Casey, J. "On the Equations and Properties--(1) of the System of Circles Touching Three Circles in a Plane; (2) of the System of Spheres Touching Four Spheres in Space; (3) of the System of Circles Touching Three Circles on a Sphere; (4) of the System of Conics Inscribed to a Conic, and Touching Three Inscribed Conics in a Plane." Proc. Roy. Irish Acad.9, 396-423, 1864-1866.Coolidge, J. L. A Treatise on the Geometry of the Circle and Sphere. New York: Chelsea, 頁. 43, 1971.Johnson, R. A. Modern Geometry: An Elementary Treatise on the Geometry of the Triangle and the Circle. Boston, MA: Houghton Mifflin, 頁. 127-128, 1929.Lachlan, R. An Elementary Treatise on Modern Pure Geometry. London: Macmillian, 頁. 254-257, 1893.Larmor, A. "Contacts of Systems of Circles." Proc. London Math. Soc.23, 136-157, 1891.
相切的內切圓 
, 
, 
, 和 
的圓三角形 
及其相關三角形。 它要麼與 
外切，與相關三角形 
, 
, 和 
的內切圓內切（如上圖所示），反之亦然。 哈特圓具有一些性質，這些性質類似於線性三角形的九點圓的性質。 與給定的圓三角形相關聯的有八個哈特圓。