81

Number
$81$ (eighty-one) is:


 * $3^4$


 * The $9$th square number after $1, 4, 9, 16, 25, 36, 49, 64$:
 * $81 = 9 \times 9$


 * The $3$rd fourth power after $1, 16$:
 * $81 = 3 \times 3 \times 3 \times 3$


 * The $13$th powerful number after $1, 4, 8, 9, 16, 25, 27, 32, 36, 49, 64, 72$


 * The $6$th number after $1, 3, 22, 66, 70$, and $2$nd square number after $1$, whose $\sigma$ value is square:


 * The $3$rd and last number after $0, 1$ whose square root equals the sum of its digits:
 * $\sqrt {81} = 9 = 8 + 1$