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URL: https://oeis.org/A014381

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A014381
Number of connected regular graphs of degree 9 with 2n nodes.
15
1, 0, 0, 0, 0, 1, 9, 88193, 113314233813, 281341168330848874, 1251392240942040452186674, 9854603833337765095207342173991, 134283276101750327256393048776114352985
OFFSET
0,7
COMMENTS
Since the nontrivial 9-regular graph with the least number of vertices is K_10, there are no disconnected 9-regular graphs with less than 20 vertices. Thus for n<20 this sequence also gives the number of all 9-regular graphs on 2n vertices. - Jason Kimberley, Sep 25 2009
REFERENCES
CRC Handbook of Combinatorial Designs, 1996, p. 648.
I. A. Faradzev, Constructive enumeration of combinatorial objects, pp. 131-135 of Problèmes combinatoires et théorie des graphes (Orsay, 9-13 Juillet 1976). Colloq. Internat. du C.N.R.S., No. 260, Centre Nat. Recherche Scient., Paris, 1978.
FORMULA
a(n) = A184993(n) + A181170(n).
EXAMPLE
The null graph on 0 vertices is vacuously connected and 9-regular; since it is acyclic, it has infinite girth. - Jason Kimberley, Feb 10 2011
CROSSREFS
Connected regular simple graphs A005177 (any degree), A068934 (triangular array), specified degree k: A002851 (k=3), A006820 (k=4), A006821 (k=5), A006822 (k=6), A014377 (k=7), A014378 (k=8), this sequence (k=9), A014382 (k=10), A014384 (k=11).
9-regular simple graphs: this sequence (connected), A185293 (disconnected).
Connected 9-regular simple graphs with girth at least g: this sequence (g=3), A181170 (g=4).
Connected 9-regular simple graphs with girth exactly g: A184993 (g=3).
Sequence in context: A058456 A184991 A184993 * A185293 A281360 A034995
KEYWORD
nonn,more,hard
EXTENSIONS
a(8) appended using the symmetry of A051031 by Jason Kimberley, Sep 25 2009
a(9)-a(10) from Andrew Howroyd, Mar 13 2020
a(10) corrected and a(11)-a(12) from Andrew Howroyd, May 19 2020
STATUS
approved