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A036325 Composite numbers n such that digits of prime factors of n are either 8 or 9. 26


%S 7921,704969,800911,8001011,8009021,8802011,8810911,8899021,62742241,

%T 71281079,79120021,80001121,80982001,88109911,88910021,712089979,

%U 712802869,783378979,784171079,791120021,791200121,792012869,800020021,800109911,800901121,800991011,809001101,809811011,880111121

%N Composite numbers n such that digits of prime factors of n are either 8 or 9.

%C All terms are a product of at least two terms of A020472. - _David A. Corneth_, Apr 30 2018

%H David A. Corneth, <a href="/A036325/b036325.txt">Table of n, a(n) for n = 1..10000</a> (first 5375 terms from Robert Israel)

%H <a href="/index/Pri#prime_factors">Index entries for sequences related to prime factors</a>

%e 7921 is in the sequence because it's composite and its distinct prime factors are 89, only having digits 8 or 9. - _David A. Corneth_, Apr 30 2018

%p N:= 9: # to get all terms of <= N digits

%p R:= 10^N: G:= {9}: S:= {1}:

%p for n from 1 to N-1 do

%p G:= map(t -> (t+8*10^n,t+9*10^n), G);

%p newprimes:= select(isprime, G);

%p for p in newprimes do

%p S:= map(s -> seq(s*p^i,i=0..floor(log[p](R/s))), S)

%p od

%p od:

%p sort(convert(remove(isprime, S minus {1}),list)); # _Robert Israel_, Apr 30 2018

%Y Cf. A020472, A036302-A036324.

%K nonn,easy,base

%O 1,1

%A _Patrick De Geest_, Dec 15 1998

%E More terms from _Robert Israel_, Apr 29 2018

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Last modified January 26 09:38 EST 2021. Contains 340435 sequences. (Running on oeis4.)