2

Number
$2$ (two) is:


 * The first (strictly) positive even number.


 * The first prime number.


 * The only even number prime number.


 * The first deficient number.


 * The second factorial after $1$:
 * $2 = 1 \times 2$


 * The $3$rd Fibonacci number:
 * $1, 1, 2, \ldots$


 * The $2$nd Ulam number after $1$.


 * The first untouchable number.


 * The $3$rd subfactorial after $0, 1, \ldots$
 * $2 = 3! \left({1 - \dfrac 1 {1!} + \dfrac 1 {2!} - \dfrac 1 {3!} }\right)$


 * The first primorial.