# Continuity of x^2 and -x^2

1. Homework Statement
f(x) = {x^2 $$x \in Q$$
-x^2 $$x \in R/Q$$
At what points is f continuous?

2. Homework Equations

continuity: for every $$\epsilon > 0$$ there exists $$\delta > 0 d(f(x),f(p)) < \epsilon$$ for all points $$x\inE$$ for which d(x,p) < $$\delta$$

3. The Attempt at a Solution
Alright my initial thought was that it would not be continuous at any point in Q, because for any two rationals there is an irrational between them (this is correct?), but then it would be continuous at all irrationals from a theorem (4.6 in Rudin) for $$p\in Q$$, lim(x-> p) f(x) = -p^2 $$\neq$$ p^2 = f(p)
However, then this function is continuous at irrationals. For $$p\in R/Q$$, lim(x-> p) f(x) = -p^2 = f(p)

is this reasoning sound ok?

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quasar987
Homework Helper
Gold Member
I didn't bother understanding what you tried (it's not very clear)... but you seem to be playing with the right idea, using the density of Q and R\Q to show failure of continuity. You should be able to use this reasoning to show that f is discontinuous everywhere but at one point.

HallsofIvy