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 A137741 #14 May 12 2017 02:55:30 %S A137741 1,16,54,119,218,360,555,814,1149,1573,2100,2745,3524,4454,5553,6840, %T A137741 8335,10059,12034,14283,16830,19700,22919,26514,30513,34945,39840, %U A137741 45229,51144,57618,64685,72380,80739,89799,99598,110175,121570,133824,146979,161078 %N A137741 Number of different strings of length n+4 obtained from "123...n" by iteratively duplicating any substring. %C A137741 See A137743 for comments and examples. %H A137741 <a href="/index/Do#repeat">Index entries for doubling substrings</a> %F A137741 a(n) = n^4 for n=1,2; a(n) = 1/24*(n+3)*(n^3+15*n^2+50*n-96) for n>2 (conjectured). %F A137741 G.f.: x*(x^6+x^5-8*x^4+x^3+16*x^2-11*x-1) / (x-1)^5 (conjectured). - _Colin Barker_, Nov 04 2013 %o A137741 (PARI) A137741(n)=if(n<2,1,n=A135473(n+4,n);n[ #n]) /* function A135473 defined in A137743 */ %Y A137741 Cf. A137740-A137743, A135473, A137744-A137748. %K A137741 nonn,easy %O A137741 1,2 %A A137741 _M. F. Hasler_, Feb 10 2008 %E A137741 More terms from _Colin Barker_, Nov 04 2013