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

Offset: 1

Author

Patrick De Geest, Dec 15 1998

Keywords

• nonn
• easy
• base

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.

Crossrefs

Cf. A003593, A020462, A036302-A036325.

Programs

Mathematica

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

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