Talk:Desargues's theorem

message: btw, there's this great free software:
Compass and Ruler is written in Java by von R Grothmann. This program is excellent. It is very well written, in Java and available for Mac or Windows or Linux, and it is free with source code too. Its home page is at http://mathsrv.ku-eichstaett.de/MGF/homes/grothmann/java/zirkel/index.html
that we can use and build example files to go along with article and theorems. Xah Lee 15:36, 2005 Mar 18 (UTC)
I don't understand from the proof... I can't see the equivalence with those scalar & cross products.
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I also have trouble with the proof. It depends on various lines being "concurrent," but "concurrent is nowhere defined.  — Preceding unsigned comment added by 119.252.188.83 (talk) 13:08, 19 August 2015 (UTC)
thread_title:  adding dynamic geometry program files 

message: There is a nice practical application in perspective drawing:  suppose you have two (or more) lines drawn in perspective, which meet at a vanishing point located far enough away from the physical page to be constructed directly.  Desargues' theorem can be used to construct additional lines which meet at the same vanishing point. --Heath 69.174.67.197 05:04, 16 July 2006 (UTC)
thread_title:  application in perspective drawing 

message: I needed a link for perspective triangles recently and was forced to resort to MathWorld instead, as the Perspective (geometry) article hadn't yet been created. I would prefer not to merge, as I think that perspective triangles are important enough, that there are enough results about them that are distinct from Desargues' theorem, and that there are enough other topics already included in the Desargues theorem article that it would be confusing to link to it under the perspective triangle name. —David Eppstein (talk) 04:55, 28 December 2007 (UTC)
replies: text:  Specifically, what results about perspective triangles other than Desargues' theorem do you have in mind? Michael Hardy (talk) 22:11, 5 January 2008 (UTC)
replies: text:  A better question might be: Specifically what other relations of Desargues' theorem to concepts in mathematics do you have in mind other than perspective triangles? The theorem should be merged into the definition if anything. Nousernameslefttalk and matrix? 02:45, 13 January 2008 (UTC)

text: Since this hasn't any opposition for quite a while, I'll just remove the template. Nousernameslefttalk and matrix? 18:44, 5 January 2008 (UTC)
thread_title:  Suggested merger from [[Perspective (geometry)]] 

message: Not a matter for concern on a par with the current state of the economy, but Wikipedia's style guide says:
:Proper nouns that end with s: There is tension in English over whether just an apostrophe, or an apostrophe and the letter s, should be added to such proper nouns (James' house or James's house, but be consistent within an article). Some forms always take an extra s (Ross's father); some, mainly Biblical and Classical, usually do not (Socrates' wife; Moses' ascent of Sinai; Jesus' last words).
Although omitting the s used to be the norm for such names (I was raised in the pro-omission tradition and have been accustomed until recently to writing it as Desargues'), more recent sources such as the 1961 edition of Coxeter's Introduction to Geometry, The Concise Oxford Dictionary of Mathematics, and Britannica (at least as far back as 1986, the publication year of my hard copy) write it as "Desargues's theorem," as do the more authoritative sources on the web, although a majority of pop writers still seem to be using the old style.  Journal editors these days seem to be encouraging its inclusion (I was recently picked on by one for using the old style and adapted accordingly).
Does anyone have any strong opinions either way?  In particular are there any strong objections to matching Wikipedia's style in the case of Desargues to the usage of the sources cited above?  --Vaughan Pratt (talk) 02:08, 8 December 2008 (UTC)
thread_title: Desargues' or Desargues's?

message: It is possible for ten points and ten lines in the plane to be so arranged that each of the ten lines passes through three of the ten points and each of the ten points lies on three of the ten lines, even though that whole configuration is not incidence-isomorphic to the Desargues configuration.  Should that be mentioned here?
: 
: 
Michael Hardy (talk) 18:22, 23 August 2009 (UTC)
replies: text: Added. As always, feel free to reword my addition. —David Eppstein (talk) 22:24, 23 August 2009 (UTC)
thread_title:  Non-Desargues configuration 

message: Hello! This is a note to let the editors of this article know that :File:Desargues theorem alt.svg will be appearing as picture of the day on November 2, 2011. You can view and edit the POTD blurb at Template:POTD/2011-11-02. If this article needs any attention or maintenance, it would be preferable if that could be done before its appearance on the Main Page so Wikipedia doesn't look bad. :) Thanks! howcheng  {chat} 20:34, 31 October 2011 (UTC)
replies: text: I agree that the lead could use a rewrite, but I can't do it in the amount of time left. The suggestion about replacing spaces by planes doesn't work, since it is precisely that two triangles in different planes which are centrally perspective are automatically axially perspective which makes the proof in 3 dimensions so easy. I've done what I can for this article - if only I had an additional 24 hours or had this occurred in a different week - Oh well! Bill Cherowitzo  (talk) 16:04, 1 November 2011 (UTC)

text: Maybe you thought of a solution but didn't actually implement it.  The caption still said "produced" when I looked at it.  I've made a change, but feel free to edit this further if you can think of a better way. Jowa fan (talk) 23:27, 1 November 2011 (UTC)
replies: text: Sorry, I meant that I changed it in the POTD box. I didn't make the same change to the actual article. —David Eppstein (talk) 23:56, 1 November 2011 (UTC)
thread_title:  [[:File:Desargues theorem alt.svg]] to appear as POTD soon 

message: Please add the date when this theorem was discovered or proved or conjectured
--Thepurplelefant (talk) 12:55, 8 June 2012 (UTC)
replies: text: Done Bill Cherowitzo  (talk) 01:07, 9 June 2012 (UTC)
thread_title:  This article needs to include dates 

message: I'd like to use the image on the t-shirts we give away to students in out local high school mathematics contest. I don't really want to write an attribution on the shirt.  Would that be okay?  — Preceding unsigned comment added by 142.27.68.70 (talk) 19:20, 31 March 2015 (UTC)
replies: text: The CC license for this image does call for attribution when it is used (remixed in your case). Try Googling Desargues theorem images, I'm sure that you'll find one that doesn't ask for attribution. Bill Cherowitzo  (talk) 22:08, 31 March 2015 (UTC)
replies: text: You might also try looking at some of the other images in :Commons:Category:Desargues' theorem. For instance, :File:DesarguesHessenberg.svg has a license that doesn't require attribution, and might fit your needs. —David Eppstein (talk) 22:11, 31 March 2015 (UTC)

text: It doesn't matter whether the projective plane is self-dual; what matters is that the statement is self-dual. That is, the dual of the statement is the same statement. So if you prove the statement in some plane (self-dual or not) you have also proved the dual of the statement. —David Eppstein (talk) 01:14, 7 February 2024 (UTC)
thread_title:   Permission to use image 

message: The 3-dimesional proof depends on various lines being "concurrent." Please define "concurrent."  — Preceding unsigned comment added by 119.252.188.83 (talk) 13:02, 19 August 2015 (UTC)
thread_title:  Definition of concurrent 

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thread_title:  External links modified 

message: The article hand-waves it away by saying it follows by duality. But some projective planes are not self-dual. --Svennik (talk) 00:41, 7 February 2024 (UTC)
replies: text: It doesn't matter whether the projective plane is self-dual; what matters is that the statement is self-dual. That is, the dual of the statement is the same statement. So if you prove the statement in some plane (self-dual or not) you have also proved the dual of the statement. —David Eppstein (talk) 01:14, 7 February 2024 (UTC)
thread_title:  Proof of converse statement?