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 A134428 #12 Aug 23 2019 15:42:12
%S A134428 23,67,1213,3637,4243,7879,9697,102103,108109,126127,138139,150151,
%T A134428 156157,180181,192193,270271,276277,312313,330331,378379,420421,
%U A134428 432433,438439,456457,522523,540541,546547,576577,600601,606607,612613,618619
%N A134428 Primes formed by concatenating k with k+1, where k+1 is a prime.
%C A134428 Subset of A030458 and A052087.
%H A134428 Paolo P. Lava, <a href="/A134428/b134428.txt">Table of n, a(n) for n = 1..2500</a>
%p A134428 P:=proc(n) local a;
%p A134428 a:=(ithprime(n)-1)*10^(ilog10(ithprime(n))+1)+ithprime(n);
%p A134428 if isprime(a) then a; fi; end: seq(P(i),i=1..10^3);
%t A134428 Select[Table[(p-1)*10^IntegerLength[p]+p,{p,Prime[Range[200]]}],PrimeQ] (* _Harvey P. Dale_, Aug 23 2019 *)
%Y A134428 Cf. A030458, A052087.
%K A134428 nonn,base,easy
%O A134428 1,1
%A A134428 _Paolo P. Lava_ and _Giorgio Balzarotti_, Jan 18 2008