%I #5 Oct 04 2012 10:28:38
%S 0,1,2,3,4,5,6,7,8,9,10,6,12,13,14,15,16,15,18,11,20,21,22
%N Threshold function for orthogonal arrays of strength 2.
%C a(n) = max b such that any parameter set satisfying the obvious necessary conditions and with <= b degrees of freedom exists, but some such parameter set with b+1 degrees of freedom does not exist.
%H E. M. Rains, N. J. A. Sloane and J. Stufken, The Lattice of N-Run Orthogonal Arrays, J. Stat. Planning Inference, 102 (2002), 477-500 (<a href="http://neilsloane.com/doc/rao.txt">Abstract</a>, <a href="http://neilsloane.com/doc/rao.pdf">pdf</a>, <a href="http://neilsloane.com/doc/rao.ps">ps</a>)
%Y Cf. A048894, A048885.
%K nonn
%O 1,3
%A _N. J. A. Sloane_, John Stufken (jstufken(AT)iastate.edu)
%E 18<=a(24)<=22 is the first open case. Sequence then continues 24,25,26,15,28,29,30,29.