Talk:Projective harmonic conjugate

Pappus harmonic theorem
Re User talk:Dickdock (query from User:Rgdboer), I have less than zero expertise on this topic. My only knowledge is from the Wolfram Mathworld site (which cites Coxeter and Greitzer's Geometry Revisited pp.67-8, a book I don't have) where I noticed the similarity between the two concepts. I've removed the "aka Pappus's harmonic theorem" assertion. I've left the redirect I added though for the moment pending research/clarification/help. Dickdock (talk) 21:28, 6 September 2008 (UTC)


 * There is a copy of Coxeter and Greitzer (1967) New Mathematical Library, in a building down the street; will check it out. Thank you for the quick response; as indicated, I'm looking at the history of developments in this aspect of geometry. Note also that at times Wolfram is unique, being contemporary in quest for authority when sources are scarce.Rgdboer (talk) 21:46, 6 September 2008 (UTC)

I managed to access (via Google books) the relevant section 3.5 "Pappus's Theorem" of Coxeter and Greitzer's Geometry Revisited (pp.67-9) cited by the Wolfram site, and it says: "The particular theory that bears his name may be stated in various ways, one of which is as follows: Theorem 3.51. If A, C, E are 3 points on one line, B, D, F on another, and if the 3 lines AB, CD, EF meet DE, FA, BC respectively, then the 3 points of intersection L, M, N are collinear." Something of a stretch to get from that to what's represented on the site as "Pappus's harmonic theorem", one might think. What's better though is that later on in Geometry Revisited ("Pappus rephrased" section 3.7 "Hexagons", p.73) appears: "...Theorem 3.51 (Pappus's theorem) may be rephrased as follows: If each set of three alternate vertices of a hexagon is a set of three collinear pts, and the three pairs of opposite sides intersect, then the three points of intersection are collinear." i.e. "Pappus's hexagon theorem". So the famous "Pappus's harmonic theorem" is actually nothing more than "Pappus's hexagon theorem"! This meme is all over the internet! Great call. Dickdock (talk) 06:00, 7 September 2008 (UTC)

Which cross ratio equals negative one?
If the points A, B, C, and D have real values a, b, c, and d respectively, is it the cross-ratio (a, b; c, d) = (a - c)(b - d) / (a - d)(b - c) that equals negative one?

If so, that should be stated explicitly. If not, the correct formula should be given. —Preceding unsigned comment added by 192.91.147.35 (talk) 22:18, 24 October 2008 (UTC)

Archytas of Tarentum
Today I removed the following from the introduction:
 * The harmonic set of points can be traced back to the concept of harmonic mean described by Archytas of Tarentum.

There are two assertions here, neither backed-up with a source. The harmonic mean is an arithmetic concept, not as primitive as the geometric construction given here. As seen above, there is a desire to connect this projective harmonic conjugate concept with ancient writers. The contributor ArepoEn has some good contributions, but this one needs substantiation.Rgdboer (talk) 21:51, 3 June 2009 (UTC)

proposed move
The page should be moved to projective harmonic conjugate. Tkuvho (talk) 01:45, 14 September 2010 (UTC)

Fano ?
The following lines were removed from the lead as unreferenced and unclear:
 * The conjugate point can be defined similarly in any projective plane satisfying the Fano axiom (ruling out the Fano plane). In the case of a projective plane over a field, this rules out fields of characteristic 2.

This space in Talk provides opportunity for anyone to bring out a reference or interpret this contribution edited out today.Rgdboer (talk) 21:59, 2 October 2010 (UTC)
 * I am not sure what needs to be interpreted here. In a projective plane over a field of characteristic different from 2, the same construction as described in the article produces the fourth harmonic point.  More generally, the Fano axiom requires the four "diagonal" points constructed from a quadrilateral, to be non-collinear.  The 7-point Fano plane does not satisfy this axiom.  If a projective plane does satisfy the axiom, then the same construction gives the fourth harmonic point.  A plane over a field will satisfy the Fano axiom if and only if its characteristic is different from 2.  This is standard material that can be found for example in Hartshorne's book.  Tkuvho (talk) 03:07, 3 October 2010 (UTC)

Thank you Tkuvho for explanation of the axiom and the reference. Will look into Hartshorne. Placement of the contribution will be adjusted. Your interest in this fundamental in geometry is appreciated.Rgdboer (talk) 02:44, 5 October 2010 (UTC)

From Russell's Principles of Mathematics (1903), page 385 one can see recognition of Fano's work! It seems important enough for a chapter of this article.Rgdboer (talk) 02:38, 8 October 2010 (UTC)


 * That sounds interesting. Could you add some material on this?  Tkuvho (talk) 08:21, 8 October 2010 (UTC)

Harmonic division
Note the article harmonic division where signs of segments are ignored. The concept is slightly less sharp than Projective harmonic conjugate. Note the sole reference of dubious quality compared to several for this article. Looking for interwiki opportunities for this article generated the "harmonic division" article. French and German versions are much the same. The Dutch version makes clear the unity of the two concepts. Its title translates as "harmonic division". Is a WP:Merge in order ? Rgdboer (talk) 01:58, 22 December 2011 (UTC)

Progression ?
Today the following text was removed:
 * The cross-ratio criterion implies that distances from any one of these points to the three remaining points form harmonic progression.

Can someone fill in an algebraic proof or provide a reference? This connection between harmonic range and harmonic progression must be verifiable.Rgdboer (talk) 02:02, 16 February 2012 (UTC)

Golden ratio
The following tagged text requires a citation and has been removed (for now) from the article: Let $$P_0,P_1,P_2$$ be three different points on the real projective line. Consider the infinite sequence of points $$P_n$$, where $$P_n$$ is the projective harmonic conjugate of $$P_{n-3}$$ with respect to $$P_{n-1},P_{n-2}$$ for $$n>2$$. This sequence is convergent. For a finite limit $$P$$ we have $$\lim_{n\rightarrow\infty}\frac{P_{n+1}P}{P_{n}P}=\Phi-2=-\Phi^{-2} $$ where $$\Phi=\frac{1+\sqrt{5}}{2}$$ is the golden ratio, i.e. $$P_{n+1}P\approx -\Phi^{-2} P_{n}P$$ for large $$n$$. For an infinite limit we have $$\lim_{n\rightarrow\infty}\frac{P_{n+2}P_{n+1}}{P_{n+1}P_{n}}=-1-\Phi =-\Phi^{2} $$. (For a proof consider the projective isomorphism $$f(z)=\frac{az+b}{cz+d}$$ with $$f((-1)^n\Phi^{2n})=P_n$$).
 * Iterated projective harmonic conjugates and the golden ratio

Comments are welcome here.Rgdboer (talk) 02:55, 29 October 2016 (UTC)

A citation is: F. Leitenberger, Iterated harmonic divisions and the golden ratio, Forum Geometricorum, 16 (2016) 429--430. — Preceding unsigned comment added by 2A00:C1A0:8604:5E00:6571:EB84:3B2D:5984 (talk) 18:46, 18 March 2018 (UTC)
 * Link to open-access article inserted. — Rgdboer (talk) 21:42, 19 March 2018 (UTC)