5050

Number
$5050$ (five thousand and fifty) is:


 * $2 \times 5^2 \times 101$


 * The $13$th Kaprekar number after $1$, $9$, $45$, $55$, $99$, $297$, $703$, $999$, $2223$, $2728$, $4879$, $4950$:
 * $5050^2 = 25 \, 502 \, 500 \to 2550 + 2500 = 5050$