The process will multiply any number by 2, so the general formula would be a0 * 2^n. If a_{0} is 1/2 it gives the sequence 1, 2, 4, .... If a_{0} = 1 then it gives the sequence 2, 4, 8, .... But a_{0} need not be a power of 2: if a_{0} = 3 you get 6, 12, 24, ....