11

Definition
$11$ (Eleven) is:


 * The $5$th prime number


 * The second repuint after the trivial case $1$


 * The only palindromic prime with an even number of digits


 * The smallest number which is the sum of a square and a prime in $3$ different ways:
 * $11 = 0 + 11 = 4 + 7 = 9 + 2$


 * The $7$th Ulam number after $1, 2, 3, 4, 6, 8$:
 * $11 = 3 + 8$

Also see

 * Divisibility by 11


 * 11 is Only Palindromic Prime with Even Number of Digits