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A087624 a(n)=0 if n is prime, A001221(n) otherwise. 6
0, 0, 0, 1, 0, 2, 0, 1, 1, 2, 0, 2, 0, 2, 2, 1, 0, 2, 0, 2, 2, 2, 0, 2, 1, 2, 1, 2, 0, 3, 0, 1, 2, 2, 2, 2, 0, 2, 2, 2, 0, 3, 0, 2, 2, 2, 0, 2, 1, 2, 2, 2, 0, 2, 2, 2, 2, 2, 0, 3, 0, 2, 2, 1, 2, 3, 0, 2, 2, 3, 0, 2, 0, 2, 2, 2, 2, 3, 0, 2, 1, 2, 0, 3, 2, 2, 2, 2, 0, 3, 2, 2, 2, 2, 2, 2, 0, 2, 2, 2, 0, 3, 0, 2, 3 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,6

COMMENTS

Number of prime divisors of n, but excluding n itself if n is prime.

Number of non-associated primes in the ring Z_n.

Also for n>1 the number of times n is crossed off in the sieve of Eratosthenes (A000040). [Reinhard Zumkeller, Oct 17 2008]

Number of primes that are proper divisors of n. [Omar E. Pol, Dec 27 2008]

LINKS

N. J. A. Sloane, Table of n, a(n) for n = 1..10000

Index entries for sequences generated by sieves [From Reinhard Zumkeller, Oct 17 2008]

FORMULA

a(n) = A001221(n) * A005171(n). - Jason Kimberley, Nov 19 2014

MAPLE

with(numtheory); f:=proc(n) if isprime(n) then nops(factorset(n))-1 else nops(factorset(n)) fi; end;

MATHEMATICA

Array[If[PrimeQ[#], 0, PrimeNu[#]]&, 110] (* Harvey P. Dale, Mar 27 2013 *)

PROG

(Haskell)

a087624 n = if a010051 n == 1 then 0 else a001221 n

-- Reinhard Zumkeller, Apr 05 2013

CROSSREFS

Cf. A001221, A010051, A087625.

A144489 gives partial sums.

Sequence in context: A114708 A084927 A072670 * A294891 A294879 A085122

Adjacent sequences:  A087621 A087622 A087623 * A087625 A087626 A087627

KEYWORD

nonn,easy

AUTHOR

Michele Dondi (bik.mido(AT)tiscalinet.it), Sep 14 2003

EXTENSIONS

Edited by N. J. A. Sloane, Dec 11 2008

STATUS

approved

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Last modified December 13 08:08 EST 2018. Contains 318082 sequences. (Running on oeis4.)