72

Number
$72$ (seventy-two) is:


 * $2^3 \times 3^2$


 * The $12$th powerful number after $1, 4, 8, 9, 16, 25, 27, 32, 36, 49, 64$


 * The $16$th semiperfect number after $6, 12, 18, 20, 24, 28, 30, 36, 40, 42, 48, 54, 56, 60, 66$:
 * $72 = 12 + 24 + 36$


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

Also see