A036320 Composite numbers whose prime factors contain no digits other than 5 and 7.

25, 35, 49, 125, 175, 245, 343, 625, 875, 1225, 1715, 2401, 2785, 2885, 3125, 3785, 3899, 4039, 4375, 5299, 6125, 8575, 12005, 13925, 14425, 15625, 16807, 18925, 19495, 20195, 21875, 26495, 27293, 27785, 28273, 30625, 37093, 37885, 38785

Offset

1,1

Comments

All terms are a product of at least two terms of A020467. - 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.

Formula

Sum_{n>=1} 1/a(n) = Product_{p in A020467} (p/(p - 1)) - Sum_{p in A020467} 1/p - 1 = 0.1179595738... . - Amiram Eldar, May 22 2022

Crossrefs

Cf. A003595, A020467, A036302-A036325.

Sequence in context: A107472 A193165 A060976 * A339520 A340096 A348423

Adjacent sequences: A036317 A036318 A036319 * A036321 A036322 A036323

Keyword

nonn,easy,base

Author

Patrick De Geest, Dec 15 1998

Status

approved