# Algebra/Chapter 10/Symmetric Polynomials/Monomial Ordering
## Definition 1

Let $(A_{1},\ldots ,A_{n})$ be an N-tuple. Let us define:
    ${\vec {A}}^{n}=(A_{1},\ldots ,A_{n})$
For functions ${\vec {F}}^{m},{\vec {G}}^{n}$ let us define:
    ${\vec {F}}^{m}({\vec {G}}^{n})={\biglF_{1}(G_{1},\ldots ,G_{n}),\ldots ,F_{m}(G_{1},\ldots ,G_{n}){\bigr )}$
This abbreviated notation will be of great use to us in the following pages and in the proof.

## Definition 2

Let $\mathbb {F}$ be a field. Then $\mathbb {F} [{\vec {X}}^{n}]$ is the polynomial space in variables $X_{1},\ldots ,X_{n}$ with coefficients in $\mathbb {F}$.
A monomial is a polynomial of the form $c\,X_{1}^{a_{1}}\!\cdots X_{n}^{a_{n}}$, such that $c\in \mathbb {F}$ and $a_{1},\ldots ,a_{n}\in \mathbb {N}$.

## Definition 3

Let $X=X_{1}\cdots X_{n}$, and let $a=(a_{1},\ldots ,a_{n})\in \mathbb {N} ^{n}$ be an exponent vector. Let us define:
* $X^{a}=X_{1}^{a_{1}}\!\cdots X_{n}^{a_{n}}$
* $\deg(X^{a})=a_{1}\!+\cdots +a_{n}$
* The degree of a non-zero polynomial is equal to the maximum of the degrees of its compsing monomials.

### Properties

Monomial multiplication maintains exponent vector addition:
    ${\begin{aligned}X^{a}\!\cdot X^{b}&=(X_{1}^{a_{1}}\!\cdots X_{n}^{a_{n}})\cdot (X_{1}^{b_{1}}\!\cdots X_{n}^{b_{n}})\\[5pt]&=(X_{1}^{a_{1}}\!\cdot X_{1}^{b_{1}})\cdots (X_{n}^{a_{n}}\!\cdot X_{n}^{b_{n}})\\[5pt]&=X_{1}^{a_{1}+b_{1}}\!\cdots X_{n}^{a_{n}+b_{n}}\\[5pt]&=X^{a+b}\end{aligned}}$

## Definition 4

Let $X^{a},X^{b}$ be monomials.
We say that $X^{a}$ is of lower order than $X^{b}$ (and denote it by $X^{a}\!\prec X^{b}$) if there exists an index $1\leq k\leq n$ such that
    ${\begin{cases}a_{i}=b_{i}&:\!1\leq i\leq k-1\\[3pt]a_{i}<b_{i}&:i=k\end{cases}}$
In other words, the vectors $(a_{1},\ldots ,a_{n}),(b_{1},\ldots ,b_{n})$ have a lexicographic ordering.
In a polynomial $F$, the monomial of maximal order is called the leading monomial, and is denoted by ${\text{L}}(F)$.

### Example

    $x_{1}^{7}x_{2}^{3}x_{3}^{10}\!\prec x_{1}^{7}x_{2}^{31}x_{3}\iff (7,3,10)\prec (7,31,1)$

## Lemma

Let $F,G\in \mathbb {F} [{\vec {X}}^{n}]$ be polynomials. Then ${\text{L}}(F\cdot G)={\text{L}}(F)\cdot {\text{L}}(G)$.

### Proof

Let $X^{a},X^{b},X^{f},X^{g}$ be monomials, with $X^{f}={\text{L}}(F),\,X^{g}={\text{L}}(G)$.
1. Let us assume that $X^{a}\!\prec X^{f}$. We will show that $X^{a+c}\!\prec X^{f+c}$ for all $X^{c}$. By definition, there exists an index $1\leq k\leq n$ such that
    ${\begin{cases}a_{i}(+\,c_{i})=f_{i}(+\,c_{i})&:\!1\leq i\leq k-1\\[3pt]a_{i}(+\,c_{i})<f_{i}(+\,c_{i})&:i=k\end{cases}}$
2. Let us assume also that $X^{b}\!\prec X^{g}$. We will show that $X^{a+b}\!\prec X^{f+g}$. By definition, there exist indexes $k_{1},k_{2}\in \{1,\ldots ,n\}$ such that respectively
    ${\begin{aligned}&{\begin{cases}a_{i}=f_{i}&:\!1\leq i\leq k_{1}-1\\[3pt]a_{i}<f_{i}&:i=k_{1}\end{cases}},\quad {\begin{cases}b_{i}=g_{i}&:\!1\leq i\leq k_{2}-1\\[3pt]b_{i}<g_{i}&:i=k_{2}\end{cases}}\\[8pt]&{\begin{cases}1\leq k_{1}\leq k_{2}\leq n\!:&(a_{k_{1}}\!<f_{k_{1}}\!)\land (b_{k_{1}}\!\leq g_{k_{1}}\!)\,\implies \,a_{k_{1}}\!+b_{k_{1}}\!<f_{k_{1}}\!+g_{k_{1}}\\[5pt]1\leq k_{2}<k_{1}\leq n\!:&(a_{k_{2}}\!=f_{k_{2}}\!)\land (b_{k_{2}}\!<g_{k_{2}}\!)\,\implies \,a_{k_{2}}\!+b_{k_{2}}\!<f_{k_{2}}\!+g_{k_{2}}\end{cases}}\!{\Bigg \}}\,\implies \,X^{a+b}\!\prec X^{f+g}\end{aligned}$
hence:
    ${\text{L}}(F\cdot G)=X^{f+g}=X^{f}\cdot X^{g}={\text{L}}(F)\cdot {\text{L}}(G)$
$\square$

## Definition 5

Let $F\in \mathbb {F} [{\vec {X}}^{n}]$ be a polynomial. Let us define:
    ${\text{D}}(F)={\Big \X^{a}\!\in \mathbb {F} [{\vec {X}}^{n}]:\deg(X^{a})\leq \deg({\text{L}}(F)),X^{a}\!\prec {\text{L}}(F){\Big \}}$
Meaning, the set of all monic monomials of degree $\leq \deg({\text{L}}(F))$ which are of lower order than ${\text{L}}(F)$.

### Example

    ${\begin{aligned}&F(x_{1},x_{2})=4+3x_{1}+2x_{1}^{2}x_{2}+x_{2}^{4}\\[3pt]&{\text{L}}(F)=2x_{1}^{2}x_{2}\\[3pt]&{\text{D}}(F)={\Big \x_{1}x_{2}^{2},x_{1}^{2},x_{1}x_{2},x_{2}^{2},x_{1},x_{2},1{\Big \}}\end{aligned}}$
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