4,2

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

R. W. Robinson, Numerical implementation of graph counting algorithms, AGRC Grant, Math. Dept., Univ. Newcastle, Australia, 1976.

Walsh, T. R. S.; Counting labeled three-connected and homeomorphically irreducible two-connected graphs. J. Combin. Theory Ser. B 32 (1982), no. 1, 1-11.

R. W. Robinson, Table of n, a(n) for n = 4..25

Row sums of A123542.

Sequence in context: A042303 A042300 A080163 this_sequence A123537 A054597 A005643

Adjacent sequences: A005641 A005642 A005643 this_sequence A005645 A005646 A005647

nonn

N. J. A. Sloane (njas(AT)research.att.com).

Corrected and extended Nov 13 2006