A036315 - OEIS

A036315

Composite numbers whose prime factors contain no digits other than 3 and 5.

9, 15, 25, 27, 45, 75, 81, 125, 135, 159, 225, 243, 265, 375, 405, 477, 625, 675, 729, 795, 1059, 1125, 1215, 1325, 1431, 1765, 1875, 2025, 2187, 2385, 2809, 3125, 3177, 3375, 3645, 3975, 4293, 5295, 5625, 6075, 6561, 6625, 7155, 8427, 8825, 9375, 9531

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OFFSET

1,1

COMMENTS

Products of at least two terms of A020462. - 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 A020462} (p/(p - 1)) - Sum_{p in A020462} 1/p - 1 = 0.3620363317... . - Amiram Eldar, May 22 2022

MATHEMATICA

Select[Range[10000], SubsetQ[{3, 5}, Union[Flatten[IntegerDigits/@ FactorInteger[ #][[All, 1]]]]]&&CompositeQ[#]&] (* Harvey P. Dale, May 30 2021 *)