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a(n) = pi(n)*pi(2n), where pi is A000720: the prime counting function.
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%I #18 Sep 08 2022 08:46:21

%S 0,2,6,8,12,15,24,24,28,32,40,45,54,54,60,66,77,77,96,96,104,112,126,

%T 135,135,135,144,144,160,170,198,198,198,209,209,220,252,252,252,264,

%U 286,299,322,322,336,336,360,360,375,375,390,405,432,448,464,464,480,480,510

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

%H Jason Kimberley, <a href="/A304483/b304483.txt">Table of n, a(n) for n = 1..10000</a>

%t Array[PrimePi[#] PrimePi[2 #] &, 59] (* _Michael De Vlieger_, May 27 2018 *)

%o (Magma) A000720:=func<n|#PrimesUpTo(n)>;

%o A304483:=func<n|A000720(n)*A000720(2*n)>;

%o [A304483(n):n in[1..59]];

%o (PARI) a(n) = primepi(n) * primepi(2*n); \\ _Michel Marcus_, May 27 2018

%Y Cf. A304484(n) = A033270(n)*A033270(2n).

%Y Cf. A291440, A291538, A291539, A291540,

%K nonn,easy

%O 1,2

%A _Jason Kimberley_, May 13 2018