A polynomials with coefficients in a field

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Homework Help Overview

The discussion revolves around proving that a polynomial of degree n with coefficients in a field has at most n roots in that field. Participants are exploring the implications of the coefficients being in a field versus an integral domain.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • One participant suggests using induction to prove the statement, questioning the necessity of the coefficients being in a field instead of an integral domain. Others discuss the implications of proving the statement for fields first and then extending it to integral domains.

Discussion Status

Participants are actively engaging with the problem, raising questions about the assumptions involved and discussing potential approaches. There is a recognition that proving the statement for fields simplifies the process for integral domains, although some express uncertainty about whether the same proof applies to integral domains directly.

Contextual Notes

One participant notes a counterexample involving polynomials with coefficients in a ring, highlighting the complexities that arise when moving beyond fields.

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Homework Statement



Prove that a polynomial f of degree n with coefficients in a field F has at most n roots in F.


Homework Equations





The Attempt at a Solution



So we could prove this by induction by using a is a root of f if and only if x-a divides f. My question is: why do the coefficients have to be in a field? Shouldn't an integral domain work? Thanks!
 
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Once you've proven it for a field, it's easy to prove it for an integral domain.

If you tried to prove it directly for integral domains, it would be trickier -- the field case has fewer details to worry about.
 
Hurkyl said:
Once you've proven it for a field, it's easy to prove it for an integral domain.

If you tried to prove it directly for integral domains, it would be trickier -- the field case has fewer details to worry about.

Hmmm...Doesn't the same proof work for an integral domain? I mean the claim that
a is a root of f(x) in R[x] if and only if x-a divides f(x) is true as long as R is an integral domain(or maybe as long as R is a ring?). Then we can just use induction.
 
It's not too hard to find a counterexample to the case of general rings.

For example, consider the polynomial x2 - 1 with coefficients in the ring of integers modulo 8. It factors in two ways:
x2-1 = (x-1)(x-7) = (x-3)(x-5)​
 

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