78

Number
$78$ (seventy-eight) is:


 * $2 \times 3 \times 13$


 * 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:
 * $\phi \left({72}\right) = \phi \left({78}\right) = \phi \left({84}\right) = \phi \left({90}\right) = 24$