An infinite series is said to converge, diverge or oscillate according as its sequence of partial sums converges, diverges or oscillates. In case converges to s, then s is called the sum of the series and we shall write or
                                   
            In this series  shall be written as
                                
            Thus the infinite series  shall be denoted by e, i.e.
                                
            By virtue of the nature of the terms of a convergent series we are sometimes able to ascertain the type of the value of the sum of the series such as it is a rational, or is an irrational number.

Example:
            The exponential number “e” defined by is an irrational number.
Solution:
            Let e be a rational number and, where m and n are natural numbers, then
                       
                 
                  Natural number
           
a contradiction as no natural number is less than .
            Hence, e is an irrational number.

            The fact that converges to s shall also be expressed as. Similarly we shall write  or according as the series diverges to. If a series is finite, say, consisting of only m terms then. Thus every finite series is a convergent series. As such investigations regarding convergence are required for infinite series only.