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 A084670 #20 Apr 09 2022 09:31:00 %S A084670 3,9,19,21,37,63,77,81,87,107,121,133,177,201,211,213,217,281,293,303, %T A084670 321,327,329,333,351,391,393,439,481,503,507,519,543,547,551,561,579, %U A084670 581,599,621,639,657,663,667,711,721,727,743,793,813,819,827,829,831,837 %N A084670 Numbers k such that concatenation of prime(k) and k is prime. %H A084670 Harvey P. Dale, <a href="/A084670/b084670.txt">Table of n, a(n) for n = 1..1000</a> %e A084670 9 is a term because prime(9) = 23 and concatenation of 23 and 9 is prime %t A084670 Select[Range[1000],PrimeQ[Prime[#]10^IntegerLength[#]+#]&] (* _Harvey P. Dale_, Apr 09 2022 *) %o A084670 (PARI) is(k) = ispseudoprime(eval(Str(prime(k), k))); \\ _Jinyuan Wang_, Apr 10 2020 %o A084670 (Python) %o A084670 from sympy import isprime, prime %o A084670 def aupto(lim): %o A084670 return [k for k in range(1, lim+1) if isprime(int(str(prime(k))+str(k)))] %o A084670 print(aupto(837)) # _Michael S. Branicky_, Mar 09 2021 %Y A084670 Cf. A075110, A084669. %K A084670 nonn,base %O A084670 1,1 %A A084670 _Zak Seidov_, Jun 29 2003 %E A084670 More terms from _Jinyuan Wang_, Apr 10 2020