31

Number
$31$ (thirty-one) is:


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


 * The $2$nd of the $5$th pair of twin primes, with $29$


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


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


 * The $3$rd emirp after $13$, $17$


 * The $8$th permutable prime after $2$, $3$, $5$, $7$, $11$, $13$, $17$


 * The $8$th happy number after $1$, $7$, $10$, $13$, $19$, $23$, $28$:
 * $31 \to 3^2 + 1^2 = 9 + 1 = 10 \to 1^2 + 0^2 = 1$


 * The $6$th of the sequence of $n$ such that $p_n \# + 1$, where $p_n \#$ denotes primorial of $n$, is prime:
 * $2, 3, 5, 7, 11, 31$


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


 * The $21$st integer $n$ such that $2^n$ contains no zero in its decimal representation:
 * $2^{31} = 2 \, 147 \, 483 \, 648$


 * The $16$th positive integer which is not the sum of $1$ or more distinct squares:
 * $2$, $3$, $6$, $7$, $8$, $11$, $12$, $15$, $18$, $19$, $22$, $23$, $24$, $27$, $28$, $31$, $\ldots$


 * The $20$th positive integer after $2$, $3$, $4$, $7$, $8$, $9$, $10$, $11$, $14$, $15$, $16$, $19$, $20$, $21$, $24$, $25$, $26$, $29$, $30$ which cannot be expressed as the sum of distinct pentagonal numbers.


 * Expressible as the sum of successive powers starting from $1$ in in $2$ different ways:
 * $31 = 1 + 5 + 5^2 = 1 + 2 + 2^2 + 2^3 + 2^4$

Also see

 * Square of Reversal of Small-Digit Number