What is the generating function of the sequence an=2na_n = 2^nan=2n?
11−2x\frac{1}{1-2x}1−2x1
11+2x\frac{1}{1+2x}1+2x1
x1−2x\frac{x}{1-2x}1−2xx
21−x\frac{2}{1-x}1−x2