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%I #13 Sep 01 2020 08:55:30
%S 3,5,7,9,11,13,15,17,21,23,25,27,29,31,33,35,39,41,43,45,47,49,53,57,
%T 59,61,63,65,67,69,71,75,77,79,81,83,85,87,89,93,95,97,103,105,107,
%U 111,113,115,117,119,121,123,125,129,131,133,135,137,139,141,143,147
%N Lower flat numbers: odd numbers k such that k-1 is a squarefree number times a power of two.
%H Amiram Eldar, <a href="/A192863/b192863.txt">Table of n, a(n) for n = 1..10000</a>
%H Kevin A. Broughan and Zhou Qizhi, <a href="https://doi.org/10.1017/S0004972710000067">Flat primes and thin primes</a>, Bulletin of the Australian Mathematical Society, Vol. 82, No. 2 (2010), pp. 282-292, <a href="http://www.math.waikato.ac.nz/~kab/papers/flatandthin4.pdf">alternative link</a>.
%F a(n) ~ Pi^2/4 * n.
%t Select[Range[3, 150, 2], SquareFreeQ[(# - 1)/2^IntegerExponent[# - 1, 2]] &] (* _Amiram Eldar_, Aug 30 2020 *)
%o (PARI) is(n)=n%2&&issquarefree((n-1)>>valuation(n-1,2)) \\ corrected by _Amiram Eldar_, Aug 30 2020
%Y Cf. A192861, A192862, A192864.
%Y Cf. A185199 (asymptotic density of this sequence).
%K nonn
%O 1,1
%A _Charles R Greathouse IV_, Jul 11 2011
%E Data corrected by _Amiram Eldar_, Aug 30 2020
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