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 A260107 #16 Jul 18 2015 12:30:51 %S A260107 1,4,5,6,13,16,19,20,21,22,23,24,25,40,41,42,49,50,51,58,61,64,67,70, %T A260107 73,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,121,124,127,128,129, %U A260107 130,131,132,133,148,151,154,155,156,157,158,159,160,175,176 %N A260107 Lexicographically first increasing sequence of positive integers such that there are exactly a(k) terms less than or equal to 3*a(k), for each k. %C A260107 See A130011 and A260139 for other variants of this self-describing sequence. %H A260107 Nathaniel Johnston, <a href="/A260107/b260107.txt">Table of n, a(n) for n = 1..10000</a> %F A260107 a(n) <= 3n-2, and there are infinitely many indices (namely, all those of the form n = a(k)+1 for some k) for which equality holds. %p A260107 l:=[1, 4]:for n from 2 to 20 do for j from l[n-1]+1 to `if`(n=2, l[n]-1, l[n]) do l:=[op(l), max(3*l[n-1], op(l))+1]: od: od: l; # _Nathaniel Johnston_, Apr 27 2011 %o A260107 (PARI) a=vector(100,i,1);i=v=1;for(k=2,#a,if(k>a[i],v=3*a[i];i++);a[k]=v++) %Y A260107 Cf. A130011, A037988, A094591. %K A260107 nonn,easy %O A260107 1,2 %A A260107 _M. F. Hasler_, Jul 16 2015