[seqfan] Re: Proof
Andrew Weimholt
andrew.weimholt at gmail.com
Tue Dec 1 10:12:32 CET 2009
Meant to say...
for some n, gcd ( n, (n-1)! ) = n
seems I can't even get the correction right the first time. :-P
Andrew
On 12/1/09, Andrew Weimholt <andrew.weimholt at gmail.com> wrote:
> Spotted an error!
>
> for some n, gcd( n, (n-1)! ) = (n-1)!
>
> This invalidates the (attempted) proof.
> Got caught doing my thinking on the keyboard instead of on paper again :-)
>
>
> Andrew
>
>
> On 11/30/09, Andrew Weimholt <andrew.weimholt at gmail.com> wrote:
> > Franklin T. Adams-Watters recently submitted A167234, and poses a
> > question in the comments:
> >
> > "What can we say about the asymptotic behavior of this sequence? Does
> > it contain every integer > 2 infinitely often?"
> >
> > The answer is yes, and here is the proof...
> >
> > By Dirichlet's Theorem, there are an infinite number of primes of the
> > form dk + b, for any positive coprimes d & b
> > Furthermore, Dirichlet showed that the primes are evenly distributed
> > over the phi(d) arithmetic progressions with difference d coprime to
> > the first term.
> > Thus the for a given d & b, the primes of the form dk + b account for
> > 1 / phi(d) of the primes.
> >
> > If we let d = (n-1)!, for n > 2
> > and b = 1,
> >
> > it follows that there are an infinite number of primes of the form
> > k*(n-1)! + 1,
> > and they account for 1 / phi( (n-1)! ) of the primes.
> >
> > In order to prove that the answer to Franklin's question is "yes", we will need
> > to show that the number of primes of the form k*(n-1)! + 1 is still
> > infinite when
> > we add the restriction that n does not divide k*(n-1)!
> >
> > For each k, where n | ( k*(n-1)! )
> >
> > there exists some t > 0, such that k = t * n / gcd(n, (n-1)! )
> >
> > Substituting into k(n-1)! + 1, we get
> >
> > t * n! / gcd ( n, (n-1)! ) + 1
> >
> > which is also an arithmetic progression for a given n, and t = 1, 2, 3, ...
> >
> > If there are not an infinite number of primes of the form k(n-1)! + 1,
> > where n does not divide k(n-1)!,
> >
> > then all of the primes of the form, k(n-1)! + 1 are also of the form t
> > * n! / gcd( n, (n-1)! )
> >
> > implying that
> >
> > phi( (n-1)! ) = phi( n! / gcd( n, (n-1)! ) )
> >
> > which we will now show is false
> >
> > case 1: n is composite:
> >
> > then n / gcd( n, (n-1)! ) is at least 2, meaning, n! / gcd( n, (n-1)!
> > ) is at least 2(n-1)!
> >
> > For x>1, there is always a prime in the interval (x, 2x).
> >
> > Therefore we have coprimes of n! / gcd( n, (n-1)! ) less than n! /
> > gcd( n, (n-1)! ) and greater than (n-1)!
> >
> > Also, since n is not prime, there are no prime factors of n, not
> > already in (n-1)!,
> > so all numbers less than (n-1)! which are coprime to (n-1)! are also
> > coprime to n! / gcd( n, (n-1)! )
> >
> > Therefore, phi( n! / gcd( n, (n-1) ) ) > phi( (n-1)! )
> >
> > case 2: n is prime:
> >
> > then phi( n! / gcd( n, (n-1)! ) ) = phi( n! ) > phi( (n-1)! )
> >
> > (Cf. A048855 : a(n) = phi ( n! ). Formula: a(n) = a(n-1)*n for
> > composite n, and a(n) = a(n-1)*(n-1) for prime n.)
> >
> > Therefore, there are an infinite number of primes of the form k(n-1)!
> > + 1 which are not congruent to 1 mod n.
> >
> > These primes are congruent to 1 mod x for all x in {1,2,...,n-1},
> > therefore, for these primes, n is the smallest modulus,
> > for which their two divisors are not congruent.
> >
> > Therefore, all numbers > 2 appear infinitely often in A167234.
> >
> >
> > Andrew
> >
>
More information about the SeqFan
mailing list