# Rings problems

1.Let R be a ring such that Z ⊂ R ⊂ Q. Show that R is a principal ideal domain.

We show that Z is a principal ideal domain, so every ideal in Z which is also in R is principal. But I'm not sure how to use that R is contained in Q.

2. Proof that X^4+1 is reducible in Z/pZ [X] for every prime p.

I have no clue for this one at all.

Could anyone please offer some insights to either of the above problems? Any help is greatly appreciated!

First convince yourself that $$x^m-1$$ divides $$x^n-1$$ if $$m\mid n$$ (think about roots of unity). If you could show that your polynomial divides a different polynomial, one with all its roots in an extension of degree less than 4, what would that mean?