OFFSET
1,1
COMMENTS
Numbers divisible by the squares of exactly three distinct primes.
Subsequence of A318720 and first differs from it at n = 123.
The asymptotic density of this sequence is (eta_1^3 - 3*eta_1*eta_2 + 2*eta_3)/Pi^2 = 0.0032920755..., where eta_j = Sum_{p prime} 1/(p^2-1)^j (Pomerance and Schinzel, 2011).
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
Carl Pomerance and Andrzej Schinzel, Multiplicative Properties of Sets of Residues, Moscow Journal of Combinatorics and Number Theory, Vol. 1, No. 1 (2011), pp. 52-66. See pp. 61-62.
EXAMPLE
900 = 2^2 * 3^2 * 5^2 is a term since it has exactly 3 prime factors, 2, 3 and 5, that are non-unitary.
MATHEMATICA
Select[Range[17000], Count[FactorInteger[#][[;; , 2]], _?(#1 > 1 &)] == 3 &]
CROSSREFS
KEYWORD
nonn
AUTHOR
Amiram Eldar, Nov 01 2020
STATUS
approved