

A048109


Numbers having equally many squarefree and nonsquarefree divisors; number of unitary divisors of n (A034444) = number of nonunitary divisors of n (A048105).


15



8, 24, 27, 40, 54, 56, 88, 104, 120, 125, 135, 136, 152, 168, 184, 189, 232, 248, 250, 264, 270, 280, 296, 297, 312, 328, 343, 344, 351, 375, 376, 378, 408, 424, 440, 456, 459, 472, 488, 513, 520, 536, 552, 568, 584, 594, 616, 621, 632, 664, 680, 686, 696
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OFFSET

1,1


COMMENTS

For these terms the number of divisors should be a special power of two because ud(n)=2^r and nud(n)=ud(n). In particular the exponent of 2 is 1+A001221(n), the number of distinct prime factors + 1. Thus this is a subsequence of A036537 where A000005(A036537(n)) = 2^s; here s=1+A001221(n).
Let us introduce a function D(n)=sigma_0(n)/(2^(alpha(1)+...+alpha(r)), sigma_0(n) number of divisors of n (A000005), prime factorization of n=p(1)^alpha(1) * ... * p(r)^alpha(r), alpha(1)+...+alpha(r) is sequence (A086436). This function splits the set of positive integers into subsets, according to the value of D(n). Squarefree numbers (A005117) has D(n)=1, other numbers are "deviated" from the squarefree ideal and have 0 < D(n) < 1. So for D(n)=1/2 we have A048109, D(n)=3/4 we have A067295.  Ctibor O. Zizka, Sep 21 2008
Integers n such that there are exactly 3 Abelian groups of order n. That is, n such that A000688(n)=3. In other words, in the prime factorization of n there is exactly one prime with exponent of 3 and the others have exponent of 1.  Geoffrey Critzer, Jun 09 2015
The asymptotic density of this sequence is (6/Pi^2) * Sum_{k>=1} 1/(prime(k)^2*(prime(k)+1)) = (1/zeta(2)) * Sum_{k>=3} (1)^(k+1) * P(k) = 0.0741777413672596019212880156082745910562809066233004356300970463709875..., where P is the prime zeta function.  Amiram Eldar, Jul 11 2020


LINKS

Robert Israel, Table of n, a(n) for n = 1..10000


FORMULA

Numbers k such that d(k) = 2^(omega(k)+1) or A000005(k) = 2^(A001221(k) + 1) = 2 * A034444(k).


EXAMPLE

n = 88 = 2*2*2*11 has 8 divisors, of which 4 are unitary divisors (because of 2 distinct prime factors) and 4 are nonunitary divisors: U={1,88,11,8} and NU = {2,44,4,22}.


MAPLE

filter:= proc(n) local F;
F:= ifactors(n)[2];
mul(t[2]+1, t=F) = 2^(1+nops(F))
end proc;
select(filter, [$1..1000]); # Robert Israel, Jun 09 2015


MATHEMATICA

Position[Table[FiniteAbelianGroupCount[n], {n, 1, 1000}], 3] // Flatten (* Geoffrey Critzer, Jun 09 2015 *)


PROG

(PARI) is(n)=select(e>e>1, factor(n)[, 2])==[3]~ \\ Charles R Greathouse IV, Jun 10 2015
(PARI) isok(n) = sumdiv(n, d, issquarefree(d)) == sumdiv(n, d, !issquarefree(d)); \\ Michel Marcus, Jun 24 2015


CROSSREFS

Cf. A000005, A001221, A034444, A036537, A048106, A048107.
Sequence in context: A336593 A176297 A175496 * A068781 A212861 A333961
Adjacent sequences: A048106 A048107 A048108 * A048110 A048111 A048112


KEYWORD

nonn


AUTHOR

Labos Elemer


EXTENSIONS

New name based on comment by Ivan Neretin, Jun 19 2015


STATUS

approved



