An N-cluster is a point lattice configuration in which the distance between every pair of points is an integer, no three points are
collinear, and no four points are concyclic.
An example is the 6-cluster (0, 0), (132, ), (546, ), (960, ), (1155, 540), (546, 1120).

Call the radius of the smallest circle centered at one of the points of an N-cluster which contains all the points in the
N-cluster the extent. Noll and Bell (1989) found 91 nonequivalent
prime 6-clusters of extent less than 20937, but found
no 7-clusters.

Kreisel and Kurz (2006) subsequently found the 7-cluster given by multiplying the coordinates of the points ,
, , , , , by , illustrated above.

Guy, R. K. Unsolved Problems in Number Theory, 2nd ed. New York: Springer-Verlag, p. 187,
1994.Kreisel, T. and Kurz, S. "There are Integral Heptagons, No
Three Points on a Line, No Four on a Circle." 7 Nov 2006. http://arxiv.org/abs/0804.1303.Noll,
L. C. and Bell, D. I. "-clusters for ." Math. Comput.53, 439-444,
1989.