9

Number
$9$ (nine) is:


 * $3^2$


 * The third square number after $1$ and $4$:
 * $9 = 3^2$
 * and therefore from Sum of Consecutive Triangular Numbers is Square, the sum of $2$ consecutive triangular numbers:
 * $9 = 3 + 6$


 * The $3$rd semiprime after $4, 6$:
 * $9 = 3 \times 3$


 * The $4$th powerful number after $1, 4, 8$


 * In ternary:
 * $100_3 = 9_{10}$


 * The first odd prime power:
 * $9 = 3^2$


 * The $4$th lucky number, and the second lucky square after $1$:
 * $1, 3, 7, 9, \ldots$


 * The $2$nd Kaprekar number after $1$:
 * $9^2 = 81 \to 8 + 1 = 9$


 * The sum of the first $3$ factorials:
 * $9 = 1! + 2! + 3!$


 * The $4$th subfactorial after $0, 1, 2$:
 * $9 = 4! \left({1 - \dfrac 1 {1!} + \dfrac 1 {2!} - \dfrac 1 {3!} + \dfrac 1 {4!} }\right)$


 * The $9$th integer $n$ after $0, 1, 2, 3, 4, 5, 6, 7$ such that both $2^n$ and $5^n$ have no zeroes:
 * $2^9 = 512, 5^9 = 1 \, 953 \, 125$


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


 * The $6$th integer after $0, 1, 3, 5, 7$ which is palindromic in both decimal and binary:
 * $9_{10} = 1001_2$

Also see

 * 9 is Only Square which is Sum of 2 Consecutive Cubes
 * Nine Regular Polyhedra
 * Dissection of Rectangle into 9 Distinct Integral Squares
 * Nine Point Circle Theorem
 * Divisibility by 9