%I

%S 9,15,19,25,31,37

%N Numbers n such that there is an arrangement of n points in the plane such that no point is on >= n/2 connecting lines.

%C Counterexamples to a conjecture of Dirac.

%D H. T. Croft, K. J. Falconer and R. K. Guy, Unsolved Problems in Geometry, F12.

%K nonn,nice,hard

%O 1,1

%A _N. J. A. Sloane_, Nov 30 2000