I need help with this problem...

prove that the limit

$\displaystyle lim x-->0 f(x)$

such that $\displaystyle f(x) = 0$ when $\displaystyle x$ is rational; $\displaystyle 1$ when $\displaystyle x$ is irrational

does not exist. Thanks to anyone who helps!

yes, I think I should learn LaTex...