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A118130 Composite lucky numbers whose divisors are also lucky numbers. 2

%I #9 Nov 16 2019 10:29:17

%S 9,21,49,63,93,111,129,169,201,219,237,259,489,511,553,559,579,723,

%T 777,961,993,1057,1101,1147,1263,1369,1389,1533,1659,1731,1737,1839,

%U 1857,1893,1963,2019,2217,2317,2479,2649,2899,2923,2973,3027,3031,3133,3261

%N Composite lucky numbers whose divisors are also lucky numbers.

%H Amiram Eldar, <a href="/A118130/b118130.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..1000 from T. D. Noe)

%t lst = Range[1, 3350, 2]; i = 2; While[i <= (len = Length@lst) && (k = lst[[i]]) <= len, lst = Drop[lst, {k, len, k}]; i++ ]; fQ[n_] := If[PrimeQ@n, False, Block[{d = Take[Divisors@n, {2, -2}], k = 1, lmt = DivisorSigma[0, n] - 1}, While[k < lmt && MemberQ[lst, d[[k]]], k++ ]; k == lmt]]; Select[ Rest@lst, fQ@# &]

%Y Cf. A000959, A043772.

%K nonn

%O 1,1

%A _Robert G. Wilson v_, May 12 2006

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Last modified March 28 15:38 EDT 2024. Contains 371254 sequences. (Running on oeis4.)