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A079275 Number of divisors of n that are semiprimes with distinct factors. 4
0, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 1, 0, 1, 1, 0, 0, 1, 0, 1, 1, 1, 0, 1, 0, 1, 0, 1, 0, 3, 0, 0, 1, 1, 1, 1, 0, 1, 1, 1, 0, 3, 0, 1, 1, 1, 0, 1, 0, 1, 1, 1, 0, 1, 1, 1, 1, 1, 0, 3, 0, 1, 1, 0, 1, 3, 0, 1, 1, 3, 0, 1, 0, 1, 1, 1, 1, 3, 0, 1, 0, 1, 0, 3, 1, 1, 1, 1, 0, 3, 1, 1, 1, 1, 1, 1, 0, 1, 1, 1, 0, 3, 0, 1, 3 (list; graph; refs; listen; history; internal format)
OFFSET

1,30

COMMENTS

a(A000961(n))=0; a(A007774(n))=1; a(A033992(n))=3; a(A033993(n))=6.

FORMULA

a(n)=omega(n)*(omega(n)-1)/2, where omega(n) is the number of distinct prime factors of n.

MATHEMATICA

f[n_]:=Module[{c=PrimeNu[n]}, (c(c-1))/2]; Array[f, 110] (* From Harvey P. Dale, Oct 05 2011 *)

CROSSREFS

Cf. A001221, A000005, A006881, A001358.

Sequence in context: A035655 A036857 A100655 * A133573 A133574 A151859

Adjacent sequences:  A079272 A079273 A079274 * A079276 A079277 A079278

KEYWORD

nonn

AUTHOR

Reinhard Zumkeller (reinhard.zumkeller(AT)gmail.com), Feb 07 2003

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Last modified February 16 10:07 EST 2012. Contains 205904 sequences.