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 A262574 #24 Nov 05 2018 17:27:55
%S A262574 1,12,123,1235,12356,123567,1235678,12356789,1235678910,123567891011,
%T A262574 12356789101112,1235678910111213,123567891011121314,
%U A262574 12356789101112131415,1235678910111213141516,123567891011121314151617,12356789101112131415161718,1235678910111213141516171819,123567891011121314151617181920
%N A262574 Concatenation of the numbers from 1 to n but omitting 4.
%C A262574 The only prime among the first 98 terms is a(8) = 12356789. Does the sequence contain any other primes?
%C A262574 No other primes through a(10000). - _Robert Price_, Nov 03 2018
%t A262574 DeleteDuplicates[Table[FromDigits[Flatten[IntegerDigits[Complement[Range[n], {4}]]]], {n, 20}]] (* _Robert Price_, Nov 05 2018 *)
%Y A262574 Cf. A007908, A262299, A262571-A262582.
%Y A262574 See A262300 for more about this problem.
%K A262574 nonn,base
%O A262574 1,2
%A A262574 _N. J. A. Sloane_, Sep 25 2015