43

Number
$43$ (forty-three) is:


 * The $14$th prime number, after $2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41$


 * The $2$nd of the $6$th pair of twin primes, with $51$


 * The $12$th lucky number:
 * $1, 3, 7, 9, 13, 15, 21, 25, 31, 33, 37, 43, \ldots$


 * The index of the $1$st non-integer term of Göbel's sequence.


 * The $5$th of $11$ primes of the form $2 x^2 + 11$:
 * $2 \times 4^2 + 11 = 43$


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


 * The $26$th positive integer after $2, 3, 4, 7, 8, \ldots, 26, 29, 30, 31, 32, 33, 37, 38, 42$ which cannot be expressed as the sum of distinct pentagonal numbers.

Also see

 * Non-Integral Value of Göbel's Sequence