A036309 Composite numbers whose prime factors contain no digits other than 1 and 9.

121, 209, 361, 1331, 2101, 2189, 2299, 3629, 3781, 3971, 6859, 10021, 10109, 10901, 14641, 17309, 17461, 18829, 21989, 23111, 24079, 25289, 36481, 37981, 38009, 39601, 39919, 41591, 43681, 68951, 71839, 75449, 101189, 110231, 111199, 119911

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 A020457. - David A. Corneth, Oct 09 2020

Links

• David A. Corneth, Table of n, a(n) for n = 1..10000 (first 360 terms from Harvey P. Dale)
• Index entries for sequences related to prime factors.

Crossrefs

Cf. A020457, A036302-A036325.

Programs

Mathematica

Select[Range[120000],CompositeQ[#]&&SubsetQ[{1,9},Union[Flatten[ IntegerDigits /@ FactorInteger[ #][[All,1]]]]]&] (* Harvey P. Dale, Mar 30 2019 *)

Formula

Sum_{n>=1} 1/a(n) = Product_{p in A020457} (p/(p - 1)) - Sum_{p in A020457} 1/p - 1 = 0.0200389643... . - Amiram Eldar, May 18 2022