231

Number
$231$ (two hundred and thirty-one) is:


 * $3 \times 7 \times 11$


 * The $21$st triangular number after $1, 3, 6, 10, 15, \ldots, 136, 153, 171, 190, 210$:
 * $231 = \displaystyle \sum_{k \mathop = 1}^{21} k = \dfrac {21 \times \left({21 + 1}\right)} 2$


 * The $11$th hexagonal number after $1, 6, 15, 28, 45, 66, 91, 120, 153, 190$:
 * $231 = \displaystyle \sum_{k \mathop = 1}^{11} \left({4 k - 3}\right) = 11 \left({2 \times 11 - 1}\right)$


 * The $44$th lucky number:
 * $1, 3, 7, 9, 13, 15, 21, \ldots, 193, 195, 201, 205, 211, 219, 223, 231, \ldots$