18

Number
$18$ (eighteen) is:
 * $2 \times 3^2$


 * The $3$rd semiperfect number after $6, 12$:
 * $18 = 3 + 6 + 9$


 * The $10$th Ulam number after $1, 2, 3, 4, 6, 8, 11, 13, 16$:
 * $18 = 2 + 16$


 * Equal to the sum of the digits of its cube:
 * $18^3 = 5832$, while $5 + 8 + 3 + 2 = 18$


 * Equal to the sum of the digits of its $6$th power:
 * $18^6 = 34 \, 012 \, 224$, while $3 + 4 + 0 + 1 + 2 + 2 + 2 + 4 = 18$


 * Equal to the sum of the digits of its $7$th power:
 * $18^7 = 612 \, 220 \, 032$, while $6 + 1 + 2 + 2 + 2 + 0 + 0 + 3 + 2 = 18$


 * $18 = 9 + 9$, and its reversal $81 = 9 \times 9$


 * $18^3 = 5832$ and $18^4 = 104 \, 976$, using all $10$ digits from $0$ to $9$ once each between them.

Also see

 * Positive Integers Equal to Sum of Digits of Cube