%I #10 Nov 21 2017 22:22:26

%S 1,1,4,19,1972,971171

%N Number of isomorphism types of symmetric configurations n_4.

%H A. Betten, <a href="http://www.mathe2.uni-bayreuth.de/home/research.html">More information</a>

%H Ed Pegg, Jr., <a href="/A023994/a023994.jpg">An example of a 27_4 configuration</a> [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}}.]

%K hard,nonn

%O 13,3

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