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A246536
a(n) = number of strings (including the empty string) over an alphabet of size n that do not have any substrings of length > 1 that appear more than once in the string.
0
3, 25, 1885, 3636297, 327094648711
OFFSET
1,1
COMMENTS
Here "substrings" have no "gaps", i.e. a substring means a subsequence of characters from the original string using contiguous indices.
The number of De Bruijn sequences B(n,2) (which has a known explicit formula) can be used to give the fairly tight lower bound that a(n) > 2*n^2*B(n,2). See A166315.
CROSSREFS
Cf. A166315.
Sequence in context: A101733 A309116 A094815 * A183248 A144788 A182135
KEYWORD
nonn,more
AUTHOR
Peter M. Huggins, Aug 28 2014
STATUS
approved