9999

Number
$9999$ (nine thousand, nine hundred and ninety-nine) is:


 * $3^2 \times 11 \times 101$


 * The $17$th Kaprekar number after $1$, $9$, $45$, $55$, $99$, $297$, $703$, $999$, $2223$, $2728$, $4879$, $4950$, $5050$, $5292$, $7272$, $7777$:
 * $9999^2 = 99 \, 980 \, 001 \to 9998 + 0001 = 9999$