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 A036324 Composite numbers whose prime factors have no digits other than 7's and 9's. 3
 49, 343, 553, 679, 2401, 3871, 4753, 5579, 6241, 6839, 6979, 7663, 9409, 16807, 27097, 33271, 39053, 43687, 47873, 48853, 53641, 62963, 65863, 77183, 77309, 78763, 94769, 96709, 117649, 189679, 232897, 273371, 305809, 335111, 341971 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS All terms are a product of at least two terms of A020471. - David A. Corneth, Oct 09 2020 LINKS David A. Corneth, Table of n, a(n) for n = 1..10000 (first 100 terms from Harvey P. Dale) FORMULA Sum_{n>=1} 1/a(n) = Product_{p in A020471} (p/(p - 1)) - Sum_{p in A020471} 1/p - 1 = 0.0287747452... . - Amiram Eldar, May 22 2022 MATHEMATICA Select[Range[342000], CompositeQ[#]&&SubsetQ[{7, 9}, Union[ Flatten[ IntegerDigits/@ FactorInteger[#][[All, 1]]]]]&] (* Harvey P. Dale, Aug 01 2019 *) CROSSREFS Cf. A020471, A036302-A036325. Sequence in context: A036318 A020323 A036322 * A211736 A211747 A211756 Adjacent sequences:  A036321 A036322 A036323 * A036325 A036326 A036327 KEYWORD nonn,easy,base,changed AUTHOR Patrick De Geest, Dec 15 1998 EXTENSIONS Definition clarified by Harvey P. Dale, Aug 01 2019 STATUS approved

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Last modified May 27 16:48 EDT 2022. Contains 354110 sequences. (Running on oeis4.)