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A036307
Composite numbers whose prime factors contain no digits other than 1 and 7.
2
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)
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
KEYWORD
nonn,easy,base
AUTHOR
Patrick De Geest, Dec 15 1998
STATUS
approved