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 A262579 #22 Nov 30 2019 16:22:45
%S A262579 1,12,123,1234,12345,123456,1234567,12345678,1234567810,123456781011,
%T A262579 12345678101112,1234567810111213,123456781011121314,
%U A262579 12345678101112131415,1234567810111213141516,123456781011121314151617,12345678101112131415161718,1234567810111213141516171819,123456781011121314151617181920
%N A262579 Concatenation of the numbers from 1 to n but omitting 9.
%C A262579 There are only two primes among the first 98 terms, namely
%C A262579 1234567810111213141516171819 and 1234567810111213141516171819202122232425262728293031323334353637.
%C A262579 Does this sequence contain any other primes?
%C A262579 No other primes through a(10000). - _Robert Price_, Nov 04 2018
%t A262579 DeleteDuplicates[Table[FromDigits[Flatten[IntegerDigits[Complement[Range[n], {9}]]]], {n, 20}]] (* _Robert Price_, Nov 05 2018 *)
%t A262579 Module[{nn=20,c},c=Drop[Range[nn],{9}];Table[FromDigits[Flatten[ IntegerDigits/@ Take[c,n]]],{n,nn-1}]] (* _Harvey P. Dale_, Nov 30 2019 *)
%Y A262579 Cf. A007908, A262299, A262571-A262582.
%Y A262579 See A262300 for more about this problem.
%K A262579 nonn,base
%O A262579 1,2
%A A262579 _N. J. A. Sloane_, Sep 25 2015