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 A299869 #14 Feb 25 2018 21:44:06
%S A299869 6,60,594,5945,59454,594535,5945351,59453514,594535135,5945351351,
%T A299869 59453513510,594535135104,5945351351035,59453513510351,
%U A299869 594535135103509,5945351351035091,59453513510350914,594535135103509135,5945351351035091351,59453513510350913508,594535135103509135082,5945351351035091350820
%N A299869 The sum of the first n terms of the sequence is the concatenation of the first n digits of the sequence, and a(1) = 6.
%C A299869 The sequence starts with a(1) = 6 and is always extended with the smallest integer not yet present in the sequence and not leading to a contradiction.
%H A299869 Jean-Marc Falcoz, <a href="/A299869/b299869.txt">Table of n, a(n) for n = 1..300</a>
%F A299869 a(n) = c(n) - c(n-1), where c(n) = concatenation of the first n digits, c(n) ~ 0.66*10^n, a(n) ~ 0.59*10^n. See A300000 for the proof. - _M. F. Hasler_, Feb 22 2018
%e A299869 6 + 60 = 66 which is the concatenation of 6 and 6.
%e A299869 6 + 60 + 594 = 660 which is the concatenation of 6, 6 and 0.
%e A299869 6 + 60 + 594 + 5945 = 6605 which is the concatenation of 6, 6, 0 and 5.
%e A299869 From n = 3 on, a(n) can be computed directly as c(n) - c(n-1), cf. formula: a(3) = 660 - 66 = 594, a(4) = 6605 - 660 = 5945, etc. - _M. F. Hasler_, Feb 22 2018
%o A299869 (PARI) a(n,show=1,a=6,c=a,d=[a])={for(n=2,n,show&&print1(a",");a=-c+c=c*10+d[1];d=concat(d[^1],if(n>2,digits(a))));a} \\ _M. F. Hasler_, Feb 22 2018
%Y A299869 A300000 is the lexicographically first sequence of this type, with a(1) = 1.
%Y A299869 Cf. A299865, ..., A299872 for variants with a(1) = 2, ..., 9.
%K A299869 nonn,base
%O A299869 1,1
%A A299869 _Eric Angelini_ and _Jean-Marc Falcoz_, Feb 21 2018