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A081770
Numbers twice their squarefree kernel (A007947).
8
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
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
KEYWORD
nonn,easy
AUTHOR
Reinhard Zumkeller, Apr 10 2003
STATUS
approved