A289279 - OEIS

%I #29 Aug 01 2017 06:14:20

%S 0,0,2,3,5,6,8,9,11,10,14,15,18,18,19,21,24,23,27,28,28,32,31,34,35,

%T 38,39,39,41,44,46,47,48,49,50,57,51,56,59,63,59,65,61,67,67,69,73,72,

%U 73,77,78,82,80,80,85,84,87,90,90,94,98,90,97,102,100

%N Number of odd composite numbers in ]2*(n-1)^2, 2*n^2[.

%F a(n) = 2n - 1 - A285786(n). - _Ralf Steiner_, Jul 30 2017

%t Table[Count[Select[Range[2(n-1)^2,2 n^2],!PrimeQ[#]&&OddQ[#]&&(#>1) &], _Integer],{n,1,100}]

%t Table[2 n - 1 - (PrimePi[2 n^2] - PrimePi[2 (n - 1)^2]), {n, 1, 100}] (* _Ralf Steiner_, Jul 30 2017 *)

%o (PARI) a(n) = sum(k=2*(n-1)^2, 2*n^2, (k%2) && (k!=1) && !isprime(k)); \\ _Michel Marcus_, Jul 01 2017

%Y Cf. A285786, A006880.

%K nonn

%O 1,3

%A _Ralf Steiner_, Jul 01 2017