login
This site is supported by donations to The OEIS Foundation.

 

Logo

Annual appeal: Please make a donation to keep the OEIS running! Over 6000 articles have referenced us, often saying "we discovered this result with the help of the OEIS".
Other ways to donate

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy .

Last modified December 13 19:20 EST 2017. Contains 295976 sequences.