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 A262573 #28 Sep 17 2019 17:25:13
%S A262573 1,12,124,1245,12456,124567,1245678,12456789,1245678910,124567891011,
%T A262573 12456789101112,1245678910111213,124567891011121314,
%U A262573 12456789101112131415,1245678910111213141516,124567891011121314151617,12456789101112131415161718,1245678910111213141516171819,124567891011121314151617181920
%N A262573 Concatenation of the numbers from 1 to n but omitting 3.
%C A262573 The only prime among the first 98 terms is a(6) = 124567. Does the sequence contain any other primes?
%C A262573 No other primes through a(10000). - _Robert Price_, Nov 04 2018
%t A262573 DeleteDuplicates[Table[FromDigits[Flatten[IntegerDigits[Complement[Range[n], {3}]]]], {n, 20}]] (* _Robert Price_, Nov 05 2018 *)
%t A262573 Join[{1,12},Table[FromDigits[Join[{1,2},Flatten[IntegerDigits/@Range[4,n]]]],{n,4,20}]] (* _Harvey P. Dale_, Sep 17 2019 *)
%Y A262573 Cf. A007908, A262299, A262571-A262582.
%Y A262573 See A262300 for more about this problem.
%K A262573 nonn,base
%O A262573 1,2
%A A262573 _N. J. A. Sloane_, Sep 25 2015