[seqfan] Re: More (composite) terms for A233281

Charles Greathouse charles.greathouse at case.edu
Wed Feb 12 05:20:08 CET 2014


This fails for (what I believe are called) Wall-Sun-Sun primes, which are
expected to be infinite in number but none are known.

Charles Greathouse
Analyst/Programmer
Case Western Reserve University


On Tue, Feb 11, 2014 at 10:57 PM, David Wilson <davidwwilson at comcast.net>wrote:

> Let Fib(f(p)) > 0 be the smallest positive Fibonacci number divisible by p.
>
> Empirically, it appears that
>
> f(2) = 3, f(4) = 6, f(2^(k+3)) = 6*2^k.
> For odd prime p, f(p) > 1, f(p^(k+1)) = f(p)*p^k.
>
> If true (which I assume it is), then f(p^k) is composite for prime p and k
> >
> 1.
> In other words, if k > 1, p^k | Fib(n) => n is composite.
> This would imply that Fib(p) for prime p is squarefree.
> As such Fib(p) is the product of k prime numbers.
> This means it has 2^k divisors, k divisors are prime, 1 divisor is 1.
> This leaves 2^k - k - 1 composite divisors.
> 2^k - k - 1 happens to be Eulerian number A(k, 1), I doubt there is a
> deeper
> connection to Eulerian numbers.
>
> We know that p divides Fib(f(p)) where f(p) = p + (0, -1, 1, 1, -1)
> according to whether p mod 5 is (0, 1, 2, 3, 4).
> Suppose p | F(q) for prime q.
> Since m | Fib(n) => m | Fib(kn), and p | Fib(f(p)), we conclude q | f(p).
> Hence f(p) = kq for some k >= 1.
> |f(p) - p| <= 1, so p must be within 1 of some multiple of q.
>
> So, with the exception of p = 5, all prime divisors of Fib(p) are of the
> form kp +- 1.
> For example, Fib(19) has prime divisors 37 = 2*19-1 and 113 = 4*19-1.
> This implies that Fib(p) cannot be divisible by a prime smaller than 2p-1.
> Not much of a lower bound, but a lower bound.
>
> > -----Original Message-----
> > From: SeqFan [mailto:seqfan-bounces at list.seqfan.eu] On Behalf Of Hans
> > Havermann
> > Sent: Monday, February 10, 2014 12:13 PM
> > To: Sequence Fanatics Discussion list
> > Subject: [seqfan] Re: More (composite) terms for A233281
> >
> > Attempting to understand this sequence better I have just posted a
> > (hopefully) complete list of composites derived from Fibonacci numbers
> with
> > prime indices, up to index 997:
> >
> > http://chesswanks.com/num/CompositeDivisorsOfPrimeIndexFibonaccis.txt
> >
> > There appear to be an Eulerian number < http://oeis.org/A000295 > of
> > solutions in each case. Antti's composites will of course be the sorted
> > collection of all these solutions to infinity. My question is still how
> one might
> > know that smallish solutions (say, up to a given size) will not come from
> > unsolved, future Fibonaccis.
> >
> > _______________________________________________
> >
> > Seqfan Mailing list - http://list.seqfan.eu/
>
>
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