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 A259937 #59 Jan 30 2024 14:33:43
%S A259937 11,1221,123321,12344321,1234554321,123456654321,12345677654321,
%T A259937 1234567887654321,123456789987654321,1234567891010987654321,
%U A259937 12345678910111110987654321,123456789101112121110987654321
%N A259937 Concatenation of the numbers from 1 to n with numbers from n down to 1.
%C A259937 For n < 700, only a(1) and a(10) are primes.
%F A259937 a(n) = A007908(n) concatenated with A000422(n).
%e A259937 a(1) = 11.
%e A259937 a(5) = 1234554321.
%t A259937 FromDigits /@ Flatten /@ IntegerDigits /@ (#~Join~Reverse@ # & /@ Table[Range@ n, {n, 12}]) (* _Michael De Vlieger_, Jul 10 2015 *)
%o A259937 (PARI) a(n)=my(r="");for(i=0,n-1,r=concat(n-i,concat(r,n-i))); eval(r); \\ _Anders Hellström_, Aug 16 2015
%o A259937 (Python)
%o A259937 from itertools import chain
%o A259937 def A259937(n): return int(''.join(str(d) for d in chain(range(1,n+1),range(n,0,-1)))) # _Chai Wah Wu_, Dec 20 2021
%Y A259937 Cf. A000422, A007908, A244861.
%K A259937 nonn,base
%O A259937 1,1
%A A259937 _José de Jesús Camacho Medina_, Jul 09 2015