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 A077309 #8 May 13 2017 10:20:49
%S A077309 1,23,345,4567,56789,67891011,78910111213,89101112131415,
%T A077309 91011121314151617,10111213141516171819,1112131415161718192021,
%U A077309 121314151617181920212223
%N A077309 Concatenation of n numbers starting with n.
%H A077309 Michael De Vlieger, <a href="/A077309/b077309.txt">Table of n, a(n) for n = 1..333</a>
%t A077309 Table[FromDigits@ Flatten@ Take[#, {n, n + (n - 1)}], {n, Ceiling[Length[#]/2]}] &@ Array[IntegerDigits, 23] (* _Michael De Vlieger_, May 13 2017 *)
%K A077309 base,easy,nonn
%O A077309 1,2
%A A077309 _Amarnath Murthy_, Nov 03 2002
%E A077309 a(7) corrected by _Michael De Vlieger_, May 13 2017