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

Offset: 1

Author

Patrick De Geest, Dec 15 1998

Keywords

• nonn
• easy
• base

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.

Crossrefs

Cf. A020468, A036302-A036325.

Programs

Mathematica

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

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