OFFSET
1,1
COMMENTS
The greatest prime factor equals the number of divisors only for 1 (as defined in A006530) and numbers of the form p^(p-1) for p a prime.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
EXAMPLE
4 has 2 as its greatest prime factor, and it has 3 factors (1, 2, 4), so it is in the sequence.
10 has 5 as its greatest prime factor, but it has only 4 factors (1, 2, 5, 10), so it is not in the sequence.
MATHEMATICA
Select[Range[300], FactorInteger[#][[-1, 1]]<DivisorSigma[0, #]&] (* Harvey P. Dale, Nov 19 2011 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Franklin T. Adams-Watters, Nov 10 2011
STATUS
approved