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Composite numbers whose prime factors contain no digits other than 5 and 7.
2

%I #22 May 22 2022 05:50:56

%S 25,35,49,125,175,245,343,625,875,1225,1715,2401,2785,2885,3125,3785,

%T 3899,4039,4375,5299,6125,8575,12005,13925,14425,15625,16807,18925,

%U 19495,20195,21875,26495,27293,27785,28273,30625,37093,37885,38785

%N Composite numbers whose prime factors contain no digits other than 5 and 7.

%C All terms are a product of at least two terms of A020467. - _David A. Corneth_, Oct 09 2020

%H David A. Corneth, <a href="/A036320/b036320.txt">Table of n, a(n) for n = 1..10000</a>

%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 A020467} (p/(p - 1)) - Sum_{p in A020467} 1/p - 1 = 0.1179595738... . - _Amiram Eldar_, May 22 2022

%Y Cf. A003595, A020467, A036302-A036325.

%K nonn,easy,base

%O 1,1

%A _Patrick De Geest_, Dec 15 1998