# Thread: How to use Euclidian Algorithm to find multiplicative inverse of a polynomial??

1. ## How to use Euclidian Algorithm to find multiplicative inverse of a polynomial??

For example, finding the multiplicative inverse of $[x] in Z_5[x]/$

or even something like

$[a+bx] in R[x]/$

I cant seem to figure out this concept.

Thank you!

2. Originally Posted by elninio
For example, finding the multiplicative inverse of $[x] in Z_5[x]/$

or even something like

$[a+bx] in R[x]/$

I cant seem to figure out this concept.

Thank you!

Divide $x^2+1$ by $bx+a$ with residue:

$x^2+1=(bx+a)(dx+c)+r$ , with $r=0\,\,\,or\,\,\,\deg(r)<1\Longrightarrow r\in\mathbb{R}\setminus{0}$.

As $x^2+1$ has no real roots, it can't be $r=0\Longrightarrow 0\ne r\in\mathbb{R}$ , and thus $(bx+a)\left(\frac{d}{r}x+\frac{c}{r}\right)=1\!\!\ !\pmod{x^2+1}\Longrightarrow (bx+a)^{-1}=\frac{d}{r}x+\frac{c}{r}\,\,\,in\,\,\,\mathbb{R }[x]\slash$

Tonio

3. Tonio, I think, the inverse of bx+a should be -(dx+c)/r.

4. Originally Posted by Shanks
Tonio, I think, the inverse of bx+a should be -(dx+c)/r.

Yes, of course: forgot the sign. Thanx

Tonio

5. Can we not write it in terms of a and b like

$(bx+a)^{-1}=\frac{a}{a^2+b^2}-\frac{b}{a^2+b^2}x$?