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 A224872 #21 May 27 2021 16:35:33 %S A224872 3,15,20,29,58,63,80,90,92,93,96,116,125,126,143,151,155,156,164,182, %T A224872 185,189,210,230,251,255,256,268,274,275,294,298,319,323,324,325,328, %U A224872 330,343,360,362,399,400,416,425,429,437,446,457,463,466,480,497,505,506 %N A224872 Numbers k such that prime(k)*pi(k)+1 is prime. %H A224872 Charles R Greathouse IV, <a href="/A224872/b224872.txt">Table of n, a(n) for n = 1..10000</a> %e A224872 Prime(126)*pi(126)+1 = 701*30+1 = 21031 which is prime, hence 126 is in the sequence. %p A224872 A224872:=proc(n) local a; a:=(ithprime(n)*numtheory[pi](n)+1) ; if isprime(a) then RETURN(n) end if:end proc: seq(A224872(n),n=1..600); %t A224872 Select[Range[2000],PrimeQ[Prime[#]*PrimePi[#]+1]&] %o A224872 (PARI) n=0;forprime(p=2,1e5,if(isprime(p*primepi(n++)+1), print1(n", "))) \\ _Charles R Greathouse IV_, Jul 23 2013 %Y A224872 Cf. A065042. %K A224872 nonn %O A224872 1,1 %A A224872 _K. D. Bajpai_, Jul 23 2013