# Math Help - Continuous Function proof

1. ## Continuous Function proof

Prove that the function f defined by f(x)=x if x is rational and f(x)=-x if x is irrational is continuous at 0 only.

2. Originally Posted by redsoxnation
Prove that the function f defined by f(x)=x if x is rational and f(x)=-x if x is irrational is continuous at 0 only.
Because this is not a homework service, you need to show us some effort on your part.
We will happily give you help on what you have already done.
But do not expect us to do your work for you.

3. Originally Posted by redsoxnation
Prove that the function f defined by f(x)=x if x is rational and f(x)=-x if x is irrational is continuous at 0 only.
Hint: If f is indeed continuous, then f^{-1}( (0,1) ) is a non-empty open subset of R. It also contains 1/2, for instance. Since 1/2 lies in an open subset of R, look at an irrational number in a neighborhood of 1/2 and show that it doesn't map into (0,1) as originally assumed.

4. Originally Posted by redsoxnation
Prove that the function f defined by f(x)=x if x is rational and f(x)=-x if x is irrational is continuous at 0 only.
I DO NOT want to do your HW for you...Plato is right, show us some work. But, a hint that may be easier than patrick is to consider that $f$ is continuous implies that $x_n\to x\implies f(x_n)\to f(x)$