Is the XOR of PRGs sharing the same input a PRG?
Thank you everybody's help. I have known that if $f$ and $f_1$ have some relation, then $g$ can not be PRG.
e.g. $f = f_1$, then we can know, after $f ⊕ f_1$, it always return 0 for $g(x)$, in this case, $g(X)$ will never be a PRG.