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%I #30 Sep 08 2022 08:44:52
%S 9,21,27,49,63,81,111,147,189,219,243,259,333,343,441,511,567,657,729,
%T 777,999,1011,1029,1119,1323,1369,1533,1701,1813,1971,2187,2199,2319,
%U 2331,2359,2401,2611,2701,2997,3033,3087,3357,3577,3969,4107,4599,5103
%N Composite numbers whose prime factors contain no digits other than 3 and 7.
%C All terms are a product of at least two terms of A020463. - _David A. Corneth_, Oct 09 2020
%H David A. Corneth, <a href="/A036316/b036316.txt">Table of n, a(n) for n = 1..10000</a> (first 800 terms from Vincenzo Librandi)
%H <a href="/index/Pri#prime_factors">Index entries for sequences related to prime factors</a>.
%F Sum_{n>=1} 1/a(n) = Product_{p in A020463} (p/(p - 1)) - Sum_{p in A020463} 1/p - 1 = 0.3143000293... . - _Amiram Eldar_, May 22 2022
%t dpfQ[n_]:=Module[{d=Union[Flatten[IntegerDigits/@Transpose[FactorInteger[n]][[1]]]]}, !PrimeQ[n]&&(d == {3}||d == {7}||d == {3, 7})]; Select[Range[6000], dpfQ] (* _Vincenzo Librandi_, Aug 25 2013 *)
%o (Magma) [n: n in [9..6000] | not IsPrime(n) and forall{f: f in PrimeDivisors(n) | Intseq(f) subset [3,7]}]; // _Bruno Berselli_, Aug 26 2013
%Y Cf. A003594, A020463, A036302-A036325.
%K nonn,easy,base
%O 1,1
%A _Patrick De Geest_, Dec 15 1998