57

Number
$57$ (fifty-seven) is:
 * $3 \times 19$


 * The $20$th semiprime after $4, 6, 9, 10, 14, 15, 21, 22, 25, 26, 33, 34, 35, 38, 39, 46, 49, 51, 55$:
 * $57 = 3 \times 19$


 * The $19$th Ulam number after $1, 2, 3, 4, 6, 8, 11, 13, 16, 18, 26, 28, 36, 38, 47, 48, 53$:
 * $57 = 4 + 53$