Paul Yiu's Introduction to the Geometry of the Triangle

Jean-Louis Ayme's Geometry * Géométrie * Geometria

Bernard Gibert's Cubics in the Triangle Plane

Quim Castellsaguer's The Triangles Web

Triangle Geometry at MathWorld

Homepage Floor van Lamoen

Triangle Centers with C.a.R.

Grinberg at MathWorld

Moses at MathWorld

Triangle Centers


Index of Biographies of Mathematicians
Historia Matematica
Triangle Geometers

Geometry Software

The Geometer's Sketchpad
Euklid DynaGeo

Journals that publish triangle geometry

Forum Geometricorum
Journal for Geometry and Graphics
American Mathematical Monthly
Mathematics Magazine
Crux Mathematicorum
Mathematical Gazette

Triangle Geometry Interest Groups (Hyacinthos and Advanced Plane Geometry

Date: Wed, 22 Dec 1999
From: "Antreas P. Hatzipolakis"
Subject: Hyacinthos Mailing List

Dear Friends,

I just created a mailing list devoted to Triangle Geometry. The name of the list is Hyacinthos
(in honor of Emile Michel Hyacinthe Lemoine).

By 7 March 2013, the number of messages had passed 21,600.

FORMERLY, subscriptions were available at HYACINTHOS. Messages have been preserved and made accessible by CÚsar Lozada. Many of these have been given new links in ETC, and others can be accessed by knowing the message number and following this example:

Simply replace 25017 by the message number that is shown in ETC. The same procedure works for ADGEOM messages, starting with


The clustering of triangle centers into families has been compared to constellations of stars, and this fact has prompted the naming of certain points after stars. For a list of such names with astronomical data, visit

SkyEye - (Un)Common Star Names

The Keepers of SkyEye, Lynne Marie Stockman and David Harper, wrote to the Keeper of ETC that there are two constellation names of particular interest: Triangulum and Triangulum Australe. The alpha star in Triangulum is sometimes called Mothallah. According to Richard Hinckley Allen in Star Names: Their Lore and Meaning (originally published in 1899 and then first republished by Dover in 1963), the constellation name was translated by Arab astronomers as Almutallath, Almutaleh, Almutlato, Mutlat, etc. He goes on to say that the constellation was known as Shalish to the Jews, from the name of a triangular-shaped musical instrument. The Romans knew the constellation as Deltotum, and it has had various other names over the centuries: Aegyptus, Nilus (both after the Nile delta of Egypt), Trigonum, Trigonus, etc. The alpha star in Triangulum Australe is called Atria.

Contact the Keeper

Your suggestions for improving ETC are welcome, as are submissions of new centers (and bicentric pairs) for possible inclusion in ETC. To submit such a center, send the simplest barycentric coordinates you can find, expressed as functions of sidelengths a,b,c or vertex angles A,B,C, or both. Also send, in quotable wording, geometric information about the center. Of course, before sending, you should check a search page, such as Search_13_6_9. Send to Clark Kimberling (University of Evansville)

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