- #1

Math Amateur

Gold Member

MHB

- 3,998

- 48

I am reading Houshang H. Sohrab's book: "Basic Real Analysis" (Second Edition).

I am focused on Chapter 4: Topology of [FONT=MathJax_AMS]R[/FONT] and Continuity ... ...

I need help in order to fully understand the proof of Theorem 4.3.4 ... ... Theorem 4.3.4 and its proof read as follows:

View attachment 9108

In the above proof by Sohrab we read the following:

" ... ... Therefore \(\displaystyle f^{ -1 } (O') = S \cap O\) for some open set \(\displaystyle O\) ... ... "Can someone please explain why the above quoted statement is true ... -----------------------------------------------------------------------------------------------------------------------------------***EDIT *** ... ... My thoughts on this matter so far ...

Since \(\displaystyle f\) is continuous at \(\displaystyle x_0\) we can find \(\displaystyle \delta\) such that

\(\displaystyle f( S \cap B_\delta ( x_0 ) ) \subseteq B_\epsilon ( f(x_0) ) \subseteq O'\) Now ... take inverse image under \(\displaystyle f\) of the above relationship (is this a legitimate move?)then we have ... \(\displaystyle S \cap B_\delta ( x_0 ) \subseteq f^{ -1 } ( B_\epsilon ( f(x_0) ) ) \subseteq f^{ -1 } ( O' )\)So that ... if we put the open set \(\displaystyle B_\delta ( x_0 )\) equal to \(\displaystyle O''\) then we get\(\displaystyle f^{ -1 } ( O' ) \supseteq S \cap O''\) ...But now ... how do we find \(\displaystyle O\) such that \(\displaystyle f^{ -1 } ( O' ) = S \cap O\) ...

---------------------------------------------------------------------------------------------------------------------------

Help will be appreciated ...

Peter

I am focused on Chapter 4: Topology of [FONT=MathJax_AMS]R[/FONT] and Continuity ... ...

I need help in order to fully understand the proof of Theorem 4.3.4 ... ... Theorem 4.3.4 and its proof read as follows:

View attachment 9108

In the above proof by Sohrab we read the following:

" ... ... Therefore \(\displaystyle f^{ -1 } (O') = S \cap O\) for some open set \(\displaystyle O\) ... ... "Can someone please explain why the above quoted statement is true ... -----------------------------------------------------------------------------------------------------------------------------------***EDIT *** ... ... My thoughts on this matter so far ...

Since \(\displaystyle f\) is continuous at \(\displaystyle x_0\) we can find \(\displaystyle \delta\) such that

\(\displaystyle f( S \cap B_\delta ( x_0 ) ) \subseteq B_\epsilon ( f(x_0) ) \subseteq O'\) Now ... take inverse image under \(\displaystyle f\) of the above relationship (is this a legitimate move?)then we have ... \(\displaystyle S \cap B_\delta ( x_0 ) \subseteq f^{ -1 } ( B_\epsilon ( f(x_0) ) ) \subseteq f^{ -1 } ( O' )\)So that ... if we put the open set \(\displaystyle B_\delta ( x_0 )\) equal to \(\displaystyle O''\) then we get\(\displaystyle f^{ -1 } ( O' ) \supseteq S \cap O''\) ...But now ... how do we find \(\displaystyle O\) such that \(\displaystyle f^{ -1 } ( O' ) = S \cap O\) ...

---------------------------------------------------------------------------------------------------------------------------

Help will be appreciated ...

Peter

#### Attachments

Last edited: