Real Number System

A real number is any number that can be mapped on the number line.

Natural or Counting Numbers  $\left(N\right)$ 
                $N=\left\{1,\ 2,\ 3,\ 4,\ ...\right\}$  

Rational Numbers  $\left(Q\right)$ 
           $Q=\left\{\frac{a}{b}:a\ and\ b\ are\ integers,\ with\ b\ne0\right\}$ 
           Examples:  $\frac{2}{3},\ -10,\ 1.25,\ \frac{11}{3},\ -9.\overline{125}$ 

Irrational Numbers  $\left(Q'\right)$ 

          Irrational numbers are numbers that cannot be expressed as a quotient (ratio) of two integers.

An even number is a number that can be written in the form  $2k$ , where  $k$  is an integer.

An odd number is a number that can be written in the form  $2k+1$ , where  $k$  is an integer.

A prime number is a natural number greater than 1, which has no other natural number divisors other than 1 and itself.

A composite number is a natural number that has at least 3 natural number divisors.