1. Proof of the inequality d2≤(R+r)(R−3r) for the distance between the centres of the circumscribed and inscribed spheres of a tetrahedron;Danielsson,1952
2. G. Egan, An n-dimensional Grace-Danielsson inequality. http://blogs.ams.org/visualinsight/2014/06/01/grace-danielsson-inequality/.
3. Tetrahedra in relation to spheres and quadrics;Grace;Proc. Lond. Math. Soc.,1918
4. http://mathoverflow.net/questions/117558/are-there-heronian-triangles-that-can-be-decomposedinto-three-smaller-ones.
5. http://mathworld.wolfram.com/Coplanar.html.