OFFSET
1,1
COMMENTS
All terms are a product of at least two terms of A020459. - David A. Corneth, Oct 09 2020
LINKS
David A. Corneth, Table of n, a(n) for n = 1..10000 (first 5340 terms from Robert Israel)
FORMULA
Sum_{n>=1} 1/a(n) = Product_{p in A020459} (p/(p - 1)) - Sum_{p in A020459} 1/p - 1 = 0.7041098484... . - Amiram Eldar, May 18 2022
MAPLE
dmax:= 4: # for terms < 2*10^dmax
P:= {2, 7}:
L:= {7}:
for d from 1 to dmax-1 do
L:= map(t -> 2*10^d+t, L) union map(t -> 7*10^d+t, L);
P:= P union select(isprime, L);
od:
R:= {1}: N:= 2*10^dmax:
for p in P do
R:= R union map(t -> seq(t*p^j, j=1..floor(log[p](N/t))), R)
od:
sort(convert(R minus P minus {1}, list)); # Robert Israel, Aug 04 2020
CROSSREFS
KEYWORD
nonn,easy,base
AUTHOR
Patrick De Geest, Dec 15 1998
STATUS
approved