5050

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


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


 * The $100$th triangular number:
 * $5050 = \displaystyle \sum_{k \mathop = 1}^{100} k = \dfrac {100 \times \left({100 + 1}\right)} 2$


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