## Re: number of selfridges?

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• I tested with pari-gp to n=5*10^9: { for(n=7,5000000000, if(n%50000000==0,print(n)); if(gcd(n,30)==1&&!issquare(n)&&!isprime(n),
Message 1 of 46 , Dec 9, 2011
I tested with pari-gp to n=5*10^9:

{
for(n=7,5000000000,
if(n%50000000==0,print(n));
if(gcd(n,30)==1&&!issquare(n)&&!isprime(n),
x=1;while(kronecker(x^2-4,n)!=-1,x=x+1);
if(Mod(Mod(1,n)*(l*x-3),l^2-x*l+1)^((n+1))+2*x^2-9==0,
print(n" "x))))
}

I converted the following to GMP with the use of a nextprime sieve from Vincent Diepeveen, taking x^2-3 outside of the loop:

{
if(gcd(n,30)==1&&!issquare(n),
x=1;while(kronecker(x^2-4,n)!=-1,x=x+1);
bin=binary(n+1);ln=length(bin);
a=x;b=-3;
for(i=2,ln,
na=a*(x*a+2*b);b=((b-a)*(b+a))%n;a=na%n;
if(bin[i],
na=(x^2-3)*a+x*b;b=-(x*a+3*b)%n;a=na%n));
print(a==0&&Mod(b,n)==9-2*x^2))
}

The new GMP test program runs ~14 times quicker than the pari-gp code. I can crunch about 2*10^9 in a 3.6GHz hour.

Without finding a counterexample (to this 2-selfridge test), I have now reached 10^10. I'll take it up to 10^11...

Paul
• ... Combining fails with the composite counterexample n=256999 and x=32768, However, I have tested the 1+1+1+2 conjecture up to n
Message 46 of 46 , Apr 14, 2012
--- In primenumbers@yahoogroups.com, "paulunderwooduk" <paulunderwood@...> wrote:
>
>
>
> --- In primenumbers@yahoogroups.com, "paulunderwooduk" <paulunderwood@> wrote:
> >
> > Hi,
> >
> > I have added a Fermat test to make a 1+1+1+2 selfridge test:
> >
> > For N>5, with gcd(6,N)==1, find an integer x:
> > gcd(x^3-x,N)==1
> > kronecker(x^2-4,N)==-1
> >
> > and check:
> > (x+2)^((N-1)/2)==kronecker(x+2,N) (mod N) (Euler)
> > (x-2)^((N-1)/2)==kronecker(x-2,N) (mod N) (Euler)
> > x^(N-1)==1 (mod N) (Fermat)
> > L^(N+1) == 1 (mod N, L^2-x*L+1) (Lucas)
> >
>
> Note: I should say gcd(30,N)==1 because gcd(x^3-x,N)==1 and kronecker(x^2-4,n)==-1.
>
>
> Now consider combining the 2 Euler tests with the Lucas test:
>
> (L*D)^((n+1)/2)==D (mod N, L^2-x*L+1) (D=x^2-4.)
>
> with the restriction kronecker(x+2,N)==-1.
>
> These together with the Fermat test makes for a 1+2-selfridge test.
>
> Can you find a counterexample?
>
> So far the near-refutation from Pinch's carmichael list is:
> N,x,gcd(x^2-1)
> ------------------
> 1909001 884658 1909001
>
> Paul
>

Combining fails with the composite counterexample n=256999 and x=32768, However, I have tested the 1+1+1+2 conjecture up to n<10^7,

Paul -- restoring symmetry
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