1. G. Ringel, Problem 25 in: Theory of Graphs and its Applications,(Proc. Internat. Sympos. Smolenice 1963 Nakl. CSAV, Praha 1964, p. 162.
2. A. Rosa, On certain valuations of the vertices of a graph, in: Theory of Graphs (Proc. Internat. Sympos. Rome 1966 ), Gordon and Breach-Dunod, N.Y.-Paris 1967, pp. 349–355.
3. S.W. Golomb, How to number of a graph, in: Graph Theory and Computing (ed. R.C. Read ), Academic Press, N.Y. (1972), pp. 23–37.
4. G.S. Bloom, A chronolgy of the Ringel-Kotzig conjecture and the continuing quest to call all trees graceful, in: Topics in Graph Theory (Annal of the N.Y. Academy of Sciences, 328,ed. F. Harary ), N.Y., 1979,pp. 32–51.
5. J.C.-Bermond, Graceful graphs, radio antennae and French windmills, in: Proc. of Open Univ. Conf., May 1978, ( Pitman, London 1979 ),pp. 13–37.