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A081770 Numbers twice their squarefree kernel (A007947). 6
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 (list; graph; refs; listen; history; text; internal format)
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

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Last modified November 30 19:02 EST 2021. Contains 349424 sequences. (Running on oeis4.)