# Algebraic Geometry - D&F Section 15.1, Exercise 24

#### Peter

##### Well-known member
MHB Site Helper
Dummit and Foote Section 15.1, Exercise 24 reads as follows:

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Let [TEX] V = \mathcal{Z} (xy - z) \subseteq \mathbb{A}^3 [/TEX].

Prove that $$\displaystyle V$$ is isomorphic to [TEX] \mathbb{A}^2 [/TEX]

and provide an explicit isomorphism [TEX] \phi [/TEX] and associated k-algebra isomorphism [TEX] \widetilde{\phi} [/TEX] from [TEX] k[V] [/TEX] to [TEX] k[ \mathbb{A}^2] [/TEX] along with their inverses.

Is [TEX] V = \mathcal{Z} (xy - z^2) [/TEX] isomorphic to [TEX] \mathbb{A}^2 [/TEX]?

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I would appreciate some help and guidance with getting started with this exercise [I suspect I might need considerable guidance! :-( ]

Some of the background and definitions are given in the attachment.

Peter

#### Peter

##### Well-known member
MHB Site Helper
Dummit and Foote Section 15.1, Exercise 24 reads as follows:

---------------------------------------------------------------------------------------------------------

Let [TEX] V = \mathcal{Z} (xy - z) \subseteq \mathbb{A}^3 [/TEX].

Prove that $$\displaystyle V$$ is isomorphic to [TEX] \mathbb{A}^2 [/TEX]

and provide an explicit isomorphism [TEX] \phi [/TEX] and associated k-algebra isomorphism [TEX] \widetilde{\phi} [/TEX] from [TEX] k[V] [/TEX] to [TEX] k[ \mathbb{A}^2] [/TEX] along with their inverses.

Is [TEX] V = \mathcal{Z} (xy - z^2) [/TEX] isomorphic to [TEX] \mathbb{A}^2 [/TEX]?

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I would appreciate some help and guidance with getting started with this exercise [I suspect I might need considerable guidance! :-( ]

Some of the background and definitions are given in the attachment.

Peter
I am still working on this problem. Here are some more thoughts I've had ... however I am struggling to make a great deal of progress and still need substantial help ...

We have to show that $$\displaystyle V$$ is isomorphic to [TEX] \mathbb{A}^2 [/TEX]

We also have to provide an explicit isomorphism [TEX] \phi [/TEX] and associated k-algebra isomorphism [TEX] \widetilde{\phi} [/TEX] from [TEX] k[V] [/TEX] to [TEX] k[ \mathbb{A}^2] [/TEX] along with their inverses!

... ... ... well, we must look for a mapping from $$\displaystyle V \subseteq \mathbb{A}^3$$ to $$\displaystyle W = \mathbb{A}^2$$, so I would say we need a morphism or polynomial map $$\displaystyle \phi : \ V \to W$$.

D&F (Section 15.1, page 662) define a morphism as follows:

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Definition. A map $$\displaystyle \phi : \ V \to W$$ is called a morphism or polynomial map or regular map of algebraic sets if there are polynomials $$\displaystyle \phi_1, ... \ ... \phi_m \in k[x_1, x_2, ... \ ... x_n]$$ such that

$$\displaystyle \phi ((a_1, a_2, ... \ ... , a_n)) = ( \phi_1(a_1, a_2, ... \ ... , a_n) ... \ ... \phi_m(a_1, a_2, ... \ ... , a_n))$$ for all $$\displaystyle (a_1, a_2, ... \ ... , a_n) \in V$$

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D&F (seemingly importantly for our problem) go on to say:

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The map $$\displaystyle \phi : \ V \to W$$ is an isomorphism of algebraic sets if there is a morphism $$\displaystyle \psi : \ W \to V$$ with $$\displaystyle \phi \circ \psi = 1_W$$ and $$\displaystyle \psi \circ \phi = 1_V$$.

... ... ...

... $$\displaystyle \phi$$ indices a well defined map from the quotient ring $$\displaystyle k[x_1, ... \ ... , x_m]/ \mathcal{I}(W)$$ to the quotient ring $$\displaystyle k[x_1, ... \ ... , x_m]/ \mathcal{I}(V)$$:

$$\displaystyle \widetilde{\phi}: \ k[W] \to k[V]$$

$$\displaystyle f \mapsto f \circ \phi$$

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So the above are the concepts I now believe need to be applied, but I lack the skills and knowledge to apply them in this specific case

I would really appreciate some help.

Peter