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A036309
Composite numbers whose prime factors contain no digits other than 1 and 9.
2
121, 209, 361, 1331, 2101, 2189, 2299, 3629, 3781, 3971, 6859, 10021, 10109, 10901, 14641, 17309, 17461, 18829, 21989, 23111, 24079, 25289, 36481, 37981, 38009, 39601, 39919, 41591, 43681, 68951, 71839, 75449, 101189, 110231, 111199, 119911
OFFSET
1,1
COMMENTS
All terms are a product of at least two terms of A020457. - David A. Corneth, Oct 09 2020
LINKS
David A. Corneth, Table of n, a(n) for n = 1..10000 (first 360 terms from Harvey P. Dale)
FORMULA
Sum_{n>=1} 1/a(n) = Product_{p in A020457} (p/(p - 1)) - Sum_{p in A020457} 1/p - 1 = 0.0200389643... . - Amiram Eldar, May 18 2022
MATHEMATICA
Select[Range[120000], CompositeQ[#]&&SubsetQ[{1, 9}, Union[Flatten[ IntegerDigits /@ FactorInteger[ #][[All, 1]]]]]&] (* Harvey P. Dale, Mar 30 2019 *)
CROSSREFS
KEYWORD
nonn,easy,base
AUTHOR
Patrick De Geest, Dec 15 1998
STATUS
approved