A036324 Composite numbers whose prime factors have no digits other than 7's and 9's.

49, 343, 553, 679, 2401, 3871, 4753, 5579, 6241, 6839, 6979, 7663, 9409, 16807, 27097, 33271, 39053, 43687, 47873, 48853, 53641, 62963, 65863, 77183, 77309, 78763, 94769, 96709, 117649, 189679, 232897, 273371, 305809, 335111, 341971

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 A020471. - David A. Corneth, Oct 09 2020

Links

• David A. Corneth, Table of n, a(n) for n = 1..10000 (first 100 terms from Harvey P. Dale)
• Index entries for sequences related to prime factors.

Crossrefs

Cf. A020471, A036302-A036325.

Programs

Mathematica

Select[Range[342000],CompositeQ[#]&&SubsetQ[{7,9},Union[ Flatten[ IntegerDigits/@ FactorInteger[#][[All,1]]]]]&] (* Harvey P. Dale, Aug 01 2019 *)

Formula

Sum_{n>=1} 1/a(n) = Product_{p in A020471} (p/(p - 1)) - Sum_{p in A020471} 1/p - 1 = 0.0287747452... . - Amiram Eldar, May 22 2022

Extensions

Definition clarified by Harvey P. Dale, Aug 01 2019