Intermediate value theorem

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Discussion Overview

The discussion revolves around the Intermediate Value Theorem (IVT), specifically addressing the conditions under which a number ##c## exists between two points ##a## and ##b## on a continuous function ##f##. Participants explore the implications of restricting ##c## to the open interval ##(a,b)## versus the closed interval ##[a,b]##.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants question why the author of the theorem restricts ##c## to ##(a,b)## instead of ##[a,b]##, suggesting that if ##M## includes the values of ##f(a)## and ##f(b)##, then ##c## should also be allowed to equal those endpoints.
  • Others argue that restricting ##c## to ##(a,b)## provides a stronger conclusion, as it implies that ##c## is strictly between the two endpoints, which conveys more information about the behavior of the function.
  • One participant notes that using "between" to mean "strictly between" is a stronger statement than including the endpoints.
  • There is a correction regarding the initial claim that ##c ∈ [a,b] ⇒ c ∈ (a,b)##, with participants acknowledging the mistake and clarifying the relationship between the intervals.

Areas of Agreement / Disagreement

Participants express differing views on the implications of the interval restrictions for ##c##. While some agree that the restriction to ##(a,b)## is stronger, the discussion does not reach a consensus on the necessity or implications of this distinction.

Contextual Notes

There are unresolved questions regarding the definitions of "between" and the implications of including endpoints in the context of the theorem, as well as the assumptions about the values of ##M##.

PFuser1232
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Suppose that ##f## is continuous on ##[a,b]## and let ##M## be any number between ##f(a)## and ##f(b)##.
Then, there exists a number ##c## (at least one) such that:
##a < c < b## and ##f(c) = M##

Why did the author restrict ##c## to ##(a,b)## rather than ##[a,b]##? After all, ##c ∈ [a,b] ⇒ c ∈ (a,b)##.
 
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MohammedRady97 said:
After all, ##c ∈ [a,b] ⇒ c ∈ (a,b)##.

False.
 
micromass said:
False.
Oops. I just realized it's the other way around.
The question remains; why did the author restrict ##c## to ##(a,b)## rather than ##[a,b]##?
 
Because it gives more information.
 
MohammedRady97 said:
Suppose that ##f## is continuous on ##[a,b]## and let ##M## be any number between ##f(a)## and ##f(b)##.
Then, there exists a number ##c## (at least one) such that:
##a < c < b## and ##f(c) = M##

Why did the author restrict ##c## to ##(a,b)## rather than ##[a,b]##? After all, ##c ∈ [a,b] ⇒ c ∈ (a,b)##.
You have to be careful here. If "M any number between f(a) and f(b)" includes f(a) and f(b), then you must include the end points a & b in the conclusion ( c is in [a,b] ). Otherwise, if M is properly between f(a) and f(b) but not equal to either, then you can conclude that c is in (a,b). And that is a stronger conclusion that c in [a,b].
 
FactChecker said:
You have to be careful here. If "M any number between f(a) and f(b)" includes f(a) and f(b), then you must include the end points a & b in the conclusion ( c is in [a,b] ). Otherwise, if M is properly between f(a) and f(b) but not equal to either, then you can conclude that c is in (a,b). And that is a stronger conclusion that c in [a,b].

Why is it a stronger conclusion? Is it because ##c ∈ [a,b]## implies more obvious conclusions like ##c = a ⇒ f(c) = M = f(a)##?
 
It is a stronger conclusion because it restricts the region in which "c" lies- it gives more information. Using "between" to mean "strictly between" is a stronger statement.
 

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