A036323 Composite numbers whose prime factors contain no digits other than 7 and 8.

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

Links

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

Index entries for sequences related to prime factors.

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

Cf. A020470, A036302-A036325.

Sequence in context: A211736 A211747 A211756 * A066429 A269327 A206255

Adjacent sequences: A036320 A036321 A036322 * A036324 A036325 A036326

Keyword

nonn,easy,base

Author

Patrick De Geest, Dec 15 1998

Status

approved