A054920 Number of connected unlabeled reflexive relations with n nodes such that complement is also connected.

2, 4, 68, 2592, 278796, 95720106, 111891292036, 457846756500066, 6664787020904248568, 349363873490889302878250, 66602024342830108271942323060, 46557190064705399729526041154647820

Offset: 1

Author

N. J. A. Sloane, May 24 2000

Keywords

• nonn
• easy

Links

• Andrew Howroyd, Table of n, a(n) for n = 1..50
• V. A. Liskovets, Some easily derivable sequences, J. Integer Sequences, 3 (2000), #00.2.2.

Crossrefs

Cf. A000595, A054919.

Programs

Mathematica

A000595 = Import["https://oeis.org/A000595/b000595.txt", "Table"][[All, 2]];
(* EulerInvTransform is defined in A022562 *)
A054919 = Join[{1}, EulerInvTransform[A000595 // Rest]];
a[n_] := 2 A054919[[n + 1]] - A000595[[n + 1]];
Array[a, 50] (* Jean-François Alcover, Sep 01 2019, updated Mar 17 2020 *)

Formula

a(n) = 2*A054919(n) - A000595(n).

Extensions

More terms from Vladeta Jovovic, Jul 17 2000