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%I #9 Sep 06 2014 01:39:35
%S 3,25,1885,3636297,327094648711
%N 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.
%C Here "substrings" have no "gaps", i.e. a substring means a subsequence of characters from the original string using contiguous indices.
%C 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.
%Y Cf. A166315.
%K nonn,more
%O 1,1
%A _Peter M. Huggins_, Aug 28 2014
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