# Noetherian Rings - R Y Sharp - Chapter 8 - exercise 8.5

#### Peter

##### Well-known member
MHB Site Helper
I am reading R. Y. Sharp: Steps in Commutative Algebra, Chapter 5 - Commutative Noetherian Rings

Exercise 8.5 on page 147 reads as follows:

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8,5 Exercise.

Show that the subring [TEX] \mathbb{Z} [ \sqrt{-5} ] [/TEX] of the field [TEX] \mathbb{C} [/TEX] is Noetherian.

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Peter

[Note: This has also been posted on MHF]

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#### mathbalarka

##### Well-known member
MHB Math Helper
From Hilbert's basis theorem, we know that since $\Bbb Z$ is Noetherian, $\Bbb Z[x]$ is also Noetherian. There is a canonical isomorphism $\Bbb Z[x]/(x^2+5) \cong \Bbb Z[\sqrt{-5}]$ given by $x \mapsto \sqrt{-5}$. As Noetherianty is invariant under taking quotients, $\Bbb Z[x]/(x^2 + 5)$ is Noetherian, hence $\Bbb Z[\sqrt{-5}]$ is also Noetherian $\blacksquare$

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