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A036305
Composite numbers whose prime factors contain no digits other than 1 and 5.
2
25, 55, 121, 125, 275, 605, 625, 755, 1331, 1375, 1661, 3025, 3125, 3775, 5755, 6655, 6875, 7555, 8305, 12661, 14641, 15125, 15625, 16621, 18271, 18875, 22801, 28775, 33275, 34375, 37775, 41525, 57755, 63305, 73205, 75625, 77555, 77755, 78125
OFFSET
1,1
COMMENTS
All terms are a product of at least two terms of A020453. - 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 A020453} (p/(p - 1)) - Sum_{p in A020453} 1/p - 1 = 0.0873463128... . - Amiram Eldar, May 18 2022
CROSSREFS
KEYWORD
nonn,easy,base
AUTHOR
Patrick De Geest, Dec 15 1998
STATUS
approved