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A065896 Number of composites <= 2*n. 1

%I

%S 0,1,2,3,5,6,7,9,10,11,13,14,16,18,19,20,22,24,25,27,28,29,31,32,34,

%T 36,37,39,41,42,43,45,47,48,50,51,52,54,56,57,59,60,62,64,65,67,69,71,

%U 72,74,75,76,78,79,80,82,83,85,87,89,91,93,95,96,98,99,101,103,104,105

%N Number of composites <= 2*n.

%H Harry J. Smith, <a href="/A065896/b065896.txt">Table of n, a(n) for n = 1..1000</a>

%F a(n) = A065855(2*n). - _Reinhard Zumkeller_, Oct 14 2014

%e a(9) = 10, since 18 is the 10th composite number: 4, 6, 8, 9, 10, 12, 14, 15, 16, 18.

%t Table[2*n-(PrimePi[2*n])-1, {n, 1, 128}]

%o (PARI) { for (n=1, 1000, f=2*n; a=f - primepi(f) - 1; write("b065896.txt", n, " ", a) ) } \\ _Harry J. Smith_, Nov 04 2009

%o (Haskell)

%o a065896 = a065855 . (* 2) -- _Reinhard Zumkeller_, Oct 14 2014

%Y Cf. A002808, A000720, A005843, A065855.

%K nonn

%O 1,3

%A _Labos Elemer_, Nov 28 2001

%E More precise definition from _Michel Marcus_, Mar 07 2021~

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Last modified May 8 13:26 EDT 2021. Contains 343666 sequences. (Running on oeis4.)