%I M1517
%S 1,2,5,18,100,1242,43425,4925635,1678993887,1613875721946,
%T 4293014800909806,31574944534364259507,644483327087699659771857,
%U 36676558984788056550610362834,5846161177591490590945591554686844,2621219060849255874034814155021919844156
%N Definition (1): Number of unlabeled strength2 Eulerian graphs with n nodes.
%C Definition (2): Number of Eulerian 2multigraphs with n nodes.
%C Definition (3): Number of switching classes of signed graphs on n unlabeled nodes.
%C Definition (4): Number of switching classes of 2multigraphs with n nodes
%C Definition (1) is same as Definition (2).  Vladeta Jovovic, Mar 15 2009
%C Definition (3) is same as Definition (4).  Vladeta Jovovic, Mar 15 2009
%C That Definition (2) is same as Definition (4) follows from Theorem 8.3 of Cameron (1977).  _N. J. A. Sloane_, Mar 22 2009
%D F. C. Bussemaker, P. J. Cameron, J. J. Seidel, and S. V. Tsaranov, Tables of signed graphs, ReportWSK 9101, Eindhoven University of Technology, Department of Mathematics and Computing Science, Eindhoven, 1991, 105 pp. (MR: 92g:05001).
%D R. W. Robinson, personal communication.
%D R. W. Robinson, Numerical implementation of graph counting algorithms, AGRC Grant, Math. Dept., Univ. Newcastle, Australia, 1980.
%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H F. C. Bussemaker, P. J. Cameron, J. J. Seidel, and S. V. Tsaranov, <a href="https://www.researchgate.net/publication/241872284_Tables_of_signed_graphs ">Tables of signed graphs</a>, ReportWSK 9101, Eindhoven University of Technology, Department of Mathematics and Computing Science, Eindhoven, 1991, 105 pp. (MR: 92g:05001).
%H P. J. Cameron, <a href="https://doi.org/10.1007/BF01215145">Cohomological aspects of twographs</a>, Math. Zeit., 157 (1977), 101119.
%Y Cf. A002854, A084565, A004102.
%K nonn,nice
%O 1,2
%A _N. J. A. Sloane_
%E Extended by _N. J. A. Sloane_ from Robinson's table, Oct 20 2006
%E Edited by _N. J. A. Sloane_, Mar 30 2009. Does the Cameron paper provide a formula? What about the labeled version? (Compare A002854.)
%E Corrected by _Vladeta Jovovic_, Apr 04 2009
