A036318 - OEIS

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A036318

Composite numbers whose prime factors contain no digits other than 4 and 7.

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

1,1

COMMENTS

All terms are a product of at least two terms of A020465. - David A. Corneth, Oct 09 2020

LINKS

David A. Corneth, Table of n, a(n) for n = 1..10000

Index entries sequences related to prime factors.

FORMULA

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

MATHEMATICA

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 *)

CROSSREFS

Cf. A020465, A036302-A036325.

Sequence in context: A340124 A017474 A335389 * A365206 A020323 A036322

Adjacent sequences: A036315 A036316 A036317 * A036319 A036320 A036321

KEYWORD

nonn,easy,base

AUTHOR

Patrick De Geest, Dec 15 1998

STATUS

approved