## 17881Re: [PrimeNumbers] Re: Cubic x^3 + x^2 + x + t prime generators

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• Mar 30, 2006
--- Mark Underwood <mark.underwood@...> wrote:
> > > x^8 - x^4 + 1 has no prime factors below 73 (!)
> >
> > Now do you see why PIES has such the incredible density of primes
> that it has?
> > The above is just Phi(24). PIES is looking at Phi(49152) and Phi
> (98304). The
> > super-fruit subprojects can have even higher densities.
> >
> > One puzzle I set the PIES guys was the following:
> > <<<
> > Mathematicians are invited to calculate the relative density of
> primes
> > of the form Phi(24576,715*b^2) compared to arbitrary numbers of the
> > same size.
> > >>>
> > Just finding out what its smallest possible factor is should be an
> eye-opener.
> >
>
> Phil,
> Something has gone whizzing over my head, and it had to do with Pies
> and Phi's and such. But it looks totally fascninating and I hope to

Phi's are cyclotomic polynomials. They are the primitive parts of the family of
polynomials f(x) = x^n-1. Equivalently they are the minimal polynomial which
vanishes at each of the primitive n-th roots of unity.
e.g.
Phi(2) = x+1, as -1 is the only primitive 2nd root of unity,
Phi(4) = x^2+1, as +/-i are the only two primitive 4th roots of unity.

> My last result with x^8 - x^4 + 1 producing primes of the form 24n+1

That 24 comes from the fact that the above is Phi(24)
(In Pari/GP, us "polcyclo" to get the n-th cyclotomic polynomial.)

It's instructive to prove that if p|Phi(n,x) then, with a few exceptions,
p == 1 (mod n).

> got me to realize that there might be application to the "Web of
> Ones" thread, and without introducing factors of five into the
> coefficients. And sure enough,
>
> x^32 - x^24 + x^16 - x^8 + 1

Phi(80,x)

> yields only prime factors ending in one.
> (Except when x=0) Furthermore the prime factors are all of the form
> 80n + 1, and there are just 4 different prime factors below 1000:
> 241,401,641 and 881.

Yup, this is why cyclotomics are rich hunting grounds for primes, as they can't
have small divisors. My GEFs are typically 2-3 times as dense as Yves' GFNs, as
I reject even more small factors.

Phil

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