Which of the following functions has a power series representation ∑n=0∞x2n\sum_{n=0}^{\infty} x^{2n}∑n=0∞x2n?
11−x\frac{1}{1-x}1−x1
11+x2\frac{1}{1+x^2}1+x21
11−x2\frac{1}{1-x^2}1−x21
(1−x2)−1(1-x^2)^{-1}(1−x2)−1