78

Number
$78$ (seventy-eight) is:


 * $2 \times 3 \times 13$


 * The $5$th sphenic number after $30$, $42$, $66$, $70$:
 * $78 = 2 \times 3 \times 13$


 * The $12$th triangular number after $1$, $3$, $6$, $10$, $15$, $21$, $28$, $36$, $45$, $55$, $66$:
 * $78 = 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 = \dfrac {12 \times \left({12 + 1}\right)} 2$


 * The $17$th semiperfect number after $6$, $12$, $18$, $20$, $24$, $28$, $30$, $36$, $40$, $42$, $48$, $54$, $56$, $60$, $66$, $72$:
 * $78 = 13 + 26 + 39$


 * The smallest positive integer which can be expressed as the sum of $2$ odd primes in $7$ ways.


 * The $2$nd 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 $8$th integer $n$ after $1$, $3$, $15$, $30$, $35$, $56$, $70$ with the property that $\tau \left({n}\right) \mathrel \backslash \phi \left({n}\right) \mathrel \backslash \sigma \left({n}\right)$:
 * $\tau \left({78}\right) = 8$, $\phi \left({78}\right) = 24$, $\sigma \left({78}\right) = 168$