is also continuous at too.
I'm trying to find . I think the function f is not continuous except at points 0 and 1. So if I can show that, that means f is not differentiable except (possibly) at 0 and 1. Am I going in the right direction?
If so, then f'(0) would be 0, but I'm not sure about f'(1)...
You have a bit of a problem here. At the starred step you made a fatal error. If you would have put you could have done that, but considering that your limit travels along a path containing both irrationals and rationals this is not permissable. Claim the limit is zero though and make and argument then that considering the limit along the path of the irrationals is enough.