Answer
This is not an arithmetic sequence.
Work Step by Step
Let's look at the terms to see if we can find a pattern:
$a _2 - a_1 = 3 - 1 = 2$
$a_3 - a_2 = 9 - 3 = 6$
$a_4 - a_3 = 27 - 9 = 18$
This is not an arithmetic sequence because the same number is not added to the previous term to get the next term, which means there is no common difference.
