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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 (list; graph; refs; listen; history; text; internal format)
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.

LINKS

Table of n, a(n) for n=1..5.

CROSSREFS

Cf. A166315.

Sequence in context: A219275 A101733 A094815 * A183248 A144788 A182135

Adjacent sequences:  A246533 A246534 A246535 * A246537 A246538 A246539

KEYWORD

nonn,more

AUTHOR

Peter M. Huggins, Aug 28 2014

STATUS

approved

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Last modified June 15 17:43 EDT 2019. Contains 324142 sequences. (Running on oeis4.)