This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A137746 #33 Jun 07 2025 13:05:30
%S A137746 0,0,0,0,0,0,1,6,26,100,360,1246,4217,14102,46861,155212,513336,
%T A137746 1697264,5614670,18594258,61671770,204907302,682110940,2275141754,
%U A137746 7603690251,25462152854,85428752530,287163766530,967046587261,3262356284310,11024401089607,37315689561280,126506891234231
%N A137746 Number of different strings of length n obtained from "abcdef" by iteratively duplicating any substring.
%C A137746 See A137743 for more comments.
%H A137746 <a href="/index/Do#repeat">Index entries for doubling substrings</a>
%e A137746 a(k) = 0 for k<6, since no shorter string can be obtained by duplication
%e A137746 a(6) = 1 = # { abcdef },
%e A137746 a(7) = 6 = # { aabcdef, abbcdef, abccdef, abcddef, abcdeef, abcdeff },
%e A137746 a(8) = 26 = # { aaabcdef, aabbcdef, aabccdef, aabcddef, aabcdeef, aabcdeff, ababcdef, abbbcdef, abbccdef, abbcddef, abbcdeef, abbcdeff, abcbcdef, abcccdef, abccddef, abccdeef, abccdeff, abcdcdef, abcdddef, abcddeef, abcddeff, abcdedef, abcdeeef, abcdeeff, abcdefef, abcdefff }.
%o A137746 (PARI) A135473(14,6) /* function defined in A137743 ... */
%Y A137746 Cf. A137740, A137741, A137742, A137743, A135473, A137744, A137745, A137746, A137747, A137748.
%K A137746 nonn
%O A137746 0,8
%A A137746 _M. F. Hasler_, Feb 10 2008
%E A137746 a(15)-a(16) from _Alois P. Heinz_, Aug 31 2011
%E A137746 a(17)-a(19) from _Lars Blomberg_, Jan 12 2013
%E A137746 a(20)-a(21) from _Michael S. Branicky_, Jan 06 2021
%E A137746 a(22)-a(23) from _Bert Dobbelaere_, Jun 11 2024
%E A137746 a(24)-a(32) from _Martin Fuller_, Jun 07 2025