47

Number
$47$ (forty-seven) is:


 * The $15$th prime number, after $2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43$


 * The $8$th Lucas number after $(2), 1, 3, 4, 7, 11, 18, 29$:
 * $47 = 18 + 29$


 * The $4$th Keith number after $14, 19, 28$:
 * $4, 7, 11, 18, 29, 47, \ldots$


 * The $4$th of $29$ primes of the form $2 x^2 + 29$:
 * $2 \times 3^2 + 29 = 47$


 * The $15$th Ulam number after $1, 2, 3, 4, 6, 8, 11, 13, 16, 18, 26, 28, 36, 38$:
 * $47 = 11 + 36$


 * The $1$st of the largest known pair of Ulam numbers which differ by $1$:
 * $47 = 11 + 36, \ 48 = 1 + 47$


 * The $4$th of the $1$st ordered quadruple of consecutive integers that have sigma values which are strictly decreasing:
 * $\sigma \left({44}\right) = 84$, $\sigma \left({45}\right) = 78$, $\sigma \left({46}\right) = 72$, $\sigma \left({47}\right) = 48$


 * $47 + 2 = 49; \ 47 \times 2 = 94$


 * The $21$st positive integer which is not the sum of $1$ or more distinct squares:
 * $2, 3, 6, 7, 8, 11, 12, 15, 18, 19, 22, 23, 24, 27, 28, 31, 32, 33, 43, 44, 47, \ldots$

Also see

 * Numbers which Multiplied by 2 are the Reverse of when Added to 2