# Do Irreducibles Induce Algebraic Extensions?

1. ### slamminsammya

15
Given $$K$$ a field, and $$f\in K[x]$$ an irreducible (monic) polynomial. Does it follow that the field $$K[x]/\left<f\right>$$ is an algebraic extension of $$K$$?

2. ### mathwonk

9,800
i think so. in general, if M is a maximal ideal in a commutative ring R, then R/M is a field. If R contains a subfield k, then R/M is an extension of k. So in your case the ring K[X] contains the subfield K, and since K[X] is a Euclidean ring, hence also a p.i.d., an irreducible polynomial f generates a maximal ideal, so K[X]/(f) is a field extension of K.

Moreover the degree (vector dimension) of the extension equals the degree of f, hence is finite, and every finite extension is definitely algebraic. so YES!

I had to think through all the details since I am old and losing my memory. hope this helps.

3. ### Hurkyl

16,089
Staff Emeritus
Isn't the answer obvious from the definition of "algebraic extension"? The interesting part is that it's a field and not just a ring.

(Hrm. I suppose there are equivalent definitions, and some would be less obvious than others. Which are you using?)