OFFSET
1,8
FORMULA
a(n) = Sum_{d|n} [[omega(d) = 2] = mu(d)^2], where [ ] is the Iverson bracket.
EXAMPLE
a(60) = 5; 60 has divisors 6,10,15 (squarefree semiprimes), 4 (=2^2), and 60 = 2^2*3*5 (has at least 3 distinct prime factors and at least 1 square divisor > 1).
CROSSREFS
KEYWORD
nonn
AUTHOR
Wesley Ivan Hurt, Feb 10 2022
STATUS
approved