721

Number
$721$ (seven hundred and twenty-one) is:


 * $7 \times 103$


 * The $16$th centered hexagonal number after $1, 7, 19, 37, 61, 91, 127, 169, 217, 271, 331, 397, 469, 547, 631$:
 * $721 = \displaystyle 1 + \sum_{k \mathop = 1}^{16 - 1} 6 k = 16^3 - 15^3$