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 A036302 Composite numbers k such that the digits of the prime factors of k are either 1 or 2. 25

%I

%S 4,8,16,22,32,44,64,88,121,128,176,242,256,352,422,484,512,704,844,

%T 968,1024,1331,1408,1688,1936,2048,2321,2662,2816,3376,3872,4096,4222,

%U 4442,4642,5324,5632,6752,7744,8192,8444,8884,9284,10648,11264,13504,14641,15488,16384

%N Composite numbers k such that the digits of the prime factors of k are either 1 or 2.

%C All terms are a product of at least two terms of A020450. - _Michel Marcus_, Oct 02 2020

%H David A. Corneth, <a href="/A036302/b036302.txt">Table of n, a(n) for n = 1..10000</a> (first 1000 terms from Alois P. Heinz)

%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 A020450} p/(p-1) - Sum_{p in A020450} 1/p - 1 = 0.616325... - _Amiram Eldar_, Oct 14 2020

%e 422 = 2 * 211 is in the sequence as the digits of its prime factors 2 and 211 are either 1 or 2. - _David A. Corneth_, Sep 26 2020

%t Select[Range[2,14650],!PrimeQ[#] && Complement[Flatten[IntegerDigits[First/@FactorInteger[#]]],{1,2}]=={} &] (* _Jayanta Basu_, May 25 2013 *)

%o (MAGMA) [k:k in [2..15000]| not IsPrime(k) and forall{a: a in PrimeDivisors(k)|Intseq(a) subset {1,2}}]; // _Marius A. Burtea_, Oct 08 2019

%Y Cf. A003596 (a subsequence), A020450, A036303-A036325.

%K nonn,base

%O 1,1

%A _Patrick De Geest_, Dec 15 1998

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Last modified November 27 06:19 EST 2020. Contains 338678 sequences. (Running on oeis4.)