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

1,1

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.

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

Mathematica

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

Crossrefs

Cf. A020471, A036302-A036325.

Sequence in context: A365206 A020323 A036322 * A211736 A211747 A211756

Adjacent sequences: A036321 A036322 A036323 * A036325 A036326 A036327

Keyword

nonn,easy,base

Author

Patrick De Geest, Dec 15 1998

Extensions

Definition clarified by Harvey P. Dale, Aug 01 2019

Status

approved