What is the nth term for the series

2 1 1/2 1/4 1/8

What is the progression of the numbers? Each one is ? of the preceding number.

Here the next term is half of previous term.
So the nth term will be (n-1)/2.

Wrong. For n = 1, 2, 3, 4, 5 ... this produces the series 0, 1/2, 1, 3/2, 2, ....