MHB Is Showing One ε Enough to Prove Discontinuity?

Joe20
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Appreciate the help needed for the attached question. Thanks!
 

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Suppose that $f$ is continuous at some rational point $x_0\in\mathbb{Q}$. Then for $\varepsilon=1/2$ there exists a $\delta>0$ such that for an open interval $U=(x_0-\delta,x_0+\delta)$ we have $f(U)\subseteq(1/2,3/2)=(f(x_0)-1/2,f(x_0)+1/2)$. But this is impossible since every neighborhood of $x_0$, including $U$, contains an irrational point that is mapped to $0$ by $f$. The case of irrational $x_0$ is considered similarly.
 
Evgeny.Makarov said:
Suppose that $f$ is continuous at some rational point $x_0\in\mathbb{Q}$. Then for $\varepsilon=1/2$ there exists a $\delta>0$ such that for an open interval $U=(x_0-\delta,x_0+\delta)$ we have $f(U)\subseteq(1/2,3/2)=(f(x_0)-1/2,f(x_0)+1/2)$. But this is impossible since every neighborhood of $x_0$, including $U$, contains an irrational point that is mapped to $0$ by $f$. The case of irrational $x_0$ is considered similarly.

Hi, may I ask how do you know that f(U)⊆(1/2,3/2) and how do u know ε=1/2? How do I answer to the f(x) = 1 and 0? I am confused.
 
He doesn't "know" that \epsilon= \frac{1}{2}, he gave it that value. To prove that a function is continuous at x= a you must show that \lim_{x\to a}f(x)= f(a). And to show that you must show that "given any \epsilon> 0 there exist \delta> 0 such that …". This must be true for any \epsilon.

But Evgeny Makarov was showing that this is not true. It was sufficient to show there there is some value of \epsilon for which this is not true. He showed it was not true for \epsilon= \frac{1}{2} and that is sufficient to show that the function is not continuous.
 
A sphere as topological manifold can be defined by gluing together the boundary of two disk. Basically one starts assigning each disk the subspace topology from ##\mathbb R^2## and then taking the quotient topology obtained by gluing their boundaries. Starting from the above definition of 2-sphere as topological manifold, shows that it is homeomorphic to the "embedded" sphere understood as subset of ##\mathbb R^3## in the subspace topology.
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