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%I #30 May 22 2022 05:49:15
%S 49,343,553,679,2401,3871,4753,5579,6241,6839,6979,7663,9409,16807,
%T 27097,33271,39053,43687,47873,48853,53641,62963,65863,77183,77309,
%U 78763,94769,96709,117649,189679,232897,273371,305809,335111,341971
%N Composite numbers whose prime factors have no digits other than 7's and 9's.
%C All terms are a product of at least two terms of A020471. - _David A. Corneth_, Oct 09 2020
%H David A. Corneth, <a href="/A036324/b036324.txt">Table of n, a(n) for n = 1..10000</a> (first 100 terms from Harvey P. Dale)
%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 A020471} (p/(p - 1)) - Sum_{p in A020471} 1/p - 1 = 0.0287747452... . - _Amiram Eldar_, May 22 2022
%t Select[Range[342000],CompositeQ[#]&&SubsetQ[{7,9},Union[ Flatten[ IntegerDigits/@ FactorInteger[#][[All,1]]]]]&] (* _Harvey P. Dale_, Aug 01 2019 *)
%Y Cf. A020471, A036302-A036325.
%K nonn,easy,base
%O 1,1
%A _Patrick De Geest_, Dec 15 1998
%E Definition clarified by _Harvey P. Dale_, Aug 01 2019
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