Encyclopedia of Triangle Centers

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The Encyclopedia of Triangle Centers (ETC) is an online list of thousands of points or "centers" associated with the geometry of a triangle. This resource is hosted at the University of Evansville. It started from a list of 400 triangle centers published in the 1998 book Triangle Centers and Central Triangles by Professor Clark Kimberling.^[1]

As of 16 October 2024^[update], the list identifies over 65,000 triangle centers^[2] and is managed cooperatively by an international team of geometry researchers.^[3]

This resource is seen as a pillar of the modern geometry, as in Gibert, Bernard. "Encyclopedia of Triangle Cubics".. In GeoGebra, this encyclopedia is provided at fingertip by a special command.^[4]

Each point in the list is identified by an index number of the form X(n) —for example, X(1) is the incenter.^[5] The information recorded about each point includes its trilinear and barycentric coordinates and its relation to lines joining other identified points. Links to The Geometer's Sketchpad diagrams are provided for key points. The Encyclopedia also includes a glossary of terms and definitions.

Each point in the list is assigned a unique name. In cases where no particular name arises from geometrical or historical considerations, the name of a star is used instead. For example, the 770th point in the list is named point Acamar.

Notable points

The first 10 points listed in the Encyclopedia are:

ETC reference	Name	Definition
X(1)	Incenter	center of the incircle
X(2)	Centroid	intersection of the three medians
X(3)	Circumcenter	center of the circumscribed circle
X(4)	orthocenter	intersection of the three altitudes
X(5)	nine-point center	center of the nine-point circle
X(6)	symmedian point	intersection of the three symmedians
X(7)	Gergonne point	symmedian point of contact triangle
X(8)	Nagel point	intersection of lines from each vertex to the corresponding semiperimeter point
X(9)	Mittenpunkt	symmedian point of the triangle formed by the centers of the three excircles
X(10)	Spieker center	center of the Spieker circle

Other points with entries in the Encyclopedia include:

ETC reference	Name
X(11)	Feuerbach point
X(13)	Fermat point
X(15), X(16)	first and second isodynamic points
X(17), X(18)	first and second Napoleon points
X(19)	Clawson point
X(20)	de Longchamps point
X(21)	Schiffler point
X(22)	Exeter point
X(39)	Brocard midpoint
X(40)	Bevan point
X(175)	Isoperimetric point
X(176)	Equal detour point

Similar, albeit shorter, lists exist for quadri-figures (quadrilaterals and systems of four lines) and polygon geometry.

References

^ Triangle Centers and Central Triangles. Congressus numerantium. Utilitas Mathematica Publishing. 1998.
^ Kimberling, Clark. "Part 31: Centers X(52001) - X(54000)". Encyclopedia of Triangle Centers. Retrieved February 6, 2024.
^ Kimberling, Clark. "Thanks". Encyclopedia of Triangle Centers. Retrieved February 6, 2024.
^ "TriangleCenter_Command". Geogebra.
^ Weisstein, Eric W. "Kimberling Center". MathWorld--A Wolfram Web Resource.

External links

Official website
Weisstein, Eric W. "Kimberling Center". MathWorld.
Implementation of ETC points as Perl subroutines by Jason Cantarella
Encyclopedia of Quadri-figures
Encyclopedia of Polygon Geometry

Categories:

Hidden categories:

Notable points

See also

References

External links