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 A262580 #20 Nov 05 2018 17:33:08
%S A262580 1,12,123,1234,12345,123456,1234567,12345678,123456789,12345678911,
%T A262580 1234567891112,123456789111213,12345678911121314,1234567891112131415,
%U A262580 123456789111213141516,12345678911121314151617,1234567891112131415161718,123456789111213141516171819,12345678911121314151617181920
%N A262580 Concatenation of the numbers from 1 to n but omitting 10.
%C A262580 The smallest prime in this sequence is the 61457-digit (probable) prime a(14512) = 123456789111213...14513 found by David Broadhurst on Sep 28 2015.
%t A262580 DeleteDuplicates[Table[FromDigits[Flatten[IntegerDigits[Complement[Range[n], {10}]]]], {n, 20}]] (* _Robert Price_, Nov 05 2018 *)
%Y A262580 Cf. A007908, A262299, A262571-A262582.
%Y A262580 See A262300 for more about this problem.
%K A262580 nonn,base
%O A262580 1,2
%A A262580 _N. J. A. Sloane_, Sep 25 2015