Answer:   , where n is a natural number.
 , where n is a natural number.
Step-by-step explanation:
A series can be represented in a summation or sigma notation. 
Greek capital letter, ∑ (sigma), is used to represent the sum. 
For example:  , where n is a natural number.
, where n is a natural number.
The given series : 1,4,9,16 which can be written as  .
 .
So , we can write it as 
 , where n is a natural number.
 , where n is a natural number.