

A036302


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


25



4, 8, 16, 22, 32, 44, 64, 88, 121, 128, 176, 242, 256, 352, 422, 484, 512, 704, 844, 968, 1024, 1331, 1408, 1688, 1936, 2048, 2321, 2662, 2816, 3376, 3872, 4096, 4222, 4442, 4642, 5324, 5632, 6752, 7744, 8192, 8444, 8884, 9284, 10648, 11264, 13504, 14641, 15488, 16384
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OFFSET

1,1


COMMENTS

All terms are a product of at least two terms of A020450.  Michel Marcus, Oct 02 2020


LINKS

David A. Corneth, Table of n, a(n) for n = 1..10000 (first 1000 terms from Alois P. Heinz)
Index entries sequences related to prime factors


FORMULA

Sum_{n>=1} 1/a(n) = Product_{p in A020450} p/(p1)  Sum_{p in A020450} 1/p  1 = 0.616325...  Amiram Eldar, Oct 14 2020


EXAMPLE

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


MATHEMATICA

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


PROG

(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


CROSSREFS

Cf. A003596 (a subsequence), A020450, A036303A036325.
Sequence in context: A312821 A312822 A053688 * A032377 A312823 A133690
Adjacent sequences: A036299 A036300 A036301 * A036303 A036304 A036305


KEYWORD

nonn,base


AUTHOR

Patrick De Geest, Dec 15 1998


STATUS

approved



