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Massimiliano Sala edited section_Bytes_The_polynomials_in__.tex
about 6 years ago
Commit id: 52e69fa8b6de4ec6faa1dded0d932c2c5ac93393
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$$\deg(f+g)\leq \deg(f), \deg(g); \qquad \deg(f+g)<\deg(f),\deg(g) \iff \deg(f)=\deg(g). $$
However, if we multiply $f,g$, their product $fg$ can be outside $S$; for example
$$
f=x^2+1,\, g=x+1
\quad \implies
\quad (x^2+1)(x+1)=x^3+x^2+x+1\,.
$$
We describe a way to make $S$ "closed with respect to multiplication",that is, we want to define a special operation on the polynomials in $S$ that allows their product to \textbf{remain} in $S$.
We do that by what is called a "polynomial relation". For
example fix example, we can define the
relations to be following relation $x^3=x+1$, or equivalently
$x^3+x+1=0$, as in the example above. We can limit the number of elements in $\Fb[x]$ in $x^3+x+1=0$.
With this
way: each relation in mind, any time we find a monomial of degree greater than or equal to $3$ we substitute $x^3$
by with $x+1$.
At the end of We iterate this
process substitution until we obtain a
new polynomial of degree strictly less than $3$. For example
\[
x^4+x^2=x(x^3)+x^2=x(x+1)+x^2=x^2+x+x^2=x. x^5+x \,=\, x^2(\underline{x^3})+x \,=^{\mathrm{substitution}\, x^2(\underline{x+1})+x\,=\,
\underline{x^3}+x^2+x=^{\mathrm{substitution} \underline{x+1}+x^2+x \,=\, x^2+1 \,.
\]
One can obtain the same result dividing the polynomial $x^4+x^2$ by the polynomial $x^3+x+1$. The remainder is $x$.
Hence the polynomials defined in the set