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 A262582 #26 Nov 05 2018 17:32:39
%S A262582 1,12,123,1234,12345,123456,1234567,12345678,123456789,12345678910,
%T A262582 1234567891011,123456789101113,12345678910111314,1234567891011131415,
%U A262582 123456789101113141516,12345678910111314151617,1234567891011131415161718,123456789101113141516171819,12345678910111314151617181920
%N A262582 Concatenation of the numbers from 1 to n but omitting 12.
%C A262582 The first (probable) prime in this sequence is a(960) = 1234567891011131415...961 (ending in 961), found by _David Broadhurst_, Sep 28 2015.
%C A262582 No other primes through a(10000). - _Robert Price_, Nov 04 2018
%t A262582 Module[{nn=30, c}, c=Drop[Range[nn], {12}]; Table[FromDigits[Flatten[IntegerDigits/@Take[c, n]]], {n, nn - 1}]] (* _Vincenzo Librandi_, Nov 05 2018 *)
%Y A262582 Cf. A007908, A262299, A262300, A262571-A262582.
%Y A262582 See A262300 for more about this problem.
%K A262582 nonn,base
%O A262582 1,2
%A A262582 _N. J. A. Sloane_, Sep 25 2015