2

Number
$2$ (two) is:


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


 * The $1$st prime number.


 * The $1$st (trivial, $1$-digit) palindromic prime.


 * The only even prime number.


 * The $1$st Sophie Germain prime:
 * $2 \times 2 + 1 = 5$, which is prime.


 * The index of the $1$st repunit prime:
 * $R_2 = 11$


 * The $2$nd highly composite number after $1$:
 * $\tau \left({2}\right) = 2$


 * 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 $1$st Fibonacci prime.


 * The $1$st permutable prime.


 * 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 term of Göbel's sequence after the $0$th term $1$:
 * $2 = \left({1 + 1^2}\right) / 1$


 * The $1$st term of the $3$-Göbel sequence after the $0$th term $1$:
 * $2 = \left({1 + 1^3}\right) / 1$


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


 * The $2$nd after $1$ of the $24$ positive integers which cannot be expressed as the sum of distinct non-pythagorean primes.


 * The $1$st positive integer which divides the sum of all smaller primes:
 * $0 = 0 \times 2$
 * (there are no primes smaller than $2$)


 * The $3$rd integer $n$ after $0, 1$ such that $2^n$ contains no zero in its decimal representation:
 * $2^2 = 4$


 * The $1$st even integer that cannot be expressed as the sum of $2$ prime numbers which are each one of a pair of twin primes.


 * The $3$rd integer after $0, 1$ which is palindromic in both decimal and ternary:
 * $2_{10} = 2_3$


 * The $2$nd of the $5$ known powers of $2$ whose digits are also all powers of $2$:
 * $1, 2, \ldots$


 * The $1$st positive integer which is not the sum of $1$ or more distinct squares.


 * The $3$rd number after $0, 1$ which is (trivially) the sum of the increasing powers of its digits taken in order:
 * $2^1 = 2$


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