58

Number
$58$ (fifty-eight) is:
 * $2 \times 29$


 * The $21$st semiprime after $4, 6, 9, 10, 14, 15, 21, 22, 25, 26, 33, 34, 35, 38, 39, 46, 49, 51, 55, 58$:
 * $58 = 2 \times 29$


 * The $6$th noncototient after $10, 26, 34, 50, 52$:
 * $\nexists m \in \Z_{>0}: m - \phi \left({m}\right) = 58$
 * where $\phi \left({m}\right)$ denotes the Euler $\phi$ function


 * The $5$th positive integer which cannot be expressed as the sum of a square and a prime:
 * $1, 10, 25, 34, 58, \ldots$


 * The $3$nd non-square positive integer which cannot be expressed as the sum of a square and a prime:
 * $10, 34, 58, \ldots$