Chapter 12 - Section 12.1 - Sequences and Summation Notation - 12.1 Exercises - Page 851: 37

$(-1)^n+1$

This is an oscillating sequence meaning it cycles in value. So to express we take advantage of the fact odd powers of negative numbers are negative but even powers aren't. So we start with the simple $(-1)^n$. Then we simply add 1 to get from -1,1,-1,1... to 0,2,0,2,0,2. The answer is then $(-1)^n+1$