

A115562


a(n) = number of distinct squarefree ternary (cyclic) sequences uniquely containing every possible lengthn substring.


0




OFFSET

1,1


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 ?


LINKS

Table of n, a(n) for n=1..10.


EXAMPLE

a(2) = 3 because the following 3 sequences contain each length2 substring {01,02,10,12,20,21} while avoiding any square {00,11,22} and are all distinct from each other:
010212
012021
012102


CROSSREFS

Sequence in context: A122830 A321296 A190902 * A127468 A173720 A173717
Adjacent sequences: A115559 A115560 A115561 * A115563 A115564 A115565


KEYWORD

hard,nonn


AUTHOR

Jim Nastos, Mar 11 2006


STATUS

approved



