A030059 Numbers that are the product of an odd number of distinct primes.

2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 30, 31, 37, 41, 42, 43, 47, 53, 59, 61, 66, 67, 70, 71, 73, 78, 79, 83, 89, 97, 101, 102, 103, 105, 107, 109, 110, 113, 114, 127, 130, 131, 137, 138, 139, 149, 151, 154, 157, 163, 165, 167, 170, 173, 174, 179, 181, 182, 186, 190, 191, 193

Offset

1,1

Comments

From Enrique Pérez Herrero, Jul 06 2012: (Start)

This sequence and A030229 partition the squarefree numbers: A005117.

Also solutions to the equation mu(n) = -1.

Sum_{n>=1} 1/a(n)^s = (zeta(s)^2 - zeta(2*s))/(2*zeta(s)*zeta(2*s)). (End) [See A088245 and the Hardy reference. - Wolfdieter Lang, Oct 18 2016]

The lexicographically least sequence of integers > 1 such that for each entry, the number of proper divisors occurring in the sequence is equal to 0 modulo 3. - Masahiko Shin, Feb 12 2018

The asymptotic density of this sequence is 3/Pi^2 (A104141). - Amiram Eldar, May 22 2020

References

B. C. Berndt & R. A. Rankin, Ramanujan: Letters and Commentary, see p. 23; AMS Providence RI 1995.

G. H. Hardy, Ramanujan, AMS Chelsea Publishing, 2002, pp. 64 - 65, (misprint on p. 65, line starting with Hence: it should be ... -1/Zeta(s) not ... -Zeta(s)).

S. Ramanujan, Collected Papers, Ed. G. H. Hardy et al., Cambridge 1927; Chelsea, NY, 1962, p. xxiv, 21.

H. S. Wilf, A Greeting; and a view of Riemann's Hypothesis, Amer. Math. Monthly, 94:1 (1987), 3-6.

Links

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

S. Ramanujan, Irregular numbers, J. Indian Math. Soc. 5 (1913) 105-106.

Eric Weisstein's World of Mathematics, Prime Factor

Eric Weisstein's World of Mathematics, Moebius Function

Eric Weisstein's World of Mathematics, Prime Sums

Formula

omega(a(n)) = A001221(a(n)) gives A005408. {primes A000040} UNION {sphenic numbers A007304} UNION {numbers that are divisible by exactly 5 different primes A051270} UNION {products of 7 distinct primes (squarefree 7-almost primes) A123321} UNION {products of 9 distinct primes; also n has exactly 9 distinct prime factors and n is squarefree A115343} UNION.... - Jonathan Vos Post, Oct 19 2007

a(n) < n*Pi^2/3 infinitely often; a(n) > n*Pi^2/3 infinitely often. - Charles R Greathouse IV, Sep 07 2017

Maple

a := n -> `if`(numtheory[mobius](n)=-1, n, NULL); seq(a(i), i=1..193); # Peter Luschny, May 04 2009
# alternative
A030059 := proc(n)
    option remember;
    local a;
    if n = 1 then
        2;
    else
        for a from procname(n-1)+1 do
            if numtheory[mobius](a) = -1 then
                return a;
            end if;
        end do:
    end if;
end proc: # R. J. Mathar, Sep 22 2020

Mathematica

Select[Range[300], MoebiusMu[#] == -1 &] (* Enrique Pérez Herrero, Jul 06 2012 *)

Prog

(PARI) is(n)=my(f=factor(n)[, 2]); #f%2 && vecmax(f)==1 \\ Charles R Greathouse IV, Oct 16 2015
(PARI) is(n)=moebius(n)==-1 \\ Charles R Greathouse IV, Jan 31 2017

Crossrefs

Cf. A000040, A001221, A005408, A007304, A008683, A030231, A051270, A123321, A030229 even nmbr. dist. primes), A005117, A088245, A104141.

Sequence in context: A359802 A028905 A365829 * A201879 A327783 A319333

Adjacent sequences: A030056 A030057 A030058 * A030060 A030061 A030062

Keyword

nonn,easy,nice

Author

N. J. A. Sloane

Extensions

More terms from David W. Wilson

Status

approved