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)
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
KEYWORD
nonn,easy,base
AUTHOR
Patrick De Geest, Dec 15 1998
EXTENSIONS
More terms from Robert Israel, Apr 29 2018
STATUS
approved