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 A034815 #11 Jul 16 2022 07:10:25
%S A034815 33,51,53,63,111,123,129,211,237,273,357,489,519,573,597,609,639,651,
%T A034815 653,657,669,681,687,747,753,819,831,873,891,987,997,1071,1611,1881,
%U A034815 2037,2049,2247,2271,2613,2763,3063,3267,3393,3573,3969,4251,4263,4293
%N A034815 Concatenations C1 and C2 are both prime (see the comment lines).
%C A034815 C1 = 'nextprime(n) followed by n'
%C A034815 C2 = 'n followed by prevprime(n)'
%e A034815 n=1881 -> next prime is 1889, previous prime is 1879, thus '18891881' and '18811879' are both primes.
%t A034815 okQ[n_]:=Module[{idn=IntegerDigits[n],ida,idb},ida=IntegerDigits[NextPrime[n,-1]];idb=IntegerDigits[NextPrime[n]];PrimeQ[FromDigits[Join[idn,ida]]]&&PrimeQ[FromDigits[Join[idb,idn]]]]
%t A034815 Select[Range[5000],okQ] (* _Harvey P. Dale_, Dec 25 2010 *)
%Y A034815 Cf. A034808-A034821.
%K A034815 nonn,base
%O A034815 0,1
%A A034815 _Patrick De Geest_, Oct 15 1998