A036325 Composite numbers whose prime factors have no digits other than 8 and 9.

7921, 704969, 800911, 8001011, 8009021, 8802011, 8810911, 8899021, 62742241, 71281079, 79120021, 80001121, 80982001, 88109911, 88910021, 712089979, 712802869, 783378979, 784171079, 791120021, 791200121, 792012869, 800020021, 800109911, 800901121, 800991011, 809001101, 809811011, 880111121

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 A020472. - David A. Corneth, Apr 30 2018

Examples

			7921 is in the sequence because it's composite and its only prime factor is 89, only having digits 8 or 9. - _David A. Corneth_, Apr 30 2018
		

Links

• David A. Corneth, Table of n, a(n) for n = 1..10000 (first 5375 terms from Robert Israel)
• Index entries for sequences related to prime factors.

Crossrefs

Cf. A020472, A036302-A036324.

Programs

Maple

N:= 9: # to get all terms of <= N digits
R:= 10^N: G:= {9}: S:= {1}:
for n from 1 to N-1 do
  G:= map(t -> (t+8*10^n,t+9*10^n), G);
  newprimes:= select(isprime, G);
  for p in newprimes do
    S:= map(s -> seq(s*p^i,i=0..floor(log[p](R/s))), S)
  od
od:
sort(convert(remove(isprime, S minus {1}),list)); # Robert Israel, Apr 30 2018

Formula

Sum_{n>=1} 1/a(n) = Product_{p in A020472} (p/(p - 1)) - Sum_{p in A020472} 1/p - 1 = 0.0001296249159... . - Amiram Eldar, May 22 2022

Extensions

More terms from Robert Israel, Apr 29 2018