Verified Answer :

Well,

This could be an arithmetic sequence.
1 + 2 = 3
3 + 3 = 6
6 + 4 = 10
10 + 5 = 15
15 + 6 = 21
...

This could also be a geometric sequence.
13=3
3
2=6
61=6
6
0=0
0-1=0
0
(anything)=0
...