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A128830 a(n) = the number of positive divisors of n which are coprime to d(n), where d(n) = the number of positive divisors of n. 2
1, 1, 2, 3, 2, 2, 2, 1, 1, 2, 2, 1, 2, 2, 4, 5, 2, 1, 2, 2, 4, 2, 2, 2, 3, 2, 4, 2, 2, 4, 2, 1, 4, 2, 4, 3, 2, 2, 4, 2, 2, 4, 2, 2, 2, 2, 2, 2, 3, 3, 4, 2, 2, 4, 4, 2, 4, 2, 2, 2, 2, 2, 2, 7, 4, 4, 2, 2, 4, 4, 2, 1, 2, 2, 3, 2, 4, 4, 2, 1, 5, 2, 2, 2, 4, 2, 4, 2, 2, 2, 4, 2, 4, 2, 4, 1, 2, 3, 2, 9, 2, 4, 2, 2, 8 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

LINKS

Harvey P. Dale, Table of n, a(n) for n = 1..1000

EXAMPLE

The 6 positive divisors of 20 are 1,2,4,5,10,20. Of these, only 1 and 5 are coprime to d(20) = 6. So a(20) = 2.

MAPLE

with(numtheory): a:=proc(n) local div, ct, i: div:=divisors(n): ct:=0: for i from 1 to tau(n) do if igcd(div[i], tau(n))=1 then ct:=ct+1 else ct:=ct: fi od: ct; end: seq(a(n), n=1..140); # Emeric Deutsch, Apr 14 2007

MATHEMATICA

cpd[n_]:=Module[{ds=DivisorSigma[0, n]}, Count[Divisors[n], _?(CoprimeQ[ #, ds]&)]]; Array[cpd, 110] (* Harvey P. Dale, Apr 21 2012 *)

CROSSREFS

Sequence in context: A112966 A286657 A160570 * A090387 A030329 A300139

Adjacent sequences:  A128827 A128828 A128829 * A128831 A128832 A128833

KEYWORD

nonn

AUTHOR

Leroy Quet, Apr 13 2007

EXTENSIONS

More terms from Emeric Deutsch, Apr 14 2007

STATUS

approved

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Last modified February 18 05:40 EST 2019. Contains 320245 sequences. (Running on oeis4.)