# Thread: congruence classes and roots of a polynomial.

1. ## congruence classes and roots of a polynomial.

I am reading a text book on elementary number theory and I am reading a chapter on congruence classes at the moment and im confused as to what it means when it asks/says that a polynomial $\displaystyle f(x)=x^2+1$ has two roots in Z_2 but none in Z_3?

Thanks very much for any help.

2. edited

3. Originally Posted by hmmmm
I am reading a text book on elementary number theory and I am reading a chapter on congruence classes at the moment and im confused as to what it means when it asks/says that a polynomial $\displaystyle f(x)=x^2+1$ has two roots in Z_2 but none in Z_3?

Thanks very much for any help.
A root is a value of x that make the polynomial equal to zero.

In $\displaystyle \mathbb{Z}_{2}$ there are only two equivalence classes.

$\displaystyle [0],[1]$

$\displaystyle f([0])=[0]^2+[1]=[1] \ne [0]$ and

$\displaystyle f([1])=[1]^2+[1]=[1] +[1]=[2] =[0]$ so it is a root and the polynomial can be factored mod 2 as

$\displaystyle (x+[1])(x+[1])=x^2+x+x+1 =x^2+[2]x+1=x^2+1$

Now try the same thing in $\displaystyle \mathbb{Z}_3$

4. Thanks very much I understand now i was just getting a bit confused.

5. For those of us who prefer very strict language, a polynomial does NOT have "roots", equations have roots. Better terminology is that the zeros of a polynomial, p(x), are the roots of the equation p(x)= 0.

6. Thanks, I probably do need to be a lot more precise with my notation/terminology