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A304483 a(n) = pi(n)*pi(2n), where pi is A000720: the prime counting function.

%I A304483 #18 Sep 08 2022 08:46:21
%S A304483 0,2,6,8,12,15,24,24,28,32,40,45,54,54,60,66,77,77,96,96,104,112,126,
%T A304483 135,135,135,144,144,160,170,198,198,198,209,209,220,252,252,252,264,
%U A304483 286,299,322,322,336,336,360,360,375,375,390,405,432,448,464,464,480,480,510
%N A304483 a(n) = pi(n)*pi(2n), where pi is A000720: the prime counting function.
%H A304483 Jason Kimberley, <a href="/A304483/b304483.txt">Table of n, a(n) for n = 1..10000</a>
%t A304483 Array[PrimePi[#] PrimePi[2 #] &, 59] (* _Michael De Vlieger_, May 27 2018 *)
%o A304483 (Magma) A000720:=func<n|#PrimesUpTo(n)>;
%o A304483 A304483:=func<n|A000720(n)*A000720(2*n)>;
%o A304483 [A304483(n):n in[1..59]];
%o A304483 (PARI) a(n) = primepi(n) * primepi(2*n); \\ _Michel Marcus_, May 27 2018
%Y A304483 Cf. A304484(n) = A033270(n)*A033270(2n).
%Y A304483 Cf. A291440, A291538, A291539, A291540,
%K A304483 nonn,easy
%O A304483 1,2
%A A304483 _Jason Kimberley_, May 13 2018