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%I #5 Apr 22 2013 14:50:23
%S 2,3,0,6,0,0,0,0,0,0
%N a(n) = number of distinct squarefree ternary (cyclic) sequences uniquely containing every possible length-n substring.
%C Sometimes called "squarefree de Bruijn sequences" Two such sequences are distinct if they are not cyclic permutations of each other. Open: do any such ternary sequences exist for n>4 ?
%e a(2) = 3 because the following 3 sequences contain each length-2 substring {01,02,10,12,20,21} while avoiding any square {00,11,22} and are all distinct from each other:
%e 010212
%e 012021
%e 012102
%K hard,nonn
%O 1,1
%A _Jim Nastos_, Mar 11 2006
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