666

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


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


 * The $36$th triangular number:
 * $666 = \dfrac {36 \times \left({36 + 1}\right)} 2$


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