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A146167 Number of odd squarefree semiprimes (A046388) <= n. 1

%I #10 Aug 23 2021 13:22:54

%S 0,0,0,0,0,0,0,0,0,0,0,0,0,0,1,1,1,1,1,1,2,2,2,2,2,2,2,2,2,2,2,2,3,3,

%T 4,4,4,4,5,5,5,5,5,5,5,5,5,5,5,5,6,6,6,6,7,7,8,8,8,8,8,8,8,8,9,9,9,9,

%U 10,10,10,10,10,10,10,10,11

%N Number of odd squarefree semiprimes (A046388) <= n.

%C A346622 is a different although very similar sequence. - _N. J. A. Sloane_, Aug 23 2021

%F a(n) = A072000(n) - A000720(floor(sqrt(n))) - A000720(floor(n/2)) + 1.

%e a(33)= 3. The semiprimes <=33 are 15, 21 and 33. Formula gives 11-pi(5)-pi(16)+1 = 3.

%t Accumulate[Table[If[OddQ[n]&&SquareFreeQ[n]&&PrimeOmega[n]==2,1,0],{n,0,100}]] (* _Harvey P. Dale_, Feb 08 2016 *)

%Y Cf. A046388, A001358 (semiprimes), A072000 (Number of semiprimes <= n), A000720 (pi(n), the number of primes <= n).

%Y Cf. also A346622.

%K nonn

%O 1,21

%A _Washington Bomfim_, Oct 27 2008

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Last modified March 28 14:38 EDT 2024. Contains 371254 sequences. (Running on oeis4.)