# Isotropic forms in field of p-adic numbers

Printable View

• November 22nd 2012, 04:41 AM
AlexanderW
Isotropic forms in field of p-adic numbers
Hi,

for which primes $p$ are the quadratic forms

$7x^2-y^2-5z^2$

and

$w^2+x^2+3y^2+11z^2$

isotropic over the field over $p$-adic numbers $\mathbb Q_p$ ?

A abbreviated form for the two quadratic forms above is $\left \langle 7,-1,5 \right \rangle$ and $\left \langle 1,1,3,11 \right \rangle$.

Define $\varepsilon (\left \langle a_1,...,a_n \right \rangle):=\prod_{1\leq i< j\leq n} (a_i,a_j) \in \left \{ \pm 1 \right \}$.

I can use the fact that a regular quadratic form $\phi$ over a field $F$ is isotropic if $dim(\phi)=3$ and
$\varepsilon(\phi)=(-1,-d)$.

[ $(. , .)$ denotes the Hilbert symbol and $d:=disc(\phi)$ the discriminant of $\phi$.]

Moreover, a regular quadratic form $\phi$ over a field $F$ is isotropic if $dim(\phi)=4$ and
$d \neq 1$ or { $d=1$ and $\varepsilon(\phi)=(-1,-1)$}.

$d \neq 1$ means actually that the residue classes of $d$ and $-1$ are not equal modulo $F^*^2$.

Let's come to the first form:
$\varepsilon (\left \langle 7,-1,-5 \right \rangle)=(7,-1)(7,-5)(-1,-5)$

$d=disc(\left \langle 7,-1,-5 \right \rangle)=35$, therefore $-d=-35$.

Now I have to check for which primes $p$
$(7,-1)(7,-5)(-1,-5) = (-1,-35)$ in $\mathbb Q_p$.

But I don't know how to solve it.

Thanks!

Regards

Alexander
• November 28th 2012, 08:48 AM
AlexanderW
Re: Isotropic forms in field of p-adic numbers
Hello,

this problem is still unsolved.
If you have any hints or ideas, it would be great if you would answer.

Bye,
Alexander
All times are GMT -8. The time now is 01:37 AM.
{"extraction_info": {"found_math": true, "script_math_tex": 0, "script_math_asciimath": 0, "math_annotations": 0, "math_alttext": 0, "mathml": 0, "mathjax_tag": 0, "mathjax_inline_tex": 0, "mathjax_display_tex": 0, "mathjax_asciimath": 0, "img_math": 0, "codecogs_latex": 31, "wp_latex": 0, "mimetex.cgi": 0, "/images/math/codecogs": 0, "mathtex.cgi": 0}, "config": {"markdown_headings": true, "markdown_code": true, "boilerplate_config": {"ratio_threshold": 0.18, "absolute_threshold": 10, "end_threshold": 15, "enable": true}, "remove_buttons": true, "remove_image_figures": true, "remove_link_clusters": true, "table_config": {"min_rows": 2, "min_cols": 3, "format": "github"}, "remove_chinese": true, "remove_edit_buttons": true, "extract_latex": true}, "english_confidence": 0.8971686363220215, "perplexity_flag": "middle"}