R. K. Guy, Unsolved Problems in Number Theory, E20.
D. P. Roselle and R. G. Stanton, Results on Davenport-Schinzel sequences, pp. 249-267 in Proceedings of the Louisiana Conference on Combinatorics, Graph Theory and Computer Science. Vol. 1, edited R. C. Mullin et al., 1970.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).