Friday, 9 June 2017

abstract algebra - Dependence of algebraic elements in a finite field

Lets work over the finite field $\mathbb{F}_p$ for a prime $p$. Consider a monic irreducible polynomial $f(X)=X^3+aX^2+bX+c$ in $\mathbb{F}_p[X]$. Let $x$ be a root of $f(x)=0$ (say, in the closure of $\mathbb{F}_p$). Consider another different monic irreducible polynomial $g(Y)=Y^3+AY^2+BY+C$ in $\mathbb{F}_p[Y]$. Let $y$ be a root of $g(Y)=0$ again in the same closure.




How do we explicity relate $x$ and $y$? Can we express $y$ as a polynomial in $x$ with coefficients from $\mathbb{F}_p$?



I know that the field extension $\mathbb{F}_{p^3}$ which can be realized as $\mathbb{F}_p[X]/f$ contains $x$ and $y$ (as it contains every root of every irreducible of degree $3$), so $y$ should be expressible in the form
$$y=\alpha x^2+ \beta x +\gamma$$
for some $\alpha, \beta, \gamma \in \mathbb{F}_p$.



But this is what I am trying to establish, without using the uniqueness of $\mathbb{F}_{p^3}$ or it being the splitting field of $X^{p^3}-X$. I would prefer not using any non-trivial facts about finite fields here. My aim is to show that every cubic irreducible has a root in the specific extension $\mathbb{F}_p[X]/f$, and I want to do this by explicitly obtaining a dependence of the form $y=\alpha x^2+ \beta x +\gamma$ whenever $y$ satisfies $y^3+Ay^2+By+C=0$.



Is there some algebraic expression or algorithm to find $\alpha, \beta, \gamma$? Something on the lines of resultants maybe.




I was thinking of considering the ideal $$ in $\mathbb{F}_p[X,Y]$ and show that there exists an element in this ideal with total degree at most $2$. But it doesn't seem to lead me anywhere much, especially since I am not clear how to incorporate the (essential) property that both the polynomials are irreducible. Would a Grobner basis of the ideal help with this? I am not too familiar with that technique, but will explore it if it is relevant here. Thanks.



(This has now been cross-posted to MathOverflow: https://mathoverflow.net/questions/243458/algebraic-dependence-of-irreducibles-in-a-finite-field)

No comments:

Post a Comment

real analysis - How to find $lim_{hrightarrow 0}frac{sin(ha)}{h}$

How to find $\lim_{h\rightarrow 0}\frac{\sin(ha)}{h}$ without lhopital rule? I know when I use lhopital I easy get $$ \lim_{h\rightarrow 0}...