A036322 Composite numbers whose prime factors contain no digits other than 6 and 7.

49, 343, 469, 2401, 3283, 4489, 4739, 16807, 22981, 31423, 33173, 45359, 117649, 160867, 219961, 232211, 300763, 317513, 458329, 474439, 536669, 537439, 823543, 1126069, 1539727, 1625477, 2105341, 2222591, 3039053, 3208303, 3321073

Offset: 1

Author

Patrick De Geest, Dec 15 1998

Keywords

• nonn
• easy
• base

Comments

The multiplicative closure of A020469 consists of itself, 1, and this sequence. - Charles R Greathouse IV, Jun 25 2014

Links

• David A. Corneth, Table of n, a(n) for n = 1..10000
• Index entries for sequences related to prime factors.

Crossrefs

Cf. A020469, A036302-A036325.

Formula

Sum_{n>=1} 1/a(n) = Product_{p in A020469} (p/(p - 1)) - Sum_{p in A020469} 1/p - 1 = 0.0268448443... . - Amiram Eldar, May 22 2022