For the recurrence an=an−1+an−2a_n = a_{n-1} + a_{n-2}an=an−1+an−2, if a0=0a_0 = 0a0=0 and a1=1a_1 = 1a1=1, what is the generating function A(x)=∑n=0∞anxnA(x) = \sum_{n=0}^{\infty} a_n x^nA(x)=∑n=0∞anxn?
x1−x−x2\frac{x}{1-x-x^2}1−x−x2x
11−x−x2\frac{1}{1-x-x^2}1−x−x21
x1+x−x2\frac{x}{1+x-x^2}1+x−x2x
x1−x+x2\frac{x}{1-x+x^2}1−x+x2x