[seqfan] Re: Comment on A151750 [erratum]

[David Wilson]

Sorry, I got a sign wrong.

----- Original Message ----- 
From: "David Wilson" <davidwwilson at comcast.net>
To: "Sequence Fanatics Discussion list" <seqfan at list.seqfan.eu>
Sent: Saturday, August 01, 2009 4:28 PM
Subject: [seqfan] Re: Comment on A151750


For b >= 2, let

    S_b = { n : b does not divide choose(2n, n) }

I believe you can prove

    S_b = { n : every digit in b-ary representation of n is <= (b-1)/2 }

This would imply that

    |{ n in S_b : n < b^k }| = ((b-1)/2)^k

which would in turn give us a density estimate for S_b.

We can then compute a density estimate S_3 intersect S_5 intersect S_7
intersect S_11 and decide whether we expect this set to be infinite or
finite.


----- Original Message ----- 
From: "peter.luschny" <peter.luschny at googlemail.com>
To: <seqfan at list.seqfan.eu>
Sent: Friday, July 31, 2009 3:44 PM
Subject: [seqfan] Comment on A151750


Inspired by Niel's yesterdays submission A151750
I tried to integrate this difficult problem in the
framework of divisibility properties of the swinging
factorial. (See also
http://www.luschny.de/math/primes/SwingingPrimes.html
---

Let S be a sequence and p a prime number.

S(n) is 'p-primefree' iff no prime <= p divides S(n).

Now we can associate to S

pf(S,i) -> seq(n, such that S(n) is ith_prime-primefree).

For example assume S to be the *odd kernel* of the
swinging factorial $ (A056040).

Then 6$ is 3-primefree, 20$ is 5-primefree, 1512$
is 7-primefree, 6320$ is 11-primefree, ...

The first few values of the associated sequences are:

pf(odd($),1) = 1,2,6,7,8,18,19,20,24,25,26,54,..
pf(odd($),2) = 1,2,20,24,54,60,61,62,72,73,74,504..
pf(odd($),3) = 1,2,20,1512,1513,1514,..
pf(odd($),4) = 1,2,6320,...
pf(odd($),5) = 1,2,...

Which of these sequences are finite? And if they are
finite, what is the largest n such that S(n) is in PF(S,i)?

i=1: infinite (Lucas),
i=2: infinite (Erdös et al.),
i=3: infinite conjectured (?),
i=4: finite conjectured, max=6320,
i>=5: finite? infinite?, max=2 always?
See the comments in A030979 and A151750.

The 3160 from A151750 and of A129489 is 6320 / 2.
A129508, doubled, is a subsequence of pf(odd($),2).
A030979, doubled, is a subsequence of pf(odd($),3).
See also A129488.

A possible attempt to concentrate these relations
in a sequence:

pf(odd($)): i -> min {pf(odd($),i) > primorial(i)}.
6,20,1512,6320,?,?,..   ( ? = 2 )
======================

Many questions open. Any comment appreciated.

Cheers Peter

--------------------------------------------------
swing := proc(n) option remember;
if n = 0 then 1 elif irem(n, 2) = 1 then
swing(n-1)*n else 4*swing(n-1)/n fi end:

oddprimorial := proc(n) option remember; local i;
mul(ithprime(i+1),i=1..n) end:

pf := proc(n,m) local v,k,p; v := 0;
p := oddprimorial(n);
for k from 1 to m do
if igcd(swing(k),p) = 1 then v := k fi;
if v > p then break fi od; [n,v] end:

submit := proc(n) pf(n,10000) end;
seq(submit(i),i=1..10);

[1, 6], [2, 20], [3, 1512], [4, 6320], [5, 2],
[6, 2], [7, 2], [8, 2], [9, 2], [10, 2], ...
--------------------------------------------------


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