Continuity of f(x) at Rational and Irrational Points

[bertram]

Homework Statement

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

Homework Equations

continuity: for every \epsilon &gt; 0 there exists \delta &gt; 0 d(f(x),f(p)) &lt; \epsilon for all points x\inE for which d(x,p) < \delta

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?

 

[quasar987]

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]

You can say that f(x) is continuous at x₀ if and only if lim f(x_n)= f(x₀) for every sequence {x_n} converging to x₀. But you can't restrict that to rational numbers only or irrational numbers only.

Notice that the f(x_n) will be close to f(x₀) for both rational and irrational x_n if and only if x₀²= -x₀². For what x₀ is that true?