OFFSET
1,1
COMMENTS
Similar to but different from A007624. Terms like 60, 72, 84, 90, 96, 108, 126, etc. are not present here.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
FORMULA
With d(n) = number of divisors(n), d(d(d(d(a(n))))) = 2 and d(d(d(a(n)))) > 2.
A036459(a(n)) = 4. - Ivan Neretin, Jan 25 2016
EXAMPLE
a(3)=20 and a(17)=63; for both x=20 and 63, d(x)=6 and d(d(x))=4, the 3rd iterates are 3 and the equilibrium value, i.e., 2 appears as 4th iterates.
PROG
(PARI) isok(n) = ((nd=numdiv(n)) != 2) && ((nd=numdiv(nd)) != 2) && ((nd=numdiv(nd)) != 2) && ((nd=numdiv(nd)) == 2); \\ Michel Marcus, Dec 30 2013 & Jan 26 2015
CROSSREFS
KEYWORD
nonn
AUTHOR
EXTENSIONS
STATUS
approved