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 A299867 #16 Feb 25 2018 21:44:06
%S A299867 4,40,396,3963,39636,396357,3963567,39635676,396356757,3963567567,
%T A299867 39635675670,396356756706,3963567567057,39635675670567,
%U A299867 396356756705673,3963567567056727,39635675670567276,396356756705672757,3963567567056727567,39635675670567275672,396356756705672756722
%N A299867 The sum of the first n terms of the sequence is the concatenation of the first n digits of the sequence, and a(1) = 4.
%C A299867 The sequence starts with a(1) = 4 and is always extended with the smallest integer not yet present in the sequence and not leading to a contradiction.
%H A299867 Jean-Marc Falcoz, <a href="/A299867/b299867.txt">Table of n, a(n) for n = 1..300</a>
%F A299867 a(n) = c(n) - c(n-1), where c(n) = concatenation of the first n digits, c(n) ~ 0.44*10^n, a(n) ~ 0.396*10^n. See A300000 for the proof. - _M. F. Hasler_, Feb 22 2018
%e A299867 4 + 40 = 44 which is the concatenation of 4 and 4.
%e A299867 4 + 40 + 396 = 440 which is the concatenation of 4, 4, and 0.
%e A299867 4 + 40 + 396 + 3963 = 4403 which is the concatenation of 4, 4, 0 and 3.
%e A299867 From n = 3 on, a(n) can be computed directly as c(n) - c(n-1), cf. formula: a(3) = 440 - 4 = 396, a(4) = 4403 - 440 = 3963, etc. - _M. F. Hasler_, Feb 22 2018
%o A299867 (PARI) a(n,show=1,a=4,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 A299867 A300000 is the lexicographically first sequence of this type, with a(1) = 1.
%Y A299867 Cf. A299865, ..., A299872 for variants with a(1) = 2, ..., 9.
%K A299867 nonn,base,easy
%O A299867 1,1
%A A299867 _Eric Angelini_ and _Jean-Marc Falcoz_, Feb 21 2018