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A079275 Number of divisors of n that are semiprimes with distinct factors. 5
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; text; internal format)
OFFSET

1,30

LINKS

G. C. Greubel, Table of n, a(n) for n = 1..1000

FORMULA

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

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] (* Harvey P. Dale, Oct 05 2011 *)

CROSSREFS

Cf. A000005, A000961, A001221, A001358, A006881, A007774, A033992, A033993.

Sequence in context: A318508 A100655 A262262 * A236314 A236322 A319419

Adjacent sequences:  A079272 A079273 A079274 * A079276 A079277 A079278

KEYWORD

nonn

AUTHOR

Reinhard Zumkeller, Feb 07 2003

STATUS

approved

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Last modified September 22 17:13 EDT 2019. Contains 327311 sequences. (Running on oeis4.)