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Numbers k such that pi(k) is prime.
3

%I #20 Jan 15 2023 02:08:53

%S 3,4,5,6,11,12,17,18,31,32,33,34,35,36,41,42,59,60,67,68,69,70,83,84,

%T 85,86,87,88,109,110,111,112,127,128,129,130,157,158,159,160,161,162,

%U 179,180,191,192,211,212,213,214,215,216,217,218,219

%N Numbers k such that pi(k) is prime.

%C A000720(a(n)) is prime. - _Omar E. Pol_, Dec 19 2008

%H T. D. Noe, <a href="/A048989/b048989.txt">Table of n, a(n) for n = 1..10000</a>

%t Select[Range[250],PrimeQ[PrimePi[#]]&] (* _Harvey P. Dale_, Mar 23 2011 *)

%o (PARI) isok(k) = isprime(primepi(k)); \\ _Michel Marcus_, Jan 15 2023

%Y Cf. A000720.

%K nonn,easy,nice

%O 1,1

%A _G. L. Honaker, Jr._