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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
OFFSET
1,1
LINKS
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. A185199 (asymptotic density of this sequence).
Sequence in context: A362680 A309325 A368729 * A143450 A225563 A373512
KEYWORD
nonn
AUTHOR
EXTENSIONS
Data corrected by Amiram Eldar, Aug 30 2020
STATUS
approved