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
KEYWORD
nonn,more
AUTHOR
Peter M. Huggins, Aug 28 2014
STATUS
approved