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 A262572 #25 Nov 05 2018 17:28:29
%S A262572 1,13,134,1345,13456,134567,1345678,13456789,1345678910,134567891011,
%T A262572 13456789101112,1345678910111213,134567891011121314,
%U A262572 13456789101112131415,1345678910111213141516,134567891011121314151617,13456789101112131415161718,1345678910111213141516171819,134567891011121314151617181920
%N A262572 Concatenation of the numbers from 1 to n but omitting 2.
%t A262572 DeleteDuplicates[Table[FromDigits[Flatten[IntegerDigits[Complement[Range[n], {2}]]]], {n, 20}]] (* _Robert Price_, Nov 05 2018 *)
%Y A262572 For primes see A262298.
%Y A262572 Cf. A007908, A262299, A262571-A262582.
%Y A262572 See A262300 for more about this problem.
%K A262572 nonn,base
%O A262572 1,2
%A A262572 _N. J. A. Sloane_, Sep 25 2015