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 A137740 #15 Oct 18 2020 14:54:19
%S A137740 1,32,138,348,700,1246,2050,3188,4749,6836,9567,13076,17514,23050,
%T A137740 29872,38188,48227,60240,74501,91308,110984,133878,160366,190852,
%U A137740 225769,265580,310779,361892,419478,484130,556476,637180,726943,826504,936641,1058172,1191956
%N A137740 Number of different strings of length n+5 obtained from "123...n" by iteratively duplicating any substring.
%C A137740 See A137743 for comments and examples.
%H A137740 <a href="/index/Do#repeat">Index entries for doubling substrings</a>
%H A137740 <a href="/index/Rec#order_06">Index entries for linear recurrences with constant coefficients</a>, signature (6,-15,20,-15,6,-1).
%F A137740 a(n) = (n+4)(n^2+3n-8)(n^2+23n+150)/5!+4 for n>4.
%F A137740 G.f.: x*(x^8+2*x^7-7*x^6-20*x^5+57*x^4-20*x^3-39*x^2+26*x+1) / (x-1)^6. - _Colin Barker_, Nov 04 2013
%t A137740 LinearRecurrence[{6,-15,20,-15,6,-1},{1,32,138,348,700,1246,2050,3188,4749},40] (* _Harvey P. Dale_, Oct 18 2020 *)
%o A137740 (PARI) A137740(n)=if(n<2,1,n=A135473(n+5,n);n[ #n]) /* function A135473 defined in A137743 */
%o A137740 (PARI) A137740(n)=if(n>4,n*(n*(n*(n*(n+30)+315)+1110)-136)/5!-36,[1,32,138,348][n])
%Y A137740 Cf. A137739-A137743, A135473, A137744-A137748.
%K A137740 nonn,easy
%O A137740 1,2
%A A137740 _M. F. Hasler_, Feb 10 2008
%E A137740 More terms from _Colin Barker_, Nov 04 2013