A224872 Numbers k such that prime(k)*pi(k)+1 is prime.

3, 15, 20, 29, 58, 63, 80, 90, 92, 93, 96, 116, 125, 126, 143, 151, 155, 156, 164, 182, 185, 189, 210, 230, 251, 255, 256, 268, 274, 275, 294, 298, 319, 323, 324, 325, 328, 330, 343, 360, 362, 399, 400, 416, 425, 429, 437, 446, 457, 463, 466, 480, 497, 505, 506

Offset: 1

Author

K. D. Bajpai, Jul 23 2013

Keywords

• nonn

Examples

			Prime(126)*pi(126)+1 = 701*30+1 = 21031 which is prime, hence 126 is in the sequence.
		

Links

• Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A065042.

Programs

Maple

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);

Mathematica

Select[Range[2000],PrimeQ[Prime[#]*PrimePi[#]+1]&]

PARI

n=0;forprime(p=2,1e5,if(isprime(p*primepi(n++)+1), print1(n", "))) \\ Charles R Greathouse IV, Jul 23 2013