The next number game!

This is a memory game where Somebody posts a series of numbers and the next member posts the number that it should be based on the sequence of numbers e.g. 1 2 4 8 ??
of course that number is 16 there are many other variances such as 5 5 4 4 5 6 5 ?
the number would be 4

I'll start off easy

1 2 1 2 3 2 1 2 3 ?

the answer is 4

okay here's mine

1 3 9 ??