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In preparation for an exam, I am trying to solve the following question:

Describe geometrically all maps from $\operatorname{Spec}(\mathbb{C}[z]/(z^2))$ to $\operatorname{Spec}(\mathbb{C}[x,y])$.

I've been able to characterize all such maps: they correspond to maps on the rings (going in the other direction). So let: $$\phi:\mathbb{C}[x,y]\longrightarrow\mathbb{C}[z]/(z^2)$$ be such a map with $\phi(1)=1$, $\phi(x)=az+b$ and $\phi(y)=cz+d$. Then we can show with a short calculation that: $$\phi(f(x,y))=f(b,d)+z\nabla f|_{b,d}\cdot\left(\begin{array}{c}a\\c\end{array}\right)$$ Now the only prime ideals of $\mathbb{C}[z]/(z^2)$ are $(z)$ and $(0)$, and they are sent to the point $(x-b,y-d)$ and to the ideal $I$ of polynomial whose zero locus contains $(b,d)$ and whose gradient at $(b,d)$ is orthogonal to $(a,c)$ (i.e. the zero locus "goes in direction $(a,c)$") respectively. Two easy examples which are in fact representative of all cases (we can reduce to those cases rotating, translating and dilating $\mathbb{C}^2$) are:

  1. $a=b=c=d=0$ gives $I=(x,y)$
  2. $a=1,\ b=c=d=0$ gives $I=(x^2,y)$

However, I don't really see what "characterize geometrically" means, how much and what I should say. Can anyone lend a hand, please?

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    A map from $Spec \mathbb C[z]/z^2$ to $\mathbb A^2$ correspond to a tangent vector of the latter. – Fredrik Meyer Aug 17 '14 at 20:30
  • @FredrikMeyer Thanks. I've upgraded my answer to include my own work on the question, and as you can see I got more or less to the same conclusion. However my "real" question is: if asked such a question during an exam, how should I answer? What should I say to convey the idea properly? What would/could be accepted as a sufficient answer for this kind of question? – Daniel Robert-Nicoud Aug 17 '14 at 20:55
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    You state "the only prime ideals of $\mathbb{C}[z]/(z^2)$ are $(z)$ and $(0)$". No, $(0)$ is not prime. – Georges Elencwajg Aug 20 '16 at 13:49

1 Answers1

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This is to follow up on Frederik Meyer's comment:

Let $X$ be variety over a field $k$, and let $x\in X(k)$. Then, there is a canonical bijection between morphisms (over $k$) $\text{Spec}(k[\varepsilon])\to X$, whose underlying point hits $x$, and $T_x X$.

Here I just use $k[\varepsilon]$ for the dual numbers. And, I use the notation $T_x X$ for the tangent space $(\mathfrak{m}_x/\mathfrak{m}_x^2)^\vee$.

To see this, note that obviously any such map $\text{Spec}(k[\varepsilon])\to X$ must factor uniquely as $\text{Spec}(k[\varepsilon])\to\text{Spec}(\mathcal{O}_{X,x})\to X$. But, maps $\text{Spec}(k[\varepsilon])\to\text{Spec}(\mathcal{O}_{X,x})$ correspond bijectively to $k$-algebra maps $\mathcal{O}_{X,x}\to k[\varepsilon]$.

But, such $k$-algebra maps correspond to $k$-derivations $\mathcal{O}_{X,x}$ 'at $x$', which are maps $\phi:\mathcal{O}_{X,x}\to k$ such that

$$\phi(fg)=\overline{f}\phi(g)+\overline{g}\phi(f)$$

where the bar denotes passage to the quotient $\mathcal{O}_{X,x}\to k$ (why?). But, I claim that such derivations correspond bijectively with $T_{X,x}$.

In particular, suppose that $\phi:\mathcal{O}_{X,x}\to k$ was such a derivation. Note that by restriction we obtain a map $\phi:\mathfrak{m}_x\to k$. But, if $f,g\in\mathfrak{m}_x$, then

$$\phi(fg)=\overline{f}\phi(g)+\overline{g}\phi(f)=0$$

since $\overline{f}=\overline{g}=0$. So, $\phi$ kills $\mathfrak{m}_x^2$, and so we get a well-defined map $\phi:\mathfrak{m}_x/\mathfrak{m}_x^2\to k$, which, since this is $k$-linear, is an element of $(\mathfrak{m}_x/\mathfrak{m}_x^2)^\vee$.

Conversely, suppose that I start with an element $\phi\in (\mathfrak{m}_x/\mathfrak{m}_x^2)^\vee$. Then, how do we extend it to a derivation of $\mathcal{O}_{X,x}$ at $x$? Well, let $f\in\mathcal{O}_{X,x}$ be arbitrary. Suppose that $\overline{f}=\alpha\in k$, then $\alpha\in\mathcal{O}_{X,x}$ and $f-\alpha\in\mathfrak{m}_x$. So, define $\phi(f):=\phi(f-\alpha+\mathfrak{m}_x^2)$. I leave it to you to check that this is a derivation of $\mathcal{O}_{X,x}$ at $x$, and that it's the inverse of the map from the last paragraph. Thus, the gray box is proven.


For you, this is helpful since any map $\text{Spec}(\mathbb{C}[\varepsilon])\to X$ (for any variety $X/\mathbb{C}$) over $\mathbb{C}$ must hit a closed point. But, since $\mathbb{C}$ is algebraically closed, closed points are the same as $\mathbb{C}$-points. Thus, the above shows that, in some sense

$$\text{Hom}_{\text{Spec}(\mathbb{C})}(\text{Spec}(\mathbb{C}[\varepsilon]),X)=\bigcup_{x\in X^\circ}T_{X,x}$$

where $X^\circ$ denotes the closed points of $X$(=$\mathbb{C}$-points). So, if you squint hard enough, the set of all such maps is like the tangent bundle.


The intuition for this, by the way is as follows. A mapping $\text{Spec}(\mathbb{C}[\varepsilon])\to X$ is essentially picking out a closed point, and then putting a little 'fuzz' (non-reduced data) which corresponds to put an infinitesimal direction, or tangent vector, on that point.

Remark: Before you ask if this is what you're supposed to put on an exam, I have no idea. That's between you and you're professor, the above is just one way to think about it. Good luck. :)

Alex Youcis
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  • Thanks, I really like your answer. – Daniel Robert-Nicoud Aug 18 '14 at 09:43
  • @AlexYoucis: I also have the same question as OP`s, and found this post. But I am finding it difficult to understand how your answer answers the question. As I understand, the first part of your answer is a proof to Ex. II. 2.8 in Hartshorne. Right? So if I am asked to describe all maps $Spec\mathbb{C}[z]/(z^2) \to Spec\mathbb{C}[x,y],$ I understand that a solution to this question is equivalent to giving a point $P \in Spec\mathbb{C}[x,y]$, which is rational over $\mathbb{C}$, and a tangent vector at $P,$ i.e. a linear map $m_p/m_p^2 \to \mathbb{C}.$ (tbc)..... –  Aug 19 '16 at 12:06
  • @AlexYoucis: (cont`d)..So let me try to give a solution: I understand that for instance $P=(x-a,y-b) \in Spec\mathbb{C}[x,y]$ is rational over $\mathbb{C}.$ In this case, how can your answer help me give a tangent vector at $P$? –  Aug 19 '16 at 12:10
  • @algeom I'm sorry, I don't actually understand your question. You gave a point--so then what? I said that a map from the spectrum of the dual numbers to the scheme corresponds to a point and a tangent vector at that point. You only gave me a point. So, instead, let me ask you a question. Let $\mathfrak{m}=(x-a,y-b)$. Then, $\mathfrak{m}/\mathfrak{m}^2$ is precisely a $2$-dimensional $k$-space with basis $x-a$ and $y-b$. So, an example of an element of the tangent space, the dual of $\mathfrak{m}/\mathfrak{m}^2$, might be $T$ which sends $x-a$ to $1$ and $y-b$ to $i$. – Alex Youcis Aug 19 '16 at 12:55
  • What is the morphism $\text{Spec}(\mathbb{C}[z]/(z^2))\to\text{Spec}(\mathbb{C}[x,y])$ in this case? – Alex Youcis Aug 19 '16 at 12:56
  • @AlexYoucis: Sorry, I think I have a logical confusion, not so much mathematical. Are you saying that the question "Describe geometrically all maps from $\operatorname{Spec}(\mathbb{C}[z]/(z^2))$ to $\operatorname{Spec}(\mathbb{C}[x,y])$" is the same as the question in Ex. II. 2.8 in Hartshorne? –  Aug 19 '16 at 13:08
  • Please tolerate my stupidity for what I am going to ask you. I am actually confused in this question of describing all maps from spectrum of the dual numbers to the scheme. I don`t know what precisely should I do to answer this question. Should I give a proof that the question is equivalent to giving a rational point in the scheme and a tangent vector at that point? (But I think this is just changing the question into another question.) Please tell me what counts as a solution to this question. If you do not understand my confusion, please let me know again. Thanks! :) –  Aug 19 '16 at 14:02
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    @algeom Then, as you've pointed out, this seems largely a question of semantics. I think that saying 'a point and a tangent vector' is a geometric description. So, yes, I think that a solution to this problem is the same thing as a solution to the Hartshorne problem. As for the second comment, I think the following. When one says "Describe all" it usually means "Find an easily describable set which is naturally equivalent to the original set". The set of morphisms from the dual numbers is not easily describable (in the sense that it's difficult to give its elements), but the set of points – Alex Youcis Aug 20 '16 at 09:15
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    and tangent vectors is easily describable (it is easy to give an element). So, yes, I think that showing the equivalence of this set is finding all maps from the spectrum of the dual numbers to the affine plane. – Alex Youcis Aug 20 '16 at 09:16
  • @AlexYoucis, wonderful! In your answer, when you describe the intuition, you said about "non-reduced data". Could you please explain what this phrase means? I know the term "reduced" only in the sense that something is nilpotents free. –  Aug 20 '16 at 09:25
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