[seqfan] Re: Numbers with squarefree exponents in their prime factorizations
Vladimir Shevelev
shevelev at bgu.ac.il
Fri Sep 11 21:28:44 CEST 2015
Dear Seq Fans,
For a fixed prime p, 1/p^j -1/p^(j+1) = (p-1)/p^(j+1)
is the density of the numbers divisible by p^j and not divisible
by p^(j+1), i.e., the numbers with p^j in their prime power
factorization (PPF). Let S be a set of nonegative integers.
Then sum_{j in S} (p-1)/p^(j+1) means the density of
the numbers having in their PPF the prime p with exponent
from S. Thus convergent product
prod_{prime p}(sum_{j in S} (p-1)/p^(j+1)) (1)
should give the density of the numbers with all exponents
from S in their PPF. For example, let S_k be set {0,1,...,k-1}.
Then sum_{j in S_k} (p-1)/p^(j+1) = 1 - 1/p^k and
prod_{prime p}(sum_{j in S} (p-1)/p^(j+1)) =
prod_{prime p}(1 - 1/p^k) = 1/zeta(k) which corresponds
to the known density of k-free numbers.
Let now S consists of 0 and squarefree numbers. Then (1)
should give the density of so-called exponentially squarefree
numbers (A209061).
On the other hand, Laszlo Toth in his Theorem 3 (see link in
A209061) obtained another formula for the same density
(without an explanation in detail (see proof)):
Prod_{prime p}(1+sum_{j>=4}((mu(j)^2-mu(j-1)^2)/p^j), (2)
where mu(n) is the Möbius function.
I asked Peter Moses to calculate this density by (1) and (2),
but he obtained different results: by (1), 0.955695..., while, by (2),
0.955923... . If anyone can verify these calculations?
Best regards,
Vladimir
________________________________________
From: SeqFan [seqfan-bounces at list.seqfan.eu] on behalf of Vladimir Shevelev [shevelev at exchange.bgu.ac.il]
Sent: 04 September 2015 14:27
To: Sequence Fanatics Discussion list
Subject: [seqfan] Re: Numbers with squarefree exponents in their prime factorizations
A better upper estimate for density of
A209061 is 1- sum_{k>=4}(1-|mu(k)|)*
(1/zeta(k+1) - 1/zeta(k))<0.95637
Best regards,
Vladimir
________________________________________
From: SeqFan [seqfan-bounces at list.seqfan.eu] on behalf of Vladimir Shevelev [shevelev at exchange.bgu.ac.il]
Sent: 03 September 2015 18:20
To: Sequence Fanatics Discussion list
Subject: [seqfan] Re: Numbers with squarefree exponents in their prime factorizations
An upper estimate for density of A209061
is 1-1/zeta(5)+1/zeta(4)=0.95955...
Best regards,
Vladimir
________________________________________
From: SeqFan [seqfan-bounces at list.seqfan.eu] on behalf of Charles Greathouse [charles.greathouse at case.edu]
Sent: 02 September 2015 22:19
To: Sequence Fanatics Discussion list
Subject: [seqfan] Numbers with squarefree exponents in their prime factorizations
I was looking at sequence A209061 today and wondered what its density was.
With A046100 as a subsequence it's at least 1/zeta(4) = 0.92..., of course.
But I can't quite figure out what Euler product to use here -- can anyone
help out to improve the sequence?
Charles Greathouse
Analyst/Programmer
Case Western Reserve University
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