|
%I #25 May 22 2022 05:51:08
%S 49,329,343,2209,2303,2401,15463,16121,16807,31129,52339,103823,
%T 108241,112847,117649,209009,217903,313439,334439,351419,366373,
%U 523229,542129,542339,544229,726761,757687,789929,823543,1463063,1525321,2104519
%N Composite numbers whose prime factors contain no digits other than 4 and 7.
%C All terms are a product of at least two terms of A020465. - _David A. Corneth_, Oct 09 2020
%H David A. Corneth, <a href="/A036318/b036318.txt">Table of n, a(n) for n = 1..10000</a>
%H <a href="/index/Pri#prime_factors">Index entries sequences related to prime factors</a>.
%F Sum_{n>=1} 1/a(n) = Product_{p in A020465} (p/(p - 1)) - Sum_{p in A020465} 1/p - 1 = 0.0279830135... . - _Amiram Eldar_, May 22 2022
%t pf47Q[n_]:=Module[{u=Union[Flatten[IntegerDigits/@Transpose[ FactorInteger[ n]][[1]]]]},!PrimeQ[n]&&(u=={4}||u=={7}||u=={4,7})];Select[ Range[ 2200000],pf47Q] (* _Harvey P. Dale_, Jun 05 2013 *)
%Y Cf. A020465, A036302-A036325.
%K nonn,easy,base
%O 1,1
%A _Patrick De Geest_, Dec 15 1998
|
