A036307 - OEIS

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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

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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.