31

Number
$31$ (thirty one) is:


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


 * The $3$rd Mersenne number and the $3$rd Mersenne prime after $3$ and $7$:
 * $31 = 2^5 - 1$


 * The $2$nd of $29$ primes of the form $2 x^2 + 29$:
 * $2 \times 1^2 + 29 = 31$