8125

Number
$8125$ (eight thousand, one hundred and twenty-five) is:


 * $5^4 \times 13$


 * The $25$th pentagonal pyramidal number after $1, 6, 12, 40, 75, \ldots, 1470, 1800, 2176, 2601, 3078, 3610, 4200, 4851, 5566, 6348, 7200$:
 * $8125 = \displaystyle \sum_{k \mathop = 1}^{25} \dfrac {k \left({3 k - 1}\right)} 2 = \dfrac {25^2 \left({25 + 1}\right)} 2$