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A030230 Numbers that have an odd number of distinct prime divisors. 19
2, 3, 4, 5, 7, 8, 9, 11, 13, 16, 17, 19, 23, 25, 27, 29, 30, 31, 32, 37, 41, 42, 43, 47, 49, 53, 59, 60, 61, 64, 66, 67, 70, 71, 73, 78, 79, 81, 83, 84, 89, 90, 97, 101, 102, 103, 105, 107, 109, 110, 113, 114, 120, 121, 125, 126, 127, 128, 130, 131, 132, 137, 138, 139, 140, 149 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

T. D. Noe, Table of n, a(n) for n = 1..1000

Mats Granvik, Mathematica program to compute the relation to the Dirichlet inverse of the Euler totient function

H. Helfgott and A. Ubis, Primos, paridad y análisis, arXiv:1812.08707 [math.NT], Dec. 2018.

FORMULA

From Benoit Cloitre, Dec 08 2002: (Start)

k such that Sum_{d|k} mu(d)*tau(d) = (-1)^omega(k) = -1 where mu(d) = A008683(d), tau(d) = A000005(d) and omega(d) = A001221(d).

k such that A023900(k) < 0. (End)

gcd(A008472(a(n)), A007947(a(n))) > 1; see A014963. - Labos Elemer, Mar 26 2003

A076479(a(n)) = -1. - Reinhard Zumkeller, Jun 01 2013

MAPLE

q:= n-> is(nops(ifactors(n)[2])::odd):

select(q, [$1..150])[];  # Alois P. Heinz, Feb 12 2021

MATHEMATICA

(* Prior to version 7.0 *) littleOmega[n_] := Length[FactorInteger[n]]; Select[ Range[2, 149], (-1)^littleOmega[#] == -1 &] (* Jean-François Alcover, Nov 30 2011, after Benoit Cloitre *)

(* Version 7.0+ *) Select[Range[2, 149], (-1)^PrimeNu[#] == -1 &]

Select[Range[1000], OddQ[PrimeNu[#]]&] (* Harvey P. Dale, Nov 27 2012 *)

PROG

(Haskell)

a030230 n = a030230_list !! (n-1)

a030230_list = filter (odd . a001221) [1..]

-- Reinhard Zumkeller, Aug 14 2011

(PARI) is(n)=omega(n)%2 \\ Charles R Greathouse IV, Sep 14 2015

CROSSREFS

Cf. A030231, A123066.

Cf. A008472, A007947, A014963.

Cf. A076479.

Cf. A008683, A000005, A001221, A023900.

Sequence in context: A331912 A326848 A328957 * A089352 A086486 A071139

Adjacent sequences:  A030227 A030228 A030229 * A030231 A030232 A030233

KEYWORD

nonn,easy,nice

AUTHOR

David W. Wilson

STATUS

approved

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Last modified October 26 06:55 EDT 2021. Contains 348257 sequences. (Running on oeis4.)