1801

Number
$1801$ (one thousand, eight hundred and one) is:
 * The $279$th prime number


 * The $4$th prime number after $1009$, $1129$, $1201$ which can be expressed in the form $x^2 + n y^2$ for all values of $n$ from $1$ to $10$:


 * The $25$th centered hexagonal number after $1$, $7$, $19$, $37$, $61$, $91$, $127$, $\ldots$, $817$, $919$, $1027$, $1141$, $1261$, $1387$, $1519$, $1657$:
 * $1801 = \ds 1 + \sum_{k \mathop = 1}^{25 - 1} 6 k = 25^3 - 24^3$