1,9

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.

A. Betten and D. Betten, Regular linear spaces, Beitraege zur Algebra und Geometrie, 38 (1997), 111-124.

A. Betten and D. Betten, Tactical decompositions and some configurations v_4, J. Geom. 66 (1999), 27-41.

A. Betten, G. Brinkmann and T. Pisanski, Counting symmetric configurations v_3, Discrete Appl. Math., 99 (2000), 331-338.

M. Boben et al., Small triangle-free configurations of points and lines, Discrete Comput. Geom., 35 (2006), 405-427.

Bokowski and Sturmfels, Comput. Synthetic Geom., Lect Notes Math. 1355, p. 41.

CRC Handbook of Combinatorial Designs, 1996, p. 255.

H. Gropp, Configurations and their realization, Discr. Math. 174 (1997), 137-151.

D. Hilbert and S. Cohn-Vossen, Geometry and the Imagination Chelsea, NY, 1952, Ch. 3.

Pisanski, T.; Boben, M.; Marusic, D.; Orbanic, A.; and Graovac, A. The 10-cages and derived configurations. Discrete Math. 275 (2004), 265-276.

Pisanski, T. and Randic, M., Bridges between Geometry and Graph Theory, in Geometry at Work: Papers in Applied Geometry (Ed. C. A. Gorini), M.A.A., Washington, DC, pp. 174-194, 2000.

B. Polster, A Geometrical Picture Book, Springer, 1998, p. 28.

Sturmfels and White, Rational realizations..., in H. Crapo et al. editors, Symbolic Computation in Geometry, IMA preprint, Univ Minn., 1988.

Sturmfels and White, All 11_3 and 12_3 configurations are rational, Aeq. Math., 39 1990 254-260.

Von Sterneck, Die Config. 11_3, Monat. f. Math. Phys. 5 325-330 1894; Die Config. 12_3, op. cit. 6 223-255 1895.

Jim Loy, Mathematics Page (see Desargues's Theorem)

Jim Loy, The configuration (10_3) arising from Desargues's theorem

Tomo Pisanski, Papers on configurations

Eric Weisstein's World of Mathematics, Link to a section of The World of Mathematics.

Example: the Fano plane is the only (7_3) configuration. It contains 7 points 1,2,...7 and 7 triples, 124, 235, 346, 457, 561, 672, 713.

The unique (8_3) configuration consists of the triples 125, 148, 167, 236, 278, 347, 358, 456.

There are three configurations (9_3), one of which is the familiar configuration arising from Desargues's theorem (see illustration).

Cf. A023994, A099999 (geometrical configurations), A100001 (self-dual configurations), A098702, A098804, A098822, A098841, A098851, A098852, A098854.

Sequence in context: A079522 A024426 A034016 this_sequence A072136 A080406 A036682

Adjacent sequences: A001400 A001401 A001402 this_sequence A001404 A001405 A001406

nonn,nice,hard

N. J. A. Sloane (njas(AT)research.att.com), D.Glynn(AT)math.canterbury.ac.nz

Von Sterneck has 228 instead of 229. His error was corrected by Gropp. The n=15 term was computed by Dieter and Anton Betten, Uni. of Kiel.

a(16)-a(18) from the Betten, Brinkmann and Pisanski article.

a(19) from the Pisanski et al. article.