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

1,1

Comments

All terms are a product of at least two terms of A020472. - 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.

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

Example

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

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

Crossrefs

Cf. A020472, A036302-A036324.

Sequence in context: A121236 A233976 A001228 * A031587 A031767 A250629

Adjacent sequences: A036322 A036323 A036324 * A036326 A036327 A036328

Keyword

nonn,easy,base

Author

Patrick De Geest, Dec 15 1998

Extensions

More terms from Robert Israel, Apr 29 2018

Status

approved