In Dummit and Foote Ch 9 Polynomial Rings, Proposition 2 (page 296 - see attached) reads as follows:

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Proposition 2: Let I be an ideal of the ring R and let (I) = I[x] denote the ideal of R[x] generated by I (the set of polynomials with co-efficients in I)

Then $\displaystyle R[x]/I[x] \cong (R/I)[x]$

In particular, if I is a prime ideal of R then I[x] is a prime ideal of R[x]

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I am concerned with the proof that $\displaystyle R[x]/I[x] \cong (R/I)[x]$.

In this proof (i think) we need the First Isomorphism Theorem. Dmmit and Foote give this theorem as part of Theorem 7 in Ch 7 on page 243 (see attached)

They state the First Isomorphism Theorem as follows: (see attached)

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If $\displaystyle \phi : R \rightarrow S $ is a homomorphism of rings, then the kernel of $\displaystyle \phi $ is an ideal of R, the image of $\displaystyle \phi $ is a subring of S, and $\displaystyle R / ker \phi$ is isomorphic as a ring to $\displaystyle \phi (R) $

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I follow the proof of down to the following statement (see attached page 296 D&F)

... ... , which is to say that ker $\displaystyle \phi $ = I[x] = (I), proving the first part of the proposition.

BUT surely we need to now use the First Isomorphism Theorem to show that we have an isomorphism ( I suspect D&F are assuming we know this bit ... )

However, by the FIT, given that ker $\displaystyle \phi $ = I[x] we have that

$\displaystyle R[x]/ ker \phi = R[x] / I[x][ \cong \phi [ R[x] ]$

So we require $\displaystyle \phi [R[x]] = (R/I) [x] $

But how do we know that the image of R[x] under $\displaystyle \phi $ is (R/I)[x] and not a subset of (R/I)[x]

Would appreciate some help in clarifying this matter.

Peter