In Euclidean geometry, Kosnita's theorem is a property of certain circles associated with an arbitrary triangle.
Let be an arbitrary triangle, its circumcenter and , , are the circumcenters of three triangles , , and respectively. The theorem asserts that the three straight lines , , and are concurrent. This result was established by the Romanian mathematician Cezar Coşniţă (1910–1962).
Their point of concurrence is known as the triangle's Kosnita point (named by Rigby in 1997). It is the isogonal conjugate of the nine-point center. It is triangle center in Clark Kimberling's list. References Ion Pătraşcu (2010), A generalization of Kosnita's theorem (in Romanian)
(as cited by Kimberling).
Clark Kimberling (2014), Encyclopedia of Triangle Centers, section X(54) = Kosnita Point. Accessed on 2014-10-08.