Talk:Rouché's theorem

message: Would it be alright if someone added information on who proved this theorem and why it is called "Rouche's Theorem"?
All the best,
Virucidal 98.206.162.69 (talk) 16:39, 4 June 2009 (UTC)
thread_title: Add History?

message: Here are a few problems that you guys should be able to fix easily.
* In the theorem statement, it says "the same number of zeros in C". I guess that should be "inside C".
* f and g are assumed to be analytic inside and on C, but they might have zeros on C.  That gives trouble with integrating the function f'/f along C.  I don't have my books handy but I guess that f is supposed to be nonzero on C as well. (Or else, the proof has to be modified.)
--Zero 01:35, 15 Jan 2005 (UTC)
replies: text: Yes, good reasoning.  My next concern is the meaning of "inside" when C is complicated  (consider a figure-of-eight).  Why not restrict it to a simple closed contour and save the hassle?  We don't need to have the most general possible statement in WP.  This goes for Argument principle too, which seems to assume a simple closed contour without saying so. In the case of a simple closed contour, the Jordan Curve Theorem says that "inside" is well-defined.  --Zero 11:22, 15 Jan 2005 (UTC)
replies: text: Well Zero, you are sharp. Not only it has to be a simple closed contour, it also has to go counter-clockwise, otherwise the signs turn out wrong.  I will fix that soon. By the way, even before you said the above, I had added a picture on the Argument principle article, which shows the situation. (As they say, a picture is worth... :)
Oleg Alexandrov 17:55, 15 Jan 2005 (UTC)
thread_title:  some problems 

message: This proof is totally unjustified, why does I(t) have to be integer valued?
131.111.213.52 (talk) 16:55, 26 October 2009 (UTC)
thread_title:  Problem 

message: A better version has been around for years.  I don't know how long but it's just the theorem that is in my complex books (undergrad and grad).  All that is required is |f(z) - g(z)| \leq |f(z)| + |g(z)|.  You'd have to redo the proof to and I don't want to write all that but if any one wants to, that would be great.  I don't remember who proved or it or when but a reference for the theorem is even available online.  Theorem 8.10 on this page has the theorem without proof.  But, this is an online version of a book, which was what I used as an undergrad, and the book has the proof.  —Preceding unsigned comment added by StatisticsMan (talk • contribs) 03:02, 31 March 2010 (UTC)
replies: text: This does not make sense: |f(z) - g(z)| \leq |f(z)| + |g(z)| holds for every f and g automatically by the triangle inequality, hence it cannot imply anything about the zeros.—Emil J. 22:23, 3 July 2014 (UTC)

text: Oh, I see, it works with the strict inequality |f(z) - g(z)| < |f(z)| + |g(z)|. And in fact, this is the “symmetric version” of Rouché’s theorem due to Esterman, whereas the statement currently presented under that name in the article is a trivial restatement of the original Rouché’s theorem.—Emil J. 11:30, 5 July 2014 (UTC)

text: And, this mixup is due to a recent misguided anonymous edit, so I’ll just revert it.—Emil J. 11:31, 5 July 2014 (UTC)

text: Now the original statement is not stated in the Article of it's name, do you really think it is better this way, further I don't think any of those two statements is better, there are cases where one is more convinient and others in which the other is.  — Preceding unsigned comment added by 130.60.188.207 (talk) 09:21, 23 January 2015 (UTC)
thread_title:  Better version 

message: Instead of referring to the tree circumference, would the dog walker example, ending the section, be a little more convincing with a walk around something like a garden (say), or a park, perhaps worded to suggest that the tree, located somewhere central in the garden, is of negligible diameter? The words "less than the circumference of the tree" could be replaced by "less than the current distance from the tree". As the example is currently worded, the statement about real life seems patently false, which isn't a very good advert for the activities of mathematicians. For example, by common sense, anyone can see that either party could walk almost twice around the tree while the other lay asleep on the grass. Also the mental image created would then better match the figure shown.  — Preceding unsigned comment added by 83.217.170.175 (talk) 14:37, 21 August 2015 (UTC)
replies: text: Perhaps you've never walked a dog on a leash. 67.198.37.16 (talk) 18:40, 4 February 2024 (UTC)
thread_title:  Geometric Explanation