A036309 - OEIS

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A036309

Composite numbers whose prime factors contain no digits other than 1 and 9.

121, 209, 361, 1331, 2101, 2189, 2299, 3629, 3781, 3971, 6859, 10021, 10109, 10901, 14641, 17309, 17461, 18829, 21989, 23111, 24079, 25289, 36481, 37981, 38009, 39601, 39919, 41591, 43681, 68951, 71839, 75449, 101189, 110231, 111199, 119911

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

COMMENTS

All terms are a product of at least two terms of A020457. - David A. Corneth, Oct 09 2020

LINKS

David A. Corneth, Table of n, a(n) for n = 1..10000 (first 360 terms from Harvey P. Dale)

Index entries for sequences related to prime factors.

FORMULA

Sum_{n>=1} 1/a(n) = Product_{p in A020457} (p/(p - 1)) - Sum_{p in A020457} 1/p - 1 = 0.0200389643... . - Amiram Eldar, May 18 2022

MATHEMATICA

Select[Range[120000], CompositeQ[#]&&SubsetQ[{1, 9}, Union[Flatten[ IntegerDigits /@ FactorInteger[ #][[All, 1]]]]]&] (* Harvey P. Dale, Mar 30 2019 *)

CROSSREFS

Cf. A020457, A036302-A036325.

Sequence in context: A364778 A037050 A275028 * A346507 A119378 A261618

Adjacent sequences: A036306 A036307 A036308 * A036310 A036311 A036312

KEYWORD

nonn,easy,base

AUTHOR

Patrick De Geest, Dec 15 1998

STATUS

approved