A036302 - OEIS

A036302

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

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/(p-1) - 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, A036303-A036325.

Sequence in context: A312822 A340298 A053688 * A032377 A358435 A312823

Adjacent sequences: A036299 A036300 A036301 * A036303 A036304 A036305

KEYWORD

nonn,base

AUTHOR

Patrick De Geest, Dec 15 1998

STATUS

approved