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A036318
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Composite numbers whose prime factors contain no digits other than 4 and 7.
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2
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49, 329, 343, 2209, 2303, 2401, 15463, 16121, 16807, 31129, 52339, 103823, 108241, 112847, 117649, 209009, 217903, 313439, 334439, 351419, 366373, 523229, 542129, 542339, 544229, 726761, 757687, 789929, 823543, 1463063, 1525321, 2104519
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OFFSET
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1,1
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COMMENTS
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LINKS
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FORMULA
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Sum_{n>=1} 1/a(n) = Product_{p in A020465} (p/(p - 1)) - Sum_{p in A020465} 1/p - 1 = 0.0279830135... . - Amiram Eldar, May 22 2022
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MATHEMATICA
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pf47Q[n_]:=Module[{u=Union[Flatten[IntegerDigits/@Transpose[ FactorInteger[ n]][[1]]]]}, !PrimeQ[n]&&(u=={4}||u=={7}||u=={4, 7})]; Select[ Range[ 2200000], pf47Q] (* Harvey P. Dale, Jun 05 2013 *)
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CROSSREFS
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KEYWORD
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nonn,easy,base
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AUTHOR
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STATUS
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approved
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