1. Balk, M.B. andBoltyanskii, V.G.: Geometry of Masses. Moscow 1987 (in Russian). P. 68.
2. Balk, M.B. andMazalov, M.Ya.: But how can this inequality be proved? ... Kvant, 1995, #6, p. 43 (in Russian).
3. Biggiogero, G.: La geometria del tetraedro. In: Enciclopedia delle Matematiche Elementari. A cura di L. Berzolari, G. Vivanti e D. Gigli.Volume II, parte I.Milano 1937. Ristampa anastatica, Maggio 1943. P. 237.
4. Coxeter, H.S.M. andGreitzer, S.L.: Geometry Revisited. Toronto ? New-York 1967. Ch.2, Section 1.
5. Danielsson, G.: Proof of the inequality d2 ? (R + r)(R-3r) for the distance between the centres of the circumscribed and inscribed spheres of a tetrahedron, Den 11te Skandinaviske Matematikerkongress, Trondheim, 1949, pp. 101?105. Johan Grundt Tanums Forlag, Oslo, 1952.