66

Number
$66$ (sixty-six) is:


 * $2 \times 3 \times 11$


 * The $11$th triangular number after $1, 3, 6, 10, 15, 21, 28, 36, 45, 55$:
 * $66 = 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 = \dfrac {11 \times \left({11 + 1}\right)} 2$


 * The $6$th hexagonal number after $1, 6, 15, 28, 45$:
 * $66 = 1 + 5 + 9 + 13 + 17 + 21 = 6 \left({2 \times 6 - 1}\right)$


 * The $15$th semiperfect number after $6, 12, 18, 20, 24, 28, 30, 36, 40, 42, 48, 54, 56, 60$:
 * $66 = 11 + 22 + 33$


 * The $\sigma$ value of $66$ is square: