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A036321
Composite numbers whose prime factors contain no digits other than 5 and 9.
2
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
OFFSET
1,1
COMMENTS
All terms are a product of at least two terms of A020468. - David A. Corneth, Oct 09 2020
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
KEYWORD
nonn,easy,base
AUTHOR
Patrick De Geest, Dec 15 1998
STATUS
approved