The OEIS is supported by the many generous donors to the OEIS Foundation.
%I #19 Sep 13 2019 08:02:29
%S 1,1,4,62,3512,664808,434127872,1019647584176,8819902136975232,
%T 284798318582888943488,34636901578294133264379392,
%U 15970288165992077917879019856128,28060652850285225214988677642354678784,188680826646319054103468721801532841166681088
%N Number of labeled connected Eulerian digraphs with n nodes and an even number of edges.
%H V. A. Liskovets, <a href="http://www.cs.uwaterloo.ca/journals/JIS/VOL3/LISK/Derseq.html">Some easily derivable sequences</a>, J. Integer Sequences, 3 (2000), #00.2.2.
%F a(n) = (A054957 + (n-1)!)/2. - _Andrew Howroyd_, Sep 10 2018
%t A054957 = Cases[Import["https://oeis.org/A054957/b054957.txt", "Table"], {_, _}][[All, 2]];
%t a[n_] := (A054957[[n]] + (n - 1)!)/2;
%t Array[a, 14] (* _Jean-François Alcover_, Sep 13 2019 *)
%Y Cf. A058338, A054955, A007080, A054959, A054956, A058337, A054957.
%K nonn,easy
%O 1,3
%A _N. J. A. Sloane_, May 24 2000
%E More terms from _Vladeta Jovovic_, Mar 06 2001
%E a(12)-a(14) from _Andrew Howroyd_, Sep 10 2018