Which of the following is an arithmetic sequence?
-2, 4, -6, 8,
-8, -6, -4, -2,
2, 4, 8, 16,

Only the second sequence is arithmetic. 

Work:
An arithmetic sequence is a sequence in which there is a common value added or subtracted between each term in the sequence. 
The first option, -2, 4, -6, 8,... is not arithmetic because the terms switch back and forth between positive and negative signs meaning that there is no common value that can be added between each term. 
The second option, -8, -6, -4, -2,... is arithmetic because there is a common difference between each term which is +2.
The third option, 2, 4, 8, 16,... is not arithmetic but is, in fact, geometric. There is no common value that is added or subtracted between each term. 

Where are the functions?