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.
-clusters for 
." Math. Comput. 53, 439-444,
1989.