A210714 Number of disconnected regular simple graphs on n vertices with girth exactly 4.

0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 3, 2, 5, 3, 15, 5, 44, 10, 203, 47, 1415, 1710, 27771, 197951, 2613710, 33117920, 463707092, 6709514282, 102306345333, 1597440872721

Offset: 0

Author

Jason Kimberley, Dec 10 2012

Keywords

• nonn
• more
• hard

Links

• Jason Kimberley, Index of sequences counting disconnected k-regular simple graphs with girth exactly g

Programs

Mathematica

A[s_Integer] := With[{s6 = StringPadLeft[ToString[s], 6, "0"]}, Cases[ Import["https://oeis.org/A" <> s6 <> "/b" <> s6 <> ".txt", "Table"], {, }][[All, 2]]];
A185214 = A@185214;
A185215 = A@185215;
a[n_] := A185214[[n + 1]] - A185215[[n + 1]];
a /@ Range[0, 31] (* Jean-François Alcover, Jan 27 2020 *)

Formula

a(n) = A185214(n) - A185215(n).

Extensions

a(31) corrected by the author, propagated from A185244, Jan 05 2013