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

1,1

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)

Links

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

Formula

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

Example

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

Mathematica

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

Prog

(PARI) is(n)=n%8==4 && issquarefree(n/4) \\ Charles R Greathouse IV, Jul 09 2013
(Haskell)
a081770 n = a081770_list !! (n-1)
a081770_list = filter ((== 1) . a008966 . (`div` 4)) a017113_list
-- Reinhard Zumkeller, Jul 13 2013

Crossrefs

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

Subsequence of A017113.

Sequence in context: A031065 A017113 A316489 * A062876 A301098 A301267

Adjacent sequences: A081767 A081768 A081769 * A081771 A081772 A081773

Keyword

nonn,easy

Author

Reinhard Zumkeller, Apr 10 2003

Status

approved