8125

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


 * $5^4 \times 13$


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