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A036323
Composite numbers whose prime factors contain no digits other than 7 and 8.
2
49, 343, 2401, 5509, 6139, 6209, 16807, 38563, 42973, 43463, 55139, 62209, 117649, 269941, 300811, 304241, 385973, 435463, 551509, 552139, 552209, 615139, 615209, 619369, 690199, 698069, 769129, 777899, 786769, 823543, 1889587, 2105677
OFFSET
1,1
COMMENTS
All terms are a product of at least two terms of A020470. - David A. Corneth, Oct 09 2020
FORMULA
Sum_{n>=1} 1/a(n) = Product_{p in A020470} (p/(p - 1)) - Sum_{p in A020470} 1/p - 1 = 0.0244618735... . - Amiram Eldar, May 22 2022
MATHEMATICA
Select[Range[25*10^5], CompositeQ[#]&&SubsetQ[{7, 8}, Flatten[ IntegerDigits/@ Transpose[ FactorInteger[#]][[1]]]]&] (* Harvey P. Dale, Jan 19 2015 *)
CROSSREFS
KEYWORD
nonn,easy,base
AUTHOR
Patrick De Geest, Dec 15 1998
STATUS
approved