, $4$, $6$, $8$ such that all smaller positive integers coprime to it are prime


 * The $7$th positive integer which is not the sum of $1$ or more distinct squares:
 * $2$, $3$, $6$, $7$, $8$, $11$, $12$, $\ldots$


 * The $8$th highly abundant number after $1$, $2$, $3$, $4$, $6$, $8$, $10$:
 * $\map \sigma {12} = 28$


 * The $8$th after $1$, $2$, $4$, $5$, $6$, $8$, $9$ of the $24$ positive integers which cannot be expressed as the sum of distinct non-pythagorean primes


 * The $9$th (strictly) positive integer after $1$, $2$, $3$, $4$, $6$, $7$, $9$, $10$ which cannot be expressed as the sum of exactly $5$ non-zero squares.


 * The $11$th Zuckerman number after $1$, $2$, $3$, $4$, $5$, $6$, $7$, $8$, $9$, $11$:
 * $12 = 6 \times 2 = 6 \times \paren {1 \times 2}$


 * The $11$th harshad number after $1$, $2$, $3$, $4$, $5$, $6$, $7$, $8$, $9$, $10$:
 * $12 = 4 \times 3 = 4 \times \paren {1 + 2}$


 * The square of the reversal of $12$ equals the reversal 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
 * Square of Reversal of Small-Digit Number