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 A158521 #10 Feb 03 2019 04:02:17
%S A158521 19,61,103,127,241,331,337,367,523,577,709,829,997,1009,1129,1213,
%T A158521 1231,1321,1381,1489,1543,1627,1861,2113,2137,2287,2347,2383,2689,
%U A158521 2851,2953,2971,3187,3499,3559,3583,3673,3967,4219,4243,4327,4363,4513,4591,4789
%N A158521 Primes which yield primes when "13" is prefixed or appended.
%C A158521 Primes in A158232.
%C A158521 It is conjectured that this sequence is infinite.
%D A158521 Richard E. Crandall, Carl Pomerance, Prime Numbers, Springer, 2005.
%D A158521 Wladyslaw Narkiewicz, The development of prime number theory, Springer, 2000.
%H A158521 Harvey P. Dale, <a href="/A158521/b158521.txt">Table of n, a(n) for n = 1..1000</a>
%F A158521 Prime p is a term if the concatenations "13p" and "p13" both yield primes.
%e A158521 Prime p=3 is not a term: "p13"=313 is prime but "13p"=133 = 7*19.
%e A158521 For p=19, both 1319 and 1913 are prime; this is the first prime that meets the requirements of the definition, so a(1)=19.
%p A158521 cat2 := proc(a,b) ndigsb := max(ilog10(b)+1,1) ; a*10^ndigsb+b ; end: for i from 1 to 800 do p := ithprime(i) ; if isprime(cat2(13,p)) and isprime(cat2(p,13)) then printf("%d,",p) ; fi; od: # _R. J. Mathar_, Apr 02 2009
%t A158521 Select[Prime[Range[1000]],AllTrue[{13*10^IntegerLength[#]+#,100#+13}, PrimeQ]&] (* The program uses the AllTrue function from Mathematica version 10 *) (* _Harvey P. Dale_, Apr 17 2015 *)
%Y A158521 Cf. A158232, A157772.
%K A158521 nonn,base
%O A158521 1,1
%A A158521 Eva-Maria Zschorn (e-m.zschorn(AT)zaschendorf.km3.de), Mar 20 2009
%E A158521 337, 1231, 1321 inserted by _R. J. Mathar_, Apr 02 2009