2

Number
$2$ (two) is:


 * The $1$st (strictly) positive even number.


 * The $1$st prime number.


 * The only even prime number.


 * The $1$st deficient number.


 * The $2$nd superabundant number after $1$:
 * $\dfrac {\sigma \left({2}\right)} 2 = \dfrac 3 2 = 1 \cdotp 5$


 * The $2$nd almost perfect number after $1$:
 * $\sigma \left({2}\right) = 3 = 4 - 1$


 * The $2$nd factorial after $1$:
 * $2 = 2! = 2 \times 1$


 * The $3$rd Fibonacci number after $1, 1$:
 * $2 = 1 + 1$


 * The $2$nd Catalan number after $(1,) \, 1$:
 * $\dfrac 1 {2 + 1} \dbinom {2 \times 2} 2 = \dfrac 1 3 \times 6 = 2$


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


 * The $1$st untouchable number.


 * The $1$st primorial.


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


 * The $1$st of the sequence of $n$ such that $p_n \# + 1$, where $p_n \#$ denotes primorial of $n$, is prime:
 * $p_2 \# + 1 = 2 + 1 = 3$


 * The $3$rd integer $n$ after $0, 1$ such that both $2^n$ and $5^n$ have no zeroes:
 * $2^2 = 4, 5^2 = 25$


 * The $2$nd (strictly) positive integer after $1$ which cannot be expressed as the sum of exactly $5$ non-zero squares.


 * The $1$st of the lucky numbers of Euler:
 * $n^2 + n + 2$ is prime for $n = 0$.


 * The $1$st even number which cannot be expressed as the sum of $2$ composite odd numbers.


 * The $1$st of $4$ numbers whose letters, when spelt in French, are in alphabetical order:
 * deux