A289279 Number of odd composite numbers in ]2*(n-1)^2, 2*n^2[.

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, 38, 39, 39, 41, 44, 46, 47, 48, 49, 50, 57, 51, 56, 59, 63, 59, 65, 61, 67, 67, 69, 73, 72, 73, 77, 78, 82, 80, 80, 85, 84, 87, 90, 90, 94, 98, 90, 97, 102, 100

Offset: 1

Author

Ralf Steiner, Jul 01 2017

Keywords

• nonn

Crossrefs

Cf. A285786, A006880.

Programs

Mathematica

Table[Count[Select[Range[2(n-1)^2,2 n^2],!PrimeQ[#]&&OddQ[#]&&(#>1) &], _Integer],{n,1,100}]
Table[2 n - 1 - (PrimePi[2 n^2] - PrimePi[2 (n - 1)^2]), {n, 1, 100}] (* Ralf Steiner, Jul 30 2017 *)

PARI

a(n) = sum(k=2*(n-1)^2, 2*n^2, (k%2) && (k!=1) && !isprime(k)); \\ Michel Marcus, Jul 01 2017

Formula

a(n) = 2n - 1 - A285786(n). - Ralf Steiner, Jul 30 2017