84

Number
$84$ (eighty-four) is:


 * $2^2 \times 3 \times 7$


 * The $7$th tetrahedral number, after $1, 4, 10, 20, 35, 56$:
 * $84 = 1 + 3 + 6 + 10 + 15 + 21 + 28 = \dfrac {7 \left({7 + 1}\right) \left({7 + 2}\right)} 6$


 * The $19$th semiperfect number after $6, 12, 18, 20, 24, 28, 30, 36, 40, 42, 48, 54, 56, 60, 66, 72, 78, 80$:
 * $84 = 14 + 28 + 42$