MHB Compact Topological Spaces .... Stromberg, Example 3.34 (c) .... ....

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I am reading Karl R. Stromberg's book: "An Introduction to Classical Real Analysis". ... ...

I am focused on Chapter 3: Limits and Continuity ... ...

I need help in order to fully understand an aspect of Example 3.34 (c) on page 102 ... ... Examples 3.34 (plus some relevant definitions ...) reads as follows:

View attachment 9122

In Example 3.34 (c) above from Stromberg we read the following:

" ... ... Let $$\mathscr{I}$$ be the collection of all open intervals $$I$$ such that $$I \subset U$$ for some $$U$$ in $$\mathscr{U}$$. Check that $$\mathscr{I}$$ is a cover of $$[a,b]$$ ... ... "

My question is as follows:

How would we go about (rigorously) checking that $$\mathscr{I}$$ is a cover of $$[a,b]$$ ... ... indeed how would we rigorously demonstrate that $$\mathscr{I}$$ is a cover of $$[a,b]$$ ... ... ?
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***EDIT***

My thoughts ... after reflecting ...$$\mathscr{U}$$ is an open cover (family of open subsets) of $$[a, b]$$ ... ...

Each set $$U \subset \mathscr{U} $$ is a countable set of pairwise disjoint open intervals ... ... (Theorem 3.18)

Therefore if $$\mathscr{I}$$ equals the collection of all open intervals $$I$$ such that $$I \subset U$$ ...

... then $$ \mathscr{I} $$ is a family of open intervals such that $$[a, b] \subset \bigcup \mathscr{I}$$ ...

Now apply Heine-Borel Theorem ...Is that correct?---------------------------------------------------------------------------------------------------------------------------------

Hope someone can help ...

Peter
========================================================================The above text from Stromberg mentions the Heine_Borel Theorem ... so I am proving the text of the (statement of ...) the theorem ... ... as follows:
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"My thoughts ... ... " above include a reference to Theorem 3.18 ... the text of the statement of Theorem 3.18 is as follows:
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Hope that helps ...

Peter
 

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  • Stromberg -  Statement of Heine-Borel Theorem 1.66 ... .png
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  • Stromberg - 1 - Statement of Theorem 3.18 ... PART 1 ... .png
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  • Stromberg - 2 - Statement of Theorem 3.18 ... PART 2  ... .png
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Last edited:
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Let $x\in[a,\,b]$. Then $x\in U$ for some $U\in\mathscr U$. As $U$ is open, there is an open interval $I_x$ such that $x\in I_x\subseteq U$. So $I_x\in\mathscr I$ and $[a,\,b]\subseteq\bigcup_xI_x$, i.e. $\mathscr I$ is a cover for $[a,\,b]$.
 
Olinguito said:
Let $x\in[a,\,b]$. Then $x\in U$ for some $U\in\mathscr U$. As $U$ is open, there is an open interval $I_x$ such that $x\in I_x\subseteq U$. So $I_x\in\mathscr I$ and $[a,\,b]\subseteq\bigcup_xI_x$, i.e. $\mathscr I$ is a cover for $[a,\,b]$.


Thanks for the help Olinguito

Peter
 
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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