Number chain game

Here are the rules
- Post a chain of 5 number in an order like 1, 3, 5, 7, 9
- Others people must guess the formula of the chain (in this case is odd number)
- No weird formula like 1, 9, 5, 4, 10 (not even valid)
- The one who guess it right can post a new chain
Anddddd very sorry for my bad english, let's play anyway
2, 5, 10, 17, 26
Not gonna be that hard heh

I don't get it lol, how does this work

1-7-9 idk............