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A192863 Lower flat numbers: odd numbers k such that k-1 is a squarefree number times a power of two. 3
3, 5, 7, 9, 11, 13, 15, 17, 21, 23, 25, 27, 29, 31, 33, 35, 39, 41, 43, 45, 47, 49, 53, 57, 59, 61, 63, 65, 67, 69, 71, 75, 77, 79, 81, 83, 85, 87, 89, 93, 95, 97, 103, 105, 107, 111, 113, 115, 117, 119, 121, 123, 125, 129, 131, 133, 135, 137, 139, 141, 143, 147 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000

Kevin A. Broughan and Zhou Qizhi, Flat primes and thin primes, Bulletin of the Australian Mathematical Society, Vol. 82, No. 2 (2010), pp. 282-292, alternative link.

FORMULA

a(n) ~ Pi^2/4 * n.

MATHEMATICA

Select[Range[3, 150, 2], SquareFreeQ[(# - 1)/2^IntegerExponent[# - 1, 2]] &] (* Amiram Eldar, Aug 30 2020 *)

PROG

(PARI) is(n)=n%2&&issquarefree((n-1)>>valuation(n-1, 2)) \\ corrected by Amiram Eldar, Aug 30 2020

CROSSREFS

Cf. A192861, A192862, A192864.

Cf. A185199 (asymptotic density of this sequence).

Sequence in context: A061808 A248608 A309325 * A143450 A225563 A294923

Adjacent sequences:  A192860 A192861 A192862 * A192864 A192865 A192866

KEYWORD

nonn

AUTHOR

Charles R Greathouse IV, Jul 11 2011

EXTENSIONS

Data corrected by Amiram Eldar, Aug 30 2020

STATUS

approved

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Last modified May 7 02:20 EDT 2021. Contains 343636 sequences. (Running on oeis4.)