100

Number
$100$ is:


 * $2^2 \times 5^2$


 * The $10$th square number after $1, 4, 9, 16, 25, 36, 49, 64, 81$:
 * $100 = 10 \times 10$


 * The $8$th noncototient after $10, 26, 34, 50, 52, 58, 86$:
 * $\nexists m \in \Z_{>0}: m - \phi \left({m}\right) = 100$
 * where $\phi \left({m}\right)$ denotes the Euler $\phi$ function


 * The $23$rd semiperfect number after $6, 12, 18, 20, 24, 28, 30, 36, 40, 42, 48, 54, 56, 60, 66, 72, 78, 80, 84, 88, 90, 96$:
 * $100 = 5 + 20 + 25 + 50$