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 A272617 #19 Dec 23 2019 18:57:06
%S A272617 11,2112,321123,43211234,5432112345,654321123456,76543211234567,
%T A272617 8765432112345678,987654321123456789,1098765432112345678910,
%U A272617 11109876543211234567891011,121110987654321123456789101112,1312111098765432112345678910111213,14131211109876543211234567891011121314
%N A272617 Concatenation of the numbers from n down to 1 with numbers from 1 to n.
%C A272617 Conjecture: a(1) is the only prime number.
%C A272617 No other prime terms up to a(8000). - _Giovanni Resta_, May 07 2016
%H A272617 Andrew Howroyd, <a href="/A272617/b272617.txt">Table of n, a(n) for n = 1..100</a>
%F A272617 a(n) = A000422(n) concatenated with A007908(n).
%e A272617 a(1) = concatenate("1", "1") = 11.
%e A272617 a(2) = concatenate("2", "1", "1", "2") = 2112.
%e A272617 a(3) = concatenate("3", "2", "1", "1", "2", "3") = 321123.
%t A272617 FromDigits@Flatten@IntegerDigits@Join[Reverse@#, #] & /@ Table[Range@n, {n, 20}]
%o A272617 (PARI) a(n)={fromdigits(concat(concat(Vecrev(vector(n,i,digits(i)))), concat(vector(n,i,digits(i)))))} \\ _Andrew Howroyd_, Dec 23 2019
%Y A272617 Cf. A259937, A007942.
%K A272617 nonn,base
%O A272617 1,1
%A A272617 _José de Jesús Camacho Medina_, May 03 2016
%E A272617 Terms a(11) and beyond from _Andrew Howroyd_, Dec 23 2019