A079275 - OEIS

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A079275

Number of divisors of n that are semiprimes with distinct factors.

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.