3

Number
$3$ (three) is:


 * The second prime number after $2$


 * The first odd prime


 * The second triangular number after $1$:
 * $3 = 1 + 2 = \dfrac {2 \left({2 + 1}\right)} 2$


 * The first prime number of the form $4 n + 3$:
 * $3 = 4 \times 0 + 3$


 * The first Mersenne number and Mersenne prime:
 * $3 = 2^2 - 1$


 * The first Fermat number and Fermat prime:
 * $3 = 2^{\left({2^0}\right)} + 1$


 * The $4$th Fibonacci number:
 * $1, 1, 2, 3, \ldots$


 * The first prime number which is the first of a pair of twin primes:
 * $3, 5$


 * The first of an arithmetic progression of primes:
 * $3, 5, 7$


 * The sum of the first two factorials:
 * $3 = 1! + 2!$


 * The first of $3$ primes of the form $2 x^2 + 3$:
 * $2 \times 0^2 + 3 = 3$


 * The $\sigma$ value of $3$ is square:

Also see

 * Trisecting the Angle
 * Three Points Describe a Circle
 * Three Regular Tessellations
 * Integer is Sum of Three Triangular Numbers
 * Integer as Sum of Three Squares
 * Divisibility by 3
 * Smallest Magic Square is of Order 3
 * Numbers whose Sigma is Square