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, 页. 43, 1971.Johnson, R. A. Modern Geometry: An Elementary Treatise on the Geometry of the Triangle and the Circle. Boston, MA: Houghton Mifflin, 页. 127-128, 1929.Lachlan, R. An Elementary Treatise on Modern Pure Geometry. London: Macmillian, 页. 254-257, 1893.Larmor, A. "Contacts of Systems of Circles." Proc. London Math. Soc.23, 136-157, 1891.
相切的内切圆 
, 
, 
, 和 
的圆三角形 
及其相关三角形。 它要么与 
外切，与相关三角形 
, 
, 和 
的内切圆内切（如上图所示），反之亦然。 哈特圆具有一些性质，这些性质类似于线性三角形的九点圆的性质。 与给定的圆三角形相关联的有八个哈特圆。