Operations on Limit: Does 0 imply Non-Existence?

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Discussion Overview

The discussion revolves around the implications of limits in the context of real analysis, specifically focusing on the behavior of the quotient of two functions as one approaches a point where the denominator approaches zero. Participants explore whether a limit of zero for the denominator necessarily implies that the limit of the quotient does not exist.

Discussion Character

  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant references a textbook stating that if the limit of g is zero, one cannot conclude that the limit of f/g does not exist, and questions if this can be extended to imply non-existence.
  • Another participant asserts that if the limit of g approaches zero, then the limit of f/g would be undefined.
  • A different participant queries whether the term "the limit does not exist" is synonymous with undefined limits.
  • One participant argues that if g approaches zero, the limit may still exist and provides examples where the limit of f/g is defined despite g approaching zero.
  • Another participant distinguishes between limits that grow indefinitely and those that oscillate without settling on a value, using sin(1/x) as an example.
  • It is noted that limits can be described as "infinity" or "negative infinity," indicating a distinction in the nature of non-existence.

Areas of Agreement / Disagreement

Participants express differing views on whether a limit of zero for the denominator leads to the non-existence of the limit of the quotient. There is no consensus on this issue, as multiple competing perspectives are presented.

Contextual Notes

Participants provide examples that illustrate the complexity of limits involving zero denominators, highlighting the nuances in defining limits and the conditions under which they may or may not exist.

quasar987
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My Real Analysis textbook says: Let f,g: D --> R be two functions of common domain D that posses a limit at x_0 an accumulation point of D. Then, f/g as a limit at x_0 and this limit is the quotient of the limit of f to the limit of g, as long as [itex]g \neq 0 \ \forall x \ \epsilon \ D[/itex] and that the limit of g is not 0.

Does this mean that if the limit of g is zero we cannot conclude or could we extend the theorem to: if the limit of g is 0, then the limit of f/g does not exist?
 
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(lim g) can't be 0 because otherwise (lim f/g) would be undefined
 
Is that a synonim of "the limit does not exist" ?
 
I would say no, that the limit would be defined as infinity. for example, the limit as 1/x goes to 0 from the right is positive infinity, because 1 goes to 1 and x goes to 0. An example of an undefined limit would be the limit as x goes to 0 of sin(1/x).
 
No. If g-> 0, then we the limit may or may not exist.

Obvious examples: Take f(x)= x, g(x)= x on the domain (0, 1). Then x= 0 is a an accumulation point. lim(x->0) f(x)= 0 and limit(x->0) g(x)= 0 so we can't use
(lim f(x))/(lim g(x)). But obviously f(x)/g(x)= 1 for all x in (0,1) so the limit as x-> 0 is just 1.

Take f(x)= x(x+a), g(x)= x on the domain (0,1). Again, g(x)-> 0 so we can't use
(lim f(x)/lim g(x)). But obviously f(x)/g(x)= x+a for all x in (0,1) so the limit as x-> 0 is just a. (The point of this example is that the "indeterminate" form 0/0 can give ANY number as limit.)

Take f(x)= x+ 1, g(x)= x on the domain (0,1). Again g(x)-> 0 so we can't use
(lim f(x)/lim g(x)). Here, for x close to 0, f(x) is close to 1 so we have f(x)/g(x)= 1/ very small number which give a very large number. There is no limit in this case.

(In general if g(x)->0 and f(x)-> non-zero number, there is no limit for f(x)/g(x). If BOTH f(x) and g(x) go to 0, then there may not be a limit or the limit may be any number.
 
maybe that's true, that limits with no bounds are said not to exist, but I think there is a big difference between a function that gets bigger and bigger in one direction as you move towards a point and one which stays finite, but does not settle on any value, such as sin(1/x) as x goes to 0.
 
Yes, that's true. That's why some times we will say the limit "is infinity" or "is negative infinity" rather than just saying the limit does not exist. The limit STILL doesn't exist but you have a little more information about why it doesn't exist.
 
Thanks everyone for those insightful posts!
 

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