This is 1/2^n
f(0) = 1/2^0 = 1
f(1) = 1/2^1 = 1/2
f(2) = 1/2^2 = 1/4
f(3) = 1/2^3 = 1/8
f(4) = 1/2^4 = 1/16
And so on. Note that the change is happening in the denominator, so I found the formula by identifying what changed with each iteration of the sequence.