# [seqfan] Re: Asymptotic for A074753

Christopher Gribble chris.eveswell at virgin.net
Fri Jan 15 17:54:55 CET 2010

I have been following this thread with interest and would like to
suggest the two following sequences:

The smallest and largest values of n for which sigma(n) = A002191(n).

I don't know whether they are in the OEIS or not but cannot find any
indication that that they are.

It may be worthwhile relating A074753 to A054973 i.e.
A074753(n) = sum(k=1..n-1, A054973(k)), for n > 2.

Best regards,

Chris Gribble

-----Original Message-----
From: seqfan-bounces at list.seqfan.eu [mailto:seqfan-bounces at list.seqfan.eu] On Behalf Of franktaw at netscape.net
Sent: 15 January 2010 1:29 AM
To: seqfan at list.seqfan.eu
Subject: [seqfan] Re: Asymptotic for A074753

Thanks to both of you.

I think the fact that A(x) is everywhere defined implies that lim
a(n)/n does actually exist, so that we don't need to talk about the lim
inf and lim sup.

-----Original Message-----
From: Matthew Conroy <jumbo at madandmoonly.com>

Thanks for that reference, Drew.

Using the full Table 1 in Wall's paper, I was able to calculate
lim inf a(n)/n > 0.6152954.

There is a lot of error in that table (there are large gaps between the
upper and lower bounds on A(x)).  More recent results suggest
these could be much improved.

Note for instance, in the Wall paper
0.2441 < A(2) < 0.2909
but this was greatly improved by Marc Deleglise to
0.2474 < A(2) < 0.2480 in his paper
"Bounds for the density of abundant integers",Marc Deléglise,
Experiment.
Math. Volume 7, Issue 2 (1998), 137-143.

Unforunately, Deleglise's paper does not have results for other A(x).

cheers,

Matt

On Jan 13, 2010, at 6:44 AM, drew at math.mit.edu wrote:

> I believe you can obtain asymptotic lower (and upper) bounds on a
> (n)/n by
> applying the results of
>
> [1] "Density Bounds for the Sum of Divisors Function", Charles R.
> Wall,
> Phillip L. Crews and Donald B. Johnson, Mathematics of Computation,
> Vol.
> 26, No. 119, 1972, pages 773-777,
>
> which builds on earlier work by Behrend and Davenport (from 1933). The
> reference above is available online at http://www.jstor.org/stable/
> 2005106.
> The bound one can obtain is certainly not tight, but it is above 0.36.
>
> I realize that not everyone on this list has convenient access to
> JSTOR, so
> I will briefly summarize the key points that are relevant to A074753.
>
> Let pi_A(N,x) count the positive integers m <= N for which sigma(m)
> >= mx,
> where x is a real number in the interval [1,5].
>
> It was proven by Davenport (see reference in [1]) that
>
>     A(x) = lim_{N\to\infty} pi_A(N,x)/N
>
> exists and is a continuous function of x. The reference [1] above
> gives a
> table of upper and lower bounds for A(x) for x = 1.0, 1.1 ....,
> 4.9, 5.0,
> for example, A(2) lies between 0.2441 and 0.2909.
>
> For a(n)=A074753(n), we may use this table to asymptotically bound a
> (n)/n.
> Indeed, for any real number x in [1,5] and positive integer n we have
>
> (*)    floor(n/x) - pi_A(n/x,x) <=  a(n)
>
> This is obtained by counting integers m <= n/x for which sigma(m) <
> mx <= n.
>
> Dividing (*) by n and taking limits as n tends to infinity yields
>
> (**)   lim inf a(n)/n >= (1-A(x)) / x.
>
> From Table 1 of [1], we have A(2.1) <= .2372, which implies that
>
> (1)   lim inf a(n)/n > 0.363238.
>
> This bound above can be likely be improved slightly by further
> subdividing
> the interval from [1,n] and applying multiple bounds on A(x).
>
> Regards,
>
> Drew
>
>> On Jan 12, 2010, at 9:22 AM, franktaw at netscape.net wrote:
>>
>> Since my last asymptotic question got no response :-(, I thought I
>> would try another one :-).
>>
>> For a(n) = http://www.research.att.com/~njas/sequences/A074753
>> (Number
>> of integers k such that sigma(k) < n), what is the limit n->infinity
>> a(n) / n?
>>
>> Up to 1 million, it appears to be about .673.
>>
>> Certainly the limit is less than 1; any 2k > 2n/3 will have sigma
>> (2k) >
>> n, so the limit must be < 5/6.  Similar arguments with other factors
>> can refine this; looking only at prime factors up to 100000, I get an
>> upper bound of .7044..., and this clearly converges to something in
>> this neighborhood.  I don't see any way to prove the actual limit
>> is >
>> 0, however.
>>
>
>
>
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>
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