A023994 - OEIS

A023994

Number of isomorphism types of symmetric configurations n_4.

1, 1, 4, 19, 1972, 971171

(list; graph; refs; listen; history; text; internal format)

OFFSET

13,3

LINKS

Table of n, a(n) for n=13..18.

A. Betten, More information

Ed Pegg, Jr., An example of a 27_4 configuration [In barycentric coordinates, both lines and points are triples. A point is on a line if point.line = 0. The following set of 27 points and 27 lines is a 27_4 configuration, since every point is on 4 lines and every line passes through 4 points: {{-2, 3, 1}, {-2, 4, -1}, {-1, -2, 4}, {-1, 0, 2}, {-1, 1, 1}, {-1, 1,2}, {-1, 2, 3}, {-1, 2, 4}, {0, 2, -1}, {0, 2, 1}, {1, -2, 3}, {1, -1, 1}, {1, 0, 2}, {1, 1, -1}, {1, 1, 2}, {1, 2, -1}, {1, 2,1}, {2, -1, 0}, {2, -1, 1}, {2, 1, 0}, {2, 1, 1}, {2, 3, -1}, {2, 4, -1}, {3, -1, 2}, {3, 1, -2}, {4, -1, -2}, {4, -1, 2}}.]

CROSSREFS

Sequence in context: A000844 A000863 A291947 * A002813 A263973 A364519

Adjacent sequences: A023991 A023992 A023993 * A023995 A023996 A023997

KEYWORD

hard,nonn

AUTHOR

Anton Betten (Anton.Betten(AT)uni-bayreuth.de)

STATUS

approved