Graduate Understanding the Weierstrass- Bolzano theorem

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The Weierstrass-Bolzano theorem is explored through the example of a bounded infinite sequence, specifically the sequence defined by a_n = 1/n. A subsequence b_n is introduced, converging to 0, raising the question of whether there are additional limit points for the original sequence. The discussion highlights that the subsequence {-1, -1, -1, ...} converges to -1, which is presented as a straightforward conclusion. The importance of applying the definition of a limit is emphasized for clarity. Understanding these concepts is crucial for grasping the implications of the Weierstrass-Bolzano theorem.
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I am currently looking at this theorem...basically it states that if you have a given sequence that is bounded and infinite then there exists atleast one limit point. Looking at the attached...i would like to know how the author concludes that the subsequence;

##{-1,-1,-1,...}## converges to ##-1##.

I guess that should follow from previous step...unless there is a mistake.


Find link here;https://math.libretexts.org/Bookshelves/Analysis/Book%3A_Real_Analysis_(Boman_and_Rogers)/07%3A_Inter
1666813869721.png
 
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Let me pick a sequence, say ##a_n=\dfrac{1}{n}## for example...this is a bounded infinite sequence. I can let my subsequence be defined by;

##b_n= \dfrac{1}{100},\dfrac{1}{200}, \dfrac{1}{300},...## the limit of this sequence will still tend to ##0##.

I am looking at it on the aspect of at least ##1## limit point. Question is, is there another limit point for the given sequence? using the given theorem?
 
chwala said:
i would like to know how the author concludes that the subsequence;

##{-1,-1,-1,...}## converges to ##-1##.

As a standalone claim, this should be exceedingly obvious. If not, apply the definition of a limit without any bells or whistles.
 
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There are probably loads of proofs of this online, but I do not want to cheat. Here is my attempt: Convexity says that $$f(\lambda a + (1-\lambda)b) \leq \lambda f(a) + (1-\lambda) f(b)$$ $$f(b + \lambda(a-b)) \leq f(b) + \lambda (f(a) - f(b))$$ We know from the intermediate value theorem that there exists a ##c \in (b,a)## such that $$\frac{f(a) - f(b)}{a-b} = f'(c).$$ Hence $$f(b + \lambda(a-b)) \leq f(b) + \lambda (a - b) f'(c))$$ $$\frac{f(b + \lambda(a-b)) - f(b)}{\lambda(a-b)}...

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