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A115562
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a(n) = number of distinct squarefree ternary (cyclic) sequences uniquely containing every possible length-n substring.
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0
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OFFSET
| 1,1
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COMMENTS
| 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 ?
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EXAMPLE
| 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:
010212
012021
012102
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CROSSREFS
| Sequence in context: A004179 A122830 A190902 * A127468 A173720 A173717
Adjacent sequences: A115559 A115560 A115561 * A115563 A115564 A115565
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KEYWORD
| hard,nonn
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AUTHOR
| Jim Nastos (nastos(AT)gmail.com), Mar 11 2006
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