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\lecture{9}{October 12, 2005}{Madhu Sudan}{Brendan Juba}
Today, we will continue our approach to factoring bivariate polynomials. We will
begin by recalling Hensel's Lifting Lemma, and we will discuss a few new things
about the Lemma. We will then give an outline of how we will perform the
factoring, and develop another concept: the resultant. Once we have these
concepts, the algorithm will be easy, and moreover, these concepts will help us
in other domains.
\section{Hensel's Lifting}
Suppose $f(x,y)=g(x,y)h(x,y)\pmod{y^k}$. We wanted a factorization for higher
powers of $y$: $f(x,y)=\tilde{g}(x,y)\tilde{h}(x,y)\pmod{y^{2k}}$. Hensel's
Lifting says we can obtain this if $g$ and $h$ are ``relatively prime,'' and
that the $\tilde{g}$ and $\tilde{h}$ we obtain are essentially unique. Formally:
\begin{lemma}[Hensel's Lifting] If $R$ is a ring, $J\subseteq R$ is an ideal,
and there exist $f,g,h,a,b$ such that
\begin{itemize}
\item[(H1) ]$f=gh\pmod{J}$
\item[(H2) ]$ag+bh=1\pmod{J}$
\end{itemize}
then there exist $\tilde{g},\tilde{h},\tilde{a},\tilde{b}$ such that
\begin{itemize}
\item[(C1) ]$g=\tilde{g}\pmod{J}$ and $h=\tilde{h}\pmod{J}$
\item[(C2) ]$f=\tilde{g}\tilde{h}\pmod{J^2}$
\item[(C3) ]$\tilde{g}\tilde{a}+\tilde{b}\tilde{h}=1\pmod{J^2}$
\end{itemize}
Uniqueness: if $g^\star$, $h^\star$ also satisfy (C1) and (C2), then $\exists u
\in J$ such that $g^\star =g(1+u)$ and $h^\star =h(1-u)$.
\end{lemma}
These conditions allow us to continue to $y^{4k}$ and so on.
We are planning to apply this with $R=\mathbb{F}_q[x,y]$ and $J=(y-y_0)^k$ for
some $y\in\mathbb{F}_q$. We'll start with a factorization in $\mathbb{F}_q[x]$,
and we'll increase the powers of $y$. We ultimately hope to get a factorization
that is not modulo any ideals.
\begin{remark}
We were given explicit formulas for $\tilde{g}$ and $\tilde{h}$. So, given $g,h,
a,b,f$, we can compute $\tilde{g},\tilde{h},\tilde{a},\tilde{b}$ in $O(1)$
operations in $R$, $\pmod{J}$, $\pmod{J^2}$, and so on.
For example, $\tilde{g}=g+g_1$ where $g_1=a(f-gh)$.
\end{remark}
Although we need to be capable of efficiently carrying out ring operations, and
the operations of reducing modulo an ideal, $\tilde{g}$ and $\tilde{h}$ don't
require any serious computations.
One more clarification: uniqueness will take on a nicer form for us.
\begin{corollary}
Suppose $R=\mathbb{F}_q[x,y]$ and $J=(y-y_0)^k$, and assume $f,g,h$ are monic in
$x$. Then, if $g$ and $h$ are relatively prime and $f=gh\pmod{J}$, there exist
monic polynomials $\tilde{g}$ and $\tilde{h}$ such that $f=\tilde{g}\tilde{h}
\bmod{(y-y_0)^{2k}}$ and $\tilde{g},\tilde{h}$ are unique $\bmod{(y-y_0)^{2k}}$.
\end{corollary}
Naturally, we can't expect any better than this modulo $(y-y_0)^{2k}$.
\begin{proof}
Existence and the fact that the polynomials are monic follow immediately.
Suppose $\tilde{g},\tilde{h},g^\star,h^\star$ satisfy the conditions. Then, by
Hensel's Lifting, $g^\star=\tilde{g}(1+u)$, $h^\star=\tilde{h}(1-u)$. If $u\neq
0\bmod{(y-y_0)^{2k}}$, then $g^\star$ is not monic.
\end{proof}
\section{Outline of Factoring}
We now turn to the question of how we can use Hensel's Lifting for bivariate
factoring---specifically, how we initiate it, and what we do after applying it.
Suppose we are given $f(x,y)$ and we wish to factor it into $f_1(x,y)f_2(x,y)
\cdots f_k(x,y)$. Suppose we pick some $y_0$ and substitute it in for $y$ to
obtain $f(x,y_0)=p^{(0)}(x)$, a polynomial of one variable, which we therefore
know how to factor into $p^{(0)}_1(x)\cdots p^{(0)}_k(x)$. Suppose now that we
are very lucky, and by picking $y_1\neq y_0$, we obtain $f(x,y_1)=p^{(1)}(x)=p^{
(1)}_1(x)\cdots p^{(1)}_k(x)$. Then we could ``interpolate'' pairs $(p^{(0)}_1
(x),p^{(1)}_1(x)), (p^{(0)}_2(x),p^{(1)}_2(x)),\ldots (p^{(0)}_k(x),p^{(1)}_k
(x))$ to obtain polynomials in $y$. Of course, we haven't formally defined what
we mean by this, and moreover, it's not clear how we should pair up the factors
of $p^{(0)}(x)$ and $p^{(1)}(x)$. Even more troublesome, depending on our choice
of $y_0$, we could get many more than $k$ factors!
For example, consider $f(x,y)=(x-5)(x-3)(x-2)+y$, which is irreducible. If we
now substitute $y=0$, we obtain $f(x,0)=(x-5)(x-3)(x-2)$, which has three
factors. We therefore see that we have a one-to-many correspondence between the
factors of $f(x,y_0)$ and the factors of $p(x)$, in general:
\[
\begin{array}{rcccccc}
f(x,y) &= &f_1(x,y)&\cdot&f_2(x,y)&\cdot\ \cdots\ \cdot&f_k(x,y)\\
p(x) &= &\overbrace{p_{11}(x)p_{12}(x)\cdots p_{1i_1}(x)}&\cdot&\overbrace{p_{21
}(x)p_{22}(x)\cdots p_{2i_2}(x)}&\cdot\ \cdots\ \cdot&\overbrace{p_{k1}(x)p_{k2}
(x)\cdot p_{ki_k}(x)}
\end{array}
\]
By Hensel's Lifting and a few other facts, we'll see that this is okay, that $p
_{11}$ and its many lifts will tell us $f_1$, provided that $p_{11}$ is
relatively prime to $p_{ij}$ for all $i$ and $j$.
Still, it is not yet clear how we will ensure that these elements are relatively
prime. We know this will not happen if $f(x,y)=f_1(x,y)^5$. More generally,
suppose $f(x,y)=f_1(x,y)^af_2(x,y)\cdots f_k(x,y)$ for $a\geq 2$. It turns out
that our intuitions from calculus hold here: suppose we take derivatives of $f$
with respect to $x$ and $y$. If $f_1(x,y)$ has positive degree in $x$, then
$\frac{\partial f}{\partial x}$ and $f$ will have a nontrivial GCD. We can argue
this the same way as before, using the sum and product rules. So, it turns out
that we don't need to worry about repeated factors in the factorization of $f$.
Now, suppose $f(x,y)=x^k+y$, and we try $f(x,0)=p(x)=x\cdot x\cdots x$. We
started with a polynomial with no repeated factors; how can we guarantee that
our choice of $y$ will not yield a polynomial with repeated factors? It's clear
that not every choice of $y$ fulfills this condition. In general, we will be
looking at the GCD of a polynomial and its derivative, so we will be studying
the GCDs of two polynomials more carefully.
{\bf We'd like reasoning of the form: }if $f(x,y)$ and $g(x,y)\ \left(=\frac{
\partial}{\partial x}f(x,y)\right)$ have no common factors, then $\exists y_0$
such that $f(x,y_0)$ and $g(x,y_0)$ have no common factors.
This begs the question, how can we derive statements about the GCDs, when
plugging in values for $y$ changes the structure of the polynomial completely?
Our answer will be the resultant.
\section{Resultants}
There are many ways of developing and talking about resultants, and our
definition may be inconsistent with some of the others, but it will work for our
purposes.
\begin{definition}
Let $R$ be a UFD with some notion of order on its elements\footnote{For $R=
\mathbb{Z}$, this order is given by the magnitude. For $R=\mathbb{F}_q[y]$, the
order is given by the degree in $y$} and let $f,g\in R[x]$. The {\em resultant
of $f$ and $g$} is then the ``minimal'' element in the ideal generated by $(f,g)
$: $\{af+bg|a,b\in R[x]\}$. That is, the element with the smallest leading
coefficient among the lowest degree polynomials in that set (excluding the zero
polynomial).
\end{definition}
When $R=\mathbb{Q}$, the lowest degree polynomial is easily obtained from the
GCD, whereas if $R=\mathbb{Z}$ (e.g., if $R$ is not a field), $f=2x^3+3x$, and
$g=9x^{10}$, then we find that $\mathrm{resultant}(f,g)$ is some multiple of $x$
(we take the smallest one).
Now consider polynomials $f,g\in\mathbb{F}_q[x,y]$ that don't share a common
factor. Let $\mathrm{Res}_x(f,g)$ denote the resultant of $f$ and $g$ when we
take $R=\mathbb{F}_q[y]$. We will see that we can say that the $x$ degree of
$R(x,y)=\mathrm{Res}_x(f(x,y),g(x,y))$ is zero.
We can see this if we consider $f,g\in\widetilde{\mathbb{F}_q[y]}[x]$. Over a
field, since $f$ and $g$ have no common factors, we know $\gcd(f,g)=1=R(x,y)$.
Thus, over $\mathbb{F}_q[y]$, we should at least be able to get some polynomial
in $y$ (which is divided out in $\widetilde{\mathbb{F}_q[y]}$). That is,
$\exists a(x,y),b(x,y)$ such that $a(x,y)f(x,y)+b(x,y)g(x,y)=R(y)$ where $R(y)
\neq 0$.
Recall that we wanted to show $f(x,y_0)$ and $g(x,y_0)$ don't have a common
factor; we generally show this via the extended GCD by demonstrating $af(x,y_0)+
bg(x,y_0)=1$. Now, if $R(y)\neq 0$, then in some extension field we will be able
to find a $y_0$ such that this condition holds. We'll guarantee this by placing
bounds on the degree of $R$.
We'd now like to find $a$ and $b$ to obtain $\mathrm{Res}(f,g)$. Note that our
ideal $\{af+bg|a,b\in R[x]\}$ is a linear space and our conditions are linear,
so we'll be able to solve for $a$ and $b$ easily.
{\bf Bounding the degree of $R(y)$: }put $\deg_xf\leq d_1$, $\deg_yf\leq d_1$,
$\deg_xg\leq d_2$, $\deg_yg\leq d_2$.
\begin{claim}
$\exists R$, a nonzero polynomial with $\deg R\leq 2d_1d_2$ such that $R(y)=a(x,
y)f(x,y)+b(x,y)g(x,y)$.
\end{claim}
\begin{proof}
Let the coefficients of $a$ and $b$ be unknowns, and $R(x,y)=a(x,y)f(x,y)+b(x,y)
g(x,y)$. Note that $R$ is temporarily a polynomial in $x$ as well as $y$. We'll
say that the coefficients of $x^iy^j$ are linear forms in the unknowns, and
require that they be zero if $i>0$. So we have a system of homogeneous linear
constraints, where we need a solution other than $a,b=0$ and $af+bg=0$. First,
we'll make the degrees in $x$ small, so there's no chance of cancellation. Then,
we'll ``shoot for'' $\deg R(y)=\Delta$. We bound $\deg_xa<\deg_xg$ (so $a\notin
\{\pm g\}$) and $\deg_xb<\deg_xf$ (which similarly rules out $b\in\{\pm f\}$).
We now further impose $\deg_ya<\Delta-d_1$ (so $\deg_y(af)<\Delta$) and $\deg_yb
<\Delta-d_2$. Thus, now, $\deg_xR<\deg_xf+\deg_xg$ and $\deg_yR<\Delta$.
\begin{claim}
$\exists a,b\neq 0$ satisfying these constraints.
\end{claim}
(e.g., with $\deg_x(af+bg)\leq 0$)
Observe:
\[
\begin{array}{rcccl}
\mathrm{\#\ of\ unknowns:\ }&
\underbrace{\deg_xg(\Delta-d_1)}&+&\underbrace{\deg_xf(\Delta-d_2)}&\geq\Delta(
\deg_xf+\deg_xg)-2d_1d_2\\&\mathrm{\#\ coeffs\ in\ }a&&\mathrm{\#\ coeffs\ in\ }
b&\\
\mathrm{\#\ of\ constraints:\ }&
\underbrace{(\deg_xf+\deg_xg-1)}&\cdot&\underbrace{\Delta}&=\Delta(\deg_xf+\deg
_xg)-\Delta\\
&\mathrm{\#\ powers\ in\ }x&&\mathrm{\#\ powers\ in\ }y&\end{array}
\]
So we see that \# unknowns $>$ \# constraints if $\Delta>2d_1d_2$: we can then
find $a$ and $b$ such that there are no powers of $x$ in $R$. All that remains
is to show that $af+bg$ is not identically zero.
\begin{claim}
We can't obtain an identically zero polynomial unless $f$ and $g$ share a common
factor.
\end{claim}
Observe that, if $f$ and $g$ have no common factors, since we are in a UFD, then
$af+bg=0$ implies that the factorizations of $af$ and $bg$ are the same. Since
$f$ and $g$ share no factors, $a$ must have all of the factors of $g$ and $b$
must have all of the factors of $f$, which can't happen by our bounds on the
degrees in $x$ of $a$ and $b$.
\end{proof}
Now, by our bound on the degree of $R(y)$, for a sufficiently large extension
field, there must be choices of $y_0$ such that $R(y_0)\neq 0$. Hence, if $f$
and $g$ have no repeated factors, for such a choice of $y_0$, $f(x,y_0)$ and $g
(x,y_0)$ have no repeated factors.\footnote{When $g=\frac{\partial f}{\partial x
}$, $\mathrm{Res}(f,g)$ is called the ``discriminant'' of $f$. For $f=ax^2+bx+c
$, $\mathrm{disc}(f)=b^2-4ac$. We ignored questions about whether the
discriminant was positive or negative, so our notions may not agree, but at
least we know that ours is nonzero if there are no repeated roots, and is zero
otherwise.} We can take the derivative or plug in $y_0$ in any order we like.
\section{Outline of Factoring, revisited}
We now give a more complete outline for factoring bivariate polynomials.
Given a monic $f(x,y)\in\mathbb{F}_q[x,y]$, we'll work with some extension field
$K$ of $\mathbb{F}_q$.
\begin{enumerate}
\item Find $y_0\in K$ such that $f(x,y_0)$ has no repeated factors. (how do we
do this? compute $\mathrm{Res}\left(f,\frac{\partial f}{\partial x}\right)$, and
plug in $y_0=1,2,\ldots$ until we find one that works.)
\item Put $p(x)=f(x,y_0)$, and factor it. Some factor should be linear; if not,
we extend $K$. We know that every field can be extended until the polynomial
splits completely, but we only need a low degree extension with one linear
factor, say $p(x)=(x-\alpha)h(x)$. We assume $(x-\alpha)$ and $h(x)$ are
relatively prime.
\item Now we start to apply Hensel's Lifting: $f(x,y)=(x-\alpha)h(x)\bmod{(y-y
_0)}$. After some iterations, $f(x,y)=(x-A(y))H(x,y)\bmod{(y-y_0)^t}$.
\item Now, from $A(y)$ we ask if we can find a nontrivial factor of $f$. We will
see that this is related to the notion of minimal polynomials. We will try to
find $f_1(x,y)$ with $\deg_xf_1<\deg_xf$ such that $f_1(x,y)=(x-A(y))H_1(x,y)
\bmod{(y-y_0)^t}$. It will turn out that, if the $f_1$ we find is the same as
$f$, then we will have a proof that $f$ has no nontrivial factors!
\end{enumerate}
We'll complete the last step by using linear algebra (which, along with the GCD,
comprises our toolbox at the moment), e.g., we'll give linear constraints and
show that solutions exist. Then, we'll say something along the lines of ``if
$\gcd(f_1,f)$ is nontrivial, report it, else $f$ is irreducible.''
\end{document}