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


 * The smallest positive integer which can be expressed as the sum of $2$ odd primes in $8$ ways.


 * The $5$th and last after $21, 29, 61, 69$ of the $5$ $2$-digit positive integers which can occur as a $5$-fold repetition at the end of a square number


 * The $5$th inconsummate number after $62, 63, 65, 75$:
 * $\nexists n \in \Z_{>0}: n = 84 \times s_{10} \left({n}\right)$


 * The $3$rd element of the $1$st set of $4$ positive integers which form an arithmetic progression which all have the same Euler $\phi$ value:
 * $\phi \left({72}\right) = \phi \left({78}\right) = \phi \left({84}\right) = \phi \left({90}\right) = 24$


 * The $44$th positive integer after $2, 3, 4, 7, 8, \ldots, 61, 65, 66, 67, 72, 77, 80, 81$ which cannot be expressed as the sum of distinct pentagonal numbers.

Also see