1. G. Ringel, \emph{Problem 25}, in Theory of Graphs and its Applications, Proc. Symposium Smolenice 1963, Prague (1964) 162.
2. A. Kotzig, \emph{On certain vertex valuations of finite graphs}, Util. Math., 4 (1973) 67-73.
3. A. Rosa, \emph{On certain valuations of the vertices of a graph}, Theory of Graphs (Internat. Symposium, Rome, July 1966), Gordon and Breach, N. Y. and Dunod Paris (1967) 349-355.
4. C. Barrientos and E. Krop, \emph{Improved bounds for relaxed graceful trees}, Graphs Combin., 33 (2017) 287-305.
5. C. Barrientos and S. Minion, \emph{New attack on Kotzig’s Conjecture}, Electron. J. Graph Theory Appl., 4 (2) (2016) 119-131.