Saturday, 13 July 2019

real analysis - Is this proof ok?

My friend and I were trying to prove that a function can't be discontinuous on the irrationals. His proof was this:





Let f:RR be continuous on all of Q. Then for any {xn}Q with xnrQ, we have lim. Now choose \{x_n\} such that \{x_n\} is Cauchy. Then for any \epsilon>0 there exists an N_0\in\mathbb{N} such that for all n,\ m\ge N_0, |x_m-x_n|<\epsilon.



Now in the interval (x_n,\ x_m) there's an irrational \gamma between them. Therefore, \gamma also satisfies the definition of continuity, and so f(x) is continuous for at least one \gamma\in\mathbb{Q}^c and so it can't be continuous on only the rationals.




Something about this doesn't sit well with me, it feels so off, but I can't tell why. If it were this simple all along, what's with all the F_\sigma and G_\delta sets and Baire Spaces and Theorems and all this other complicated stuff?

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