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 A262581 #28 Nov 05 2018 17:23:59
%S A262581 1,12,123,1234,12345,123456,1234567,12345678,123456789,12345678910,
%T A262581 1234567891012,123456789101213,12345678910121314,1234567891012131415,
%U A262581 123456789101213141516,12345678910121314151617,1234567891012131415161718,123456789101213141516171819,12345678910121314151617181920
%N A262581 Concatenation of the numbers from 1 to n but omitting 11.
%C A262581 There are only two primes among the first 98 terms, namely 123456789101213 and
%C A262581 1234567891012131415161718192021222324252627282930313233343536\
%C A262581 373839404142434445464748495051525354555657585960616263.
%C A262581 Does this sequence contain any other primes?
%C A262581 No other primes through a(10000). - _Robert Price_, Nov 05 2018
%t A262581 DeleteDuplicates[Table[FromDigits[Flatten[IntegerDigits[Complement[Range[n], {11}]]]], {n, 20}]] (* _Robert Price_, Nov 05 2018 *)
%o A262581 (PARI) terms(n) = my(s="", i=0); for(k=1, n, if(k!=11, s=concat(s, Str(k)); print1(eval(s), ", "); i++); if(i==n, break))
%o A262581 /* Print initial 20 terms as follows */
%o A262581 terms(20) \\ _Felix Fröhlich_, Nov 05 2018
%Y A262581 Cf. A007908, A262299, A262571-A262582.
%Y A262581 See A262300 for more about this problem.
%K A262581 nonn,base
%O A262581 1,2
%A A262581 _N. J. A. Sloane_, Sep 25 2015