OFFSET
1,2
COMMENTS
Numbers which are the product of up to two primes (not necessarily distinct) or the cube of a prime. Alternatively, numbers having prime decomposition p*q, where q either is distinct from p or equals p^k for 0 <= k <= 2.
Corresponds to numbers having at most four divisors. (For numbers with exactly four divisors see A030513.) - Lekraj Beedassy, Sep 23 2003
For n > 3: numbers that can occur as fourth divisors; union of A000040, A001248, A006881 and A030078. - Reinhard Zumkeller, May 15 2006
REFERENCES
Liu Hongyan and Zhang Wenpeng, On the simple numbers and the mean value properties, Smarandache Notions (Book Series, Vol. 14), American Research Press, 2004; pp. 171-175.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
F. Smarandache, Only Problems, Not Solutions! (listing the sequence as the "simple numbers" in Unsolved Problem: 23 p.23).
MATHEMATICA
Select[Range[100], DivisorSigma[0, #] < 5 &] (* Amiram Eldar, Apr 30 2020 *)
PROG
(PARI) is(n)=numdiv(n)<5 \\ Charles R Greathouse IV, Sep 16 2015
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
R. Muller
EXTENSIONS
Description corrected by Henry Bottomley, Nov 24 2000
STATUS
approved