- On Sunday 03 April 2005 23:48, you wrote:
> The prime 100999 is of the mod8=7 variety. So

Fermat has proven that all primes p == 1 mod 4 are uniquely represented as a

> x^2 + y^2 = 100999 should (and does) have no solution.

sum of two squares, while primes p == 3 mod 4 have no such representation. p

== 7 mod 8 is also == 3 mod 4.

> x^2 + 2*y^2 = 100999 should (and does) have no solution.

I believe this is related to the theory of imaginary quadratic fields (and in

particular elliptic curves with complex multiplication, which I'm a little

bit more familiar with, despite what David might rightly say after my

blunders on the primeform list). Now, someone correct me if I'm wrong, but

you can show that there's indeed only one solution, related to the

decomposition of 100999 in the field Q(sqrt(-2)). You can use Cornacchia's

algorithm to efficiently search for solutions or show that there are none.

> x^2 - 2*y^2 = 100999 should (and does) have exactly one solution.

Now this is out of my territory, but by symmetry I would risk the very

uneducated guess that it is related to the theory of real quadratic fields.

> Compare this to the composite number 100521.

I believe this is related to the fact that 100521 can be factorized in two

>

> x^2 + y^2 = 100521 has 2 solutions.

different ways (up to units, of course) in the ring of Gaussian integers

Z[i]: 3^4*(4 - i)*(3 + 8i) and 3^4*(4 + i)*(3 + 8*i).

> x^2 + 2*y^2 = 100521 has 10 solutions.

Similarly, I believe this should be factorable in Q(sqrt(-2)) in 10 different

ways up to units.

> x^2 - 2*y^2 = 100521 has 2 solutions.

Won't comment here, but I'm hoping the idea about real quadratic fields and

its extension here is correct.

Décio

[Non-text portions of this message have been removed] - --- In primenumbers@yahoogroups.com, "Mark Underwood"

<mark.underwood@s...> wrote:>

...

> x^2 - 2*y^2 = 100999 should (and does) have exactly one solution.

...

>

> Mark

Solutions of x^2 - 2*y^2 = 100999:

>

x0=357, y0=115

Xn+1 = 3 Xn + 4 Yn

Yn+1 = 2 Xn + 3 Yn

so we can deduce:

x1=1531, y1=1059

x2=8829, y2=6239

x3=51443, y3=36375

x4=299829, y4=212011

x5=1747531, y5=1235691

x6=10185357, y6=7202135

x7=59364611, y7=41977119

x8=346002309, y8=244660579

x9=2016649243, y9=1425986355

....

We can also negate x_n and/or y_n.

You can find the solution using my Quadratic Diophantine Equation

Solver at:

http://www.alpertron.com.ar/QUAD.HTM

Best regards,

Dario Alpern

Buenos Aires - Argentina - Thank you Decio and Dario

Decio I looked up Fermat's "4n+1" Theorem and it is as you say, and

proved by Euler. (That is, every prime of the form 4n+1 can be

uniquely expressed as x^2 + y^2.) Thanks for the input on the other

forms. I have a lot to learn.

Dario, your data was great, thanks. Because of it I am going to

investigate something further. I had only done solutions counts, not

investigated the properties of the actual solutions before.

Your data showed many solutions, and I said their was only one

solution for p = x^2 -2*y^2 , p = 8m+7. What gives?

Brings out another one of my mistakes! I had said that y<p/2, but

what I meant to say was that y < p^2 /2. In such a case there is only

one solution.

I've book marked your very cool site BTW!

Mark

--- In primenumbers@yahoogroups.com, "Dario Alpern" <alpertron@h...>

wrote:>

> --- In primenumbers@yahoogroups.com, "Mark Underwood"

> <mark.underwood@s...> wrote:

> >

> ...

> > x^2 - 2*y^2 = 100999 should (and does) have exactly one solution.

> >

> ...

> > Mark

> >

>

> Solutions of x^2 - 2*y^2 = 100999:

>

> x0=357, y0=115

>

> Xn+1 = 3 Xn + 4 Yn

> Yn+1 = 2 Xn + 3 Yn

>

> so we can deduce:

>

> x1=1531, y1=1059

> x2=8829, y2=6239

> x3=51443, y3=36375

> x4=299829, y4=212011

> x5=1747531, y5=1235691

> x6=10185357, y6=7202135

> x7=59364611, y7=41977119

> x8=346002309, y8=244660579

> x9=2016649243, y9=1425986355

> ....

>

> We can also negate x_n and/or y_n.

>

> You can find the solution using my Quadratic Diophantine Equation

> Solver at:

>

> http://www.alpertron.com.ar/QUAD.HTM

>

> Best regards,

>

> Dario Alpern

> Buenos Aires - Argentina