## Roots of Polynomials I

When we consider a polynomial as a function, we’re particularly interested in those field elements so that . We call such an a “zero” or a “root” of the polynomial .

One easy way to get this to happen is for to have a factor of . Indeed, in that case if we write for some other polynomial then we evaluate to find

The interesting thing is that this is the *only* way for a root to occur, other than to have the zero polynomial. Let’s say we have the polynomial

and let’s also say we’ve got a root so that . But that means

This is not just a field element — it’s the zero polynomial! So we can subtract it from to find

Now for any we can use the identity

to factor out from each term above. This gives the factorization we were looking for.

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[…] can actually tease out more information from the factorization we constructed yesterday. Bur first we need a little […]

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[…] with Too Few Roots Okay, we saw that roots of polynomials exactly correspond to linear factors, and that a polynomial can have at most as many roots as its degree. In fact, there’s an […]

Pingback by Polynomials with Too Few Roots « The Unapologetic Mathematician | August 6, 2008 |