Math Amateur
Gold Member
MHB
- 3,920
- 48
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 the proof of Theorem 3.40 on page 104 ... ... Theorem 3.40 and its proof read as follows:
View attachment 9141
In the above proof by Stromberg we read the following:
" ... ... To see that $$S$$ is bounded, consider $$\mathscr{U} = \{ B_k (0) \ : \ k \in \mathbb{N} \}$$. Plainly $$\mathscr{U}$$ is an open cover of $$\mathbb{R}^n$$ and, in particular of $$S$$. Thus there is a $$k_0 \in \mathbb{N}$$ such that $$S \subset B_{ k_0 } (0)$$; whence $$\text{diam}S \leq 2 k_0 \lt \infty$$. ... ... "
I am troubled by the apparent (to me) lack of a rigorous argument to demonstrate that $$S$$ is bounded ...It appears to me that $$\mathbb{R}^n$$ is "infinitely large", that is unbounded ... and hence $$S$$ can be "infinitely large" and hence unbounded ... for example if $$S$$ is the $$x_1$$ axis of $$\mathbb{R}^n$$ then $$S \subset \mathbb{R}^n$$ and $$S$$ is unbounded ...
Can someone please demonstrate rigorously that $$S$$ is bounded ...
*** EDIT ***Just a thought after reflection ... even though open covers of \mathbb{R}^n and S may be "infinite" (in dimension) because S is compact it must have a finite subcover ... and hence S is bounded ... is that the correct argument?***************************************************************************************************Help will be appreciated ...
Peter
I am focused on Chapter 3: Limits and Continuity ... ...
I need help in order to fully understand the proof of Theorem 3.40 on page 104 ... ... Theorem 3.40 and its proof read as follows:
View attachment 9141
In the above proof by Stromberg we read the following:
" ... ... To see that $$S$$ is bounded, consider $$\mathscr{U} = \{ B_k (0) \ : \ k \in \mathbb{N} \}$$. Plainly $$\mathscr{U}$$ is an open cover of $$\mathbb{R}^n$$ and, in particular of $$S$$. Thus there is a $$k_0 \in \mathbb{N}$$ such that $$S \subset B_{ k_0 } (0)$$; whence $$\text{diam}S \leq 2 k_0 \lt \infty$$. ... ... "
I am troubled by the apparent (to me) lack of a rigorous argument to demonstrate that $$S$$ is bounded ...It appears to me that $$\mathbb{R}^n$$ is "infinitely large", that is unbounded ... and hence $$S$$ can be "infinitely large" and hence unbounded ... for example if $$S$$ is the $$x_1$$ axis of $$\mathbb{R}^n$$ then $$S \subset \mathbb{R}^n$$ and $$S$$ is unbounded ...
Can someone please demonstrate rigorously that $$S$$ is bounded ...
*** EDIT ***Just a thought after reflection ... even though open covers of \mathbb{R}^n and S may be "infinite" (in dimension) because S is compact it must have a finite subcover ... and hence S is bounded ... is that the correct argument?***************************************************************************************************Help will be appreciated ...
Peter
Attachments
Last edited: