53

Number
$53$ (fifty-three) is:


 * The $16$th prime number, after $2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47$


 * The $17$th Ulam number after $1, 2, 3, 4, 6, 8, 11, 13, 16, 18, 26, 28, 36, 38, 47, 48$:
 * $53 = 6 + 47$