Define f : R → R by f(x) = 1 if x ∈ Q and f(x) = 0 if x /∈ Q. Prove that f is not continuous at any x0 ∈ R. (This function f is called the Dirichlet function.)
Added by Aaron M.
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Since \(\mathbb{Q}\) is dense in \(\mathbb{R}\), this is possible. Then \(x_n\) converges to \(x_0\), but \(f(x_0) = 0\). ** Show more…
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