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A100001 Number of self-dual combinatorial configurations of type (n_3). 3
0, 0, 0, 0, 0, 0, 1, 1, 3, 10, 25, 95, 366, 1433, 5802, 24105, 102479, 445577, 1992044 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,9
COMMENTS
A combinatorial configuration of type (n_3) consists of an (abstract) set of n points together with a set of n triples of points, called lines, such that each point belongs to 3 lines and each line contains 3 points.
Interchanging the roles of points and lines gives the dual configuration. A configuration is self-dual if there is an isomorphism from it to its dual.
LINKS
A. Betten, G. Brinkmann and T. Pisanski, Counting symmetric configurations v_3, Discrete Appl. Math., 99 (2000), 331-338.
T. Pisanski, M. Boben, D. Marušic, A. Orbanic, A. Graovac, The 10-cages and derived configurations, Discrete Math. 275 (2004), 265-276.
EXAMPLE
Example: the Fano plane is the only (7_3) configuration and is self-dual. It contains 7 points 1,2,...7 and 7 triples, 124, 235, 346, 457, 561, 672, 713.
The unique (8_3) configuration is also self-dual. It consists of the triples 125, 148, 167, 236, 278, 347, 358, 456.
CROSSREFS
Cf. A001403 (configurations (n_3), with many further references), A099999, A023994.
Sequence in context: A131433 A110713 A098702 * A227937 A299754 A190529
KEYWORD
nonn,nice,hard,more
AUTHOR
N. J. A. Sloane, Nov 05 2004
EXTENSIONS
a(1)-a(18) from the Betten, Brinkmann and Pisanski article.
a(19) from the Pisanski et al. article.
STATUS
approved

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Last modified February 29 21:35 EST 2024. Contains 370428 sequences. (Running on oeis4.)