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A351416
Number of divisors of n that are either squarefree semiprimes, numbers of the form p^k (p prime, k>1), or numbers with at least one square divisor > 1 and 3 or more distinct prime factors.
0
0, 0, 0, 1, 0, 1, 0, 2, 1, 1, 0, 2, 0, 1, 1, 3, 0, 2, 0, 2, 1, 1, 0, 3, 1, 1, 2, 2, 0, 3, 0, 4, 1, 1, 1, 3, 0, 1, 1, 3, 0, 3, 0, 2, 2, 1, 0, 4, 1, 2, 1, 2, 0, 3, 1, 3, 1, 1, 0, 5, 0, 1, 2, 5, 1, 3, 0, 2, 1, 3, 0, 4, 0, 1, 2, 2, 1, 3, 0, 4, 3, 1, 0, 5, 1, 1, 1, 3, 0, 5, 1, 2, 1, 1, 1
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
Cf. A001221 (omega), A008683 (mu), A351414.
Sequence in context: A276806 A308427 A252736 * A253559 A136167 A140748
KEYWORD
nonn
AUTHOR
Wesley Ivan Hurt, Feb 10 2022
STATUS
approved