666

Number
$666$ (six hundred and sixty-six) is:


 * $2 \times 3^2 \times 37$


 * The $36$th triangular number after $1$, $3$, $6$, $10$, $15$, $\ldots$, $325$, $351$, $378$, $406$, $435$, $465$, $496$, $528$, $561$, $595$, $630$:
 * $666 = \displaystyle \sum_{k \mathop = 1}^{36} k = \dfrac {36 \times \left({36 + 1}\right)} 2$


 * The $8$th palindromic triangular number after $1$, $3$, $6$, $55$, $66$, $171$, $595$.


 * The $3$rd and last after $55, 66$ of the $3$ repdigit numbers which are also triangular.