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 6 February 2024[update], the list identifies over 61,000 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.