M. A. Harrison, On the number of classes of switching networks, J. Franklin Instit., 276 (1963), 313-327. (Available on-line at http://deepblue.lib.umich.edu/handle/2027.42/5406/.)
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).