A024556 Odd squarefree composite numbers.

15, 21, 33, 35, 39, 51, 55, 57, 65, 69, 77, 85, 87, 91, 93, 95, 105, 111, 115, 119, 123, 129, 133, 141, 143, 145, 155, 159, 161, 165, 177, 183, 185, 187, 195, 201, 203, 205, 209, 213, 215, 217, 219, 221, 231, 235, 237, 247, 249, 253, 255, 259, 265, 267, 273

Offset: 1

Author

N. J. A. Sloane, May 22 2000

Keywords

• nonn
• easy

Comments

Composite numbers n such that Sum_{k=1..n-1} floor(k^3/n) = (1/4)*(n-2)*(n^2-1) (equality also holds for all primes). - Benoit Cloitre, Dec 08 2002

Links

• Zak Seidov, Table of n, a(n) for n = 1..11999.
• Eric Weisstein's World of Mathematics, Lehmer's Constant
• Eric Weisstein's World of Mathematics, Prime Sums

Crossrefs

Intersection of A056911 and A071904.

Subsequence of A061346.

Cf. A010051, A046388, A078837.

Programs

Haskell

a024556 n = a024556_list !! (n-1)
a024556_list = filter ((== 0) . a010051) $ tail a056911_list
-- Reinhard Zumkeller, Apr 12 2012

Mathematica

Complement[Select[Range[3,281,2],SquareFreeQ],Prime[Range[PrimePi[281]]]] (* Harvey P. Dale, Jan 26 2011 *)

PARI

is(n)=n>1&&n%2&&!isprime(n)&&issquarefree(n) \\ Charles R Greathouse IV, Apr 12 2012

PARI

forstep(n=3,273,2,k=omega(n);if(k>1&&bigomega(n)==k,print1(n,", "))) \\ Hugo Pfoertner, Dec 19 2018

Formula

a(n) = (Pi^2/4)*n + O(n/log n). - Charles R Greathouse IV, Mar 12 2025

Extensions

More terms from James Sellers, May 22 2000