19

Number
$19$ (nineteen) is:


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


 * The $3$rd centered hexagonal number after $1$ and $7$:
 * $19 = 1 + 6 + 12 = 27^3 - 8^3$


 * The $2$nd Keith number:
 * $1, 9, 10, 19, \ldots$


 * The $3$rd prime number whose period is of maximum length:
 * $\dfrac 1 {19} = 0 \cdotp \dot 05263 \, 15789 \, 47368 \, 42 \dot 1$


 * The $3$rd of $11$ primes of the form $2 x^2 + 11$:
 * $2 \times 2^2 + 11 = 19$

Also see

 * Period of Reciprocal of 19 is of Maximal Length