53

Number
$53$ (fifty-three) is:


 * The $16$th prime number, after $2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47$


 * The $8$th Sophie Germain prime after $2, 3, 5, 11, 23, 29, 41$:
 * $2 \times 53 + 1 = 107$, which is prime.


 * The $17$th Ulam number after $1, 2, 3, 4, 6, 8, 11, 13, 16, 18, 26, 28, 36, 38, 47, 48$:
 * $53 = 6 + 47$


 * The $5$th term of the $1$st $5$-tuple of consecutive integers have the property that they are not values of the $\sigma$ function $\sigma \left({n}\right)$ for any $n$:
 * $\left({49, 50, 51, 52, 53}\right)$


 * The $1$st prime number which cannot be expressed as the difference between a power of $2$ and a power of $3$.


 * The $1$st prime number $p$ the period of whose reciprocal is $\dfrac {p - 1} 4$:
 * $\dfrac 1 {53} = 0 \cdotp \dot 01886 \, 79245 \, 28 \dot 3$


 * The $2$nd prime number after $5$ which is the arithmetic mean of the prime numbers on either side of it:
 * $53 = \dfrac {47 + 59} 2$


 * The probability that out of $53$ people, no $2$ of them share the same birthday is approximately $\dfrac 1 {53}$.

Also see

 * Period of Reciprocal of 53 is of Quarter Maximal Length
 * Probability of no 2 People out of 53 Sharing the Same Birthday