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 A138957 #8 May 26 2022 09:53:10
%S A138957 1,12,123,1234,12345,123456,1234567,12345678,123456789,12345678901,
%T A138957 1234567890111,123456789011121,12345678901112131,1234567890111213141,
%U A138957 123456789011121314151,12345678901112131415161
%N A138957 Concatenation of the reversed digits of numbers from 1 to n.
%C A138957 There are no primes in this sequence for n<=7000
%t A138957 b = {}; a = {}; Do[w = RealDigits[n]; w = First[w]; Do[AppendTo[a, w[[Length[w] - k + 1]]], {k, 1, Length[w]}]; p = FromDigits[a]; AppendTo[b, p], {n, 1, 21}]; b
%t A138957 (* or *)
%t A138957 Table[FromDigits[Flatten[Reverse/@IntegerDigits[Range[n]]]],{n,20}] (* _Harvey P. Dale_, Oct 22 2013 *)
%Y A138957 Cf. A104759, A000422, A116504, A007908, A116505, A104759, A138789, A138790.
%K A138957 nonn,base
%O A138957 1,2
%A A138957 _Artur Jasinski_, Apr 04 2008, Apr 05 2008