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 A068715 #16 Sep 08 2022 08:45:05
%S A068715 11,31,271,811,21871,196831,5314411,3874204891,313810596091,
%T A068715 46383976865881019793281501678905914543189676980091,
%U A068715 2738927449953408337773479392637715347860807235997334411
%N A068715 Primes of the form 10*3^k + 1.
%H A068715 Vincenzo Librandi, <a href="/A068715/b068715.txt">Table of n, a(n) for n = 1..19</a>
%e A068715 21871 is a member as it is a concatenation of 2187 (= 3^7) and 1.
%t A068715 Select[Table[10*3^n+1,{n,0,300}],PrimeQ](* _Vincenzo Librandi_, Dec 08 2011 *)
%o A068715 (Magma) [a: n in [0..200] | IsPrime(a) where a is 10*3^n + 1]; // _Vincenzo Librandi_, Dec 08 2011
%Y A068715 Cf. A068712, A068714.
%K A068715 nonn
%O A068715 1,1
%A A068715 _Amarnath Murthy_, Mar 05 2002
%E A068715 More terms from _Sascha Kurz_, Mar 17 2002