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

Author

Patrick De Geest, Dec 15 1998

Keywords

• nonn
• easy
• base

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.

Crossrefs

Cf. A020470, A036302-A036325.

Programs

Mathematica

Select[Range[25*10^5],CompositeQ[#]&&SubsetQ[{7,8},Flatten[ IntegerDigits/@ Transpose[ FactorInteger[#]][[1]]]]&] (* Harvey P. Dale, Jan 19 2015 *)

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