OFFSET
1,1
COMMENTS
Verified using the known values of f(n) up to a limit determined by the upper bound for divisor function 2*sqrt(n). The lower bound for f(n) is n/d(n), which can be combined with d(n) <= 2*sqrt(n) to yield f(n) >= sqrt(n)/2, and n <= 4*f(n)^2. E.g. for f(n) = 18, this means that checking f(n) for n <= 1296 is sufficient to verify it's never the value of f(n).
It appears that all numbers 9*p, p prime, are in this sequence.
LINKS
Viliam Furík, Table of n, a(n) for n = 1..13935
CROSSREFS
KEYWORD
nonn,new
AUTHOR
Viliam Furík, Dec 16 2024
STATUS
approved