171

Number
$171$ (one hundred and seventy-one) is:


 * $3^2 \times 19$


 * The $18$th triangular number after $1, 3, 6, 10, 15, 21, 28, 36, 45, 55, 66, 78, 91, 105, 120, 136, 153$:
 * $171 = \displaystyle \sum_{k \mathop = 1}^{18} k = \dfrac {18 \times \left({18 + 1}\right)} 2$


 * The $6$th palindromic triangular number after $1, 3, 6, 55, 66$.


 * The $35$th lucky number:
 * $1, 3, 7, 9, 13, 15, 21, \ldots, 133, 135, 141, 151, 159, 163, 169, 171, \ldots$


 * The $10$th palindromic lucky number:
 * $1, 3, 7, 9, 33, 99, 111, 141, 151, 171, \ldots$


 * The $1$st of the $6$th pair of consecutive integers which both have $6$ divisors:
 * $\tau \left({171}\right) = \tau \left({172}\right) = 6$