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 A034809 #20 Apr 06 2021 01:56:22 %S A034809 4,5,9,10,16,24,33,36,42,46,51,53,56,59,63,66,67,69,75,76,78,81,87,96, %T A034809 102,106,108,111,114,116,123,125,129,130,135,137,144,145,147,148,153, %U A034809 156,159,170,171,177,179,180,184,187,190,192,195,196,198,207,211,214 %N A034809 Numbers k such that the concatenation of k and previous_prime(k) is a prime. %H A034809 Jon E. Schoenfield, <a href="/A034809/b034809.txt">Table of n, a(n) for n = 1..10000</a> (first 1001 terms from Harvey P. Dale) %e A034809 k=156 is a term because the largest prime < 156 is 151 and '156151' is a prime. %t A034809 Select[Range[250],PrimeQ[FromDigits[Join[IntegerDigits[#], IntegerDigits[ NextPrime[ #,-1]]]]]&] (* _Harvey P. Dale_, Jul 10 2017 *) %o A034809 (Python) %o A034809 from sympy import isprime, prevprime %o A034809 def ok(n): return isprime(int(str(n) + str(prevprime(n)))) %o A034809 print(list(filter(ok, range(3, 215)))) # _Michael S. Branicky_, Apr 05 2021 %Y A034809 Cf. A034808-A034821. %K A034809 nonn,base %O A034809 1,1 %A A034809 _Patrick De Geest_, Oct 15 1998