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 A034821 #7 Oct 06 2019 18:17:15
%S A034821 5,17,103,127,157,191,200,201,202,203,204,205,206,207,208,209,210,227,
%T A034821 234,235,236,237,238,258,259,260,261,262,307,347,354,355,356,357,358,
%U A034821 431,524,525,526,527,528,529,530,531,532,533,534,535,536,537,538,539
%N A034821 Concatenations C1 and C2 are both prime (see the comment lines).
%C A034821 C1 = 'prevprime(n) followed by nextprime(n)'
%C A034821 C2 = 'nextprime(n) followed by prevprime(n)'
%e A034821 n=17 -> next prime is 19, previous prime is 13, thus '1319' and '1913' are both primes.
%t A034821 c1c2pQ[n_]:=Module[{c1=NextPrime[n,-1],c2=NextPrime[n]},AllTrue[ {c1* 10^IntegerLength[ c2]+c2,c2*10^IntegerLength[c1]+c1},PrimeQ]]; Select[ Range[600],c1c2pQ] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Aug 23 2019 *)
%Y A034821 Cf. A034808-A034820.
%K A034821 nonn,base
%O A034821 0,1
%A A034821 _Patrick De Geest_, Oct 15 1998