A036307 Composite numbers whose prime factors contain no digits other than 1 and 7.

49, 77, 119, 121, 187, 289, 343, 497, 539, 781, 833, 847, 1207, 1309, 1331, 2023, 2057, 2401, 3179, 3479, 3773, 4913, 5041, 5467, 5831, 5929, 7819, 8197, 8449, 8591, 9163, 9317, 12287, 12439, 12881, 13277, 14161, 14399, 14641, 16807, 18989, 19547

Offset

1,1

Comments

All terms are a product of at least two terms of A020455. - David A. Corneth, Oct 09 2020

Links

David A. Corneth, Table of n, a(n) for n = 1..10000 (first 1000 terms from Alois P. Heinz)

Index entries for sequences related to prime factors.

Formula

Sum_{n>=1} 1/a(n) = Product_{p in A020455} (p/(p - 1)) - Sum_{p in A020455} 1/p - 1 = 0.0775663737... . - Amiram Eldar, May 18 2022

Crossrefs

Cf. A003599, A020455, A036302-A036325.

Sequence in context: A063163 A345356 A103216 * A294028 A056938 A267986

Adjacent sequences: A036304 A036305 A036306 * A036308 A036309 A036310

Keyword

nonn,easy,base

Author

Patrick De Geest, Dec 15 1998

Status

approved