The number of edges of radius-invariant graphs
Vacek Ondrej TUZDFMDG - Katedra matematiky a deskriptívnej geometrie
Mathematica Slovaca. Vol. 59, no. 2 (2009), p. 201-220
xcla - ČLÁNKY
https://www.degruyter.com/document/doi/10.2478/s12175-009-0118-3/html