13

Number
$13$ (thirteen) is:


 * The $6$th prime number, after $2$, $3$, $5$, $7$, $11$


 * The $2$nd of the $3$rd pair of twin primes, with $11$


 * The second Pythagorean prime after $5$, and so by Fermat's Two Squares Theorem the sum of two squares uniquely:
 * $13 = 4 \times 3 + 1 = 4 + 9 = 2^2 + 3^2$


 * The $5$th lucky number:
 * $1$, $3$, $7$, $9$, $13$, $\ldots$


 * The $7$th Fibonacci number, after $1$, $1$, $2$, $3$, $5$, $8$:
 * $13 = 5 + 8$


 * The $4$th Fibonacci prime after $2$, $3$, $5$.


 * The $8$th Ulam number after $1$, $2$, $3$, $4$, $6$, $8$, $11$:
 * $13 = 2 + 11$


 * The $4$th happy number after $1$, $7$, $10$:
 * $13 \to 1^2 + 3^2 = 1 + 9 = 10 \to 1^2 + 0^2 = 1$


 * The $2$nd prime number $p$ after $3$ the period of whose reciprocal is $\dfrac {p - 1} 2$:
 * $\dfrac 1 {13} = 0 \cdot 076923 \, 076923 \ldots$


 * The smallest emirp:
 * $13$, $17$, $31$, $37$, $71$, $73$, $79$, $97$, $107$, $113$, $\ldots$


 * The $6$th permutable prime after $2$, $3$, $5$, $7$, $11$.


 * The $2$nd Wilson prime after $5$:
 * $13^2 \mathrel \backslash \left({13 - 1}\right)! + 1 = 479 \, 001 \, 601$


 * The hypotenuse of the $5-12-13$ Pythagorean triangle.


 * The square of the reverse of $13$ equals the reverse of the square of $13$:
 * $13^2 = 169$
 * $31^2 = 961$


 * The $4$th of the sequence of $n$ such that $p_n \# - 1$, where $p_n \#$ denotes primorial of $n$, is prime, after $3$, $5$, $11$:
 * $p_{13} \# - 1 = 30 \, 029$


 * The $2$nd of $11$ primes of the form $2 x^2 + 11$:
 * $2 \times 1^2 + 13 = 13$


 * The $3$rd of $5$ primes of the form $2 x^2 + 5$:
 * $2 \times 2^2 + 5 = 13$


 * The $2$nd integer after $7$ the decimal representation of whose square can be split into two parts which are each themselves square:
 * $13^2 = 169; 16 = 4^2, 9 = 3^2$


 * The $9$th after $1$, $2$, $4$, $5$, $6$, $8$, $9$, $12$ of the $24$ positive integers which cannot be expressed as the sum of distinct non-pythagorean primes.


 * The $11$th integer $n$ after $0$, $1$, $2$, $3$, $4$, $5$, $6$, $7$, $8$, $9$ such that $2^n$ contains no zero in its decimal representation:
 * $2^{13} = 8192$


 * The $2$nd after $4$ in the sequence formed by adding the squares of the first $n$ primes:
 * $13 = \displaystyle \sum_{i \mathop = 1}^2 {p_i}^2 = 2^2 + 3^2$


 * With $x = 239$, the only $y$ which is the solution of the indeterminate Diophantine equation $x^2 + 1 = 2 y^4$:
 * $239^2 + 1 = 2 \times 13^4$

Also see

 * Recurring Parts of Multiples of One Thirteenth
 * Twelve Factorial plus One is divisible by 13 Squared
 * Square of Reversal of Small-Digit Number
 * Solution of Diophantine Equation $x^2 + 1 = 2 y^4$