A036321 - OEIS

A036321

Composite numbers whose prime factors contain no digits other than 5 and 9.

25, 125, 295, 625, 1475, 2995, 3125, 3481, 7375, 14975, 15625, 17405, 35341, 36875, 74875, 78125, 87025, 176705, 184375, 205379, 299995, 358801, 374375, 390625, 435125, 479795, 497795, 883525, 921875, 1026895, 1499975, 1794005, 1871875

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OFFSET

1,1

COMMENTS

All terms are a product of at least two terms of A020468. - David A. Corneth, Oct 09 2020

LINKS

David A. Corneth, Table of n, a(n) for n = 1..10000

Index entries for sequences related to prime factors.

FORMULA

Sum_{n>=1} 1/a(n) = Product_{p in A020468} (p/(p - 1)) - Sum_{p in A020468} 1/p - 1 = 0.0550718517... . - Amiram Eldar, May 22 2022

MATHEMATICA

Select[Range[1872000], CompositeQ[#]&&SubsetQ[{5, 9}, Flatten[ IntegerDigits/@ FactorInteger[#][[All, 1]]]]&] (* Harvey P. Dale, Sep 17 2019 *)

CROSSREFS

Cf. A020468, A036302-A036325.

Sequence in context: A044357 A044738 A062672 * A289337 A211581 A226231

Adjacent sequences: A036318 A036319 A036320 * A036322 A036323 A036324

KEYWORD

nonn,easy,base

AUTHOR

Patrick De Geest, Dec 15 1998

STATUS

approved