A081770 Numbers twice their squarefree kernel (A007947).

4, 12, 20, 28, 44, 52, 60, 68, 76, 84, 92, 116, 124, 132, 140, 148, 156, 164, 172, 188, 204, 212, 220, 228, 236, 244, 260, 268, 276, 284, 292, 308, 316, 332, 340, 348, 356, 364, 372, 380, 388, 404, 412, 420, 428, 436, 444, 452, 460, 476, 492, 508, 516, 524

Offset: 1

Author

Reinhard Zumkeller, Apr 10 2003

Keywords

• nonn
• easy

Comments

From Amiram Eldar, Nov 02 2020: (Start)

Numbers k such that A280292(k) = 2.

The asymptotic density of this sequence is 1/Pi^2 (A092742). (End)

Examples

			84=2*2*3*7=2*(2*3*7)=2*rad(84), therefore 84 is a term.
		

Links

• Reinhard Zumkeller, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A001414, A005117, A007947, A008472, A008966, A013929, A039956, A056911, A092742, A280292.

Subsequence of A017113.

Programs

Haskell

a081770 n = a081770_list !! (n-1)
a081770_list = filter ((== 1) . a008966 . (`div` 4)) a017113_list
-- Reinhard Zumkeller, Jul 13 2013

Mathematica

4 * Select[Range[1, 100, 2], SquareFreeQ] (* Amiram Eldar, Nov 02 2020 *)

PARI

is(n)=n%8==4 && issquarefree(n/4) \\ Charles R Greathouse IV, Jul 09 2013

Formula

a(n) = 2*A039956(n) = 4*A056911(n).