Studying Deformation Theory of Schemes

• Versal Property
• Local Deformation Space
• Mini-versal deformation space

I came across these words while studying these papers a) Desingularization of moduli varities for vector bundles on curves, Int. Symp on Algebraic Geometry by C. S. Seshadri and b) Cohomology of certain moduli spaces of vector bundles Proc. Indian Acad. Sci. by V. Balaji

So, I Googled and I ended up trying to understand Deformation Theory.

I have tried reading few lecture notes, for example:

1) Notes on Deformation Theory by Nitin Nitsure

2) Deformation Theory by M. Doubek, M. Markl and P. Zima

3) A glimpse on Deformation theory by Brian Osserman

4) Robin Hartshorne's book on Deformation Theory

Nothing helped me to understand what is deformation theory actually.

I am finding it difficult to understand why everyone suddenly starts talking about artinian local algebras. All the lectures seems to be very abstract to me.

May be I am missing some points for understanding.

I would appreciate if someone writes an answer either stating 1) Why to study deformation theory? 2) What is deformation theory? or Someone can point out any another nice reference to study Deformation Theory.

I understand what is meant by Moduli Space. Some of the above mentioned notes say that deformation theory is somehow related to Moduli Theory. But I have no clue how.

• I think a nice introduction is Artin's old lectures Deformations of Singularities, which TATA has retyped and made available freely. Everything is done in a special case and shown to follow from basic algebra. – Hoot Aug 22 '16 at 14:31

What follows is an attempt to motivate this beautiful and difficult (in my opinion) subject. It is just an attempt, I cannot promise it will be useful.

Suppose you have a family of curves over $\mathbb A^1=\textrm{Spec }\mathbb C[t]$, like for instance the family $$\pi:\textrm{Spec }\mathbb C[x,y,t]/(xy-t)\to \mathbb A^1$$ given by $t\mapsto t$. As it is explained very well in Hartshorne's book, deformation theory is:

$\textbf{the infinitesimal study of a family in a neighborhood of one of its members.}$

For instance, the member corresponding to $t=0$ is very special in the above family, as it is the only singular fiber of $\pi$: the smooth hyperbolae degenerate, or rather, deform to a singular conic, the union of two lines at a point (draw a picture!). In a "neighborhood" of this member of the family, all other curves are smooth conics, so when we stare at this unique, very special singular conic, the natural question arises: $$\textrm{How could that happen?}$$ The curiosity towards the answer to such a question could be one motivation for deformation theory.

Now you can already see the relation to moduli: we just finished talking about a "family of curves"...

Now let me tell you something very naive. Let $x$ be a (closed) point on a variety $X$. What does it mean to deform $x$ in $X$? Well, pretend you are a point on a sphere, then to "deform yourself" you have to look around you in all possible directions and see what surrounds you - but you need to do this infinitesimally, first because you are a point, and second because deformation theory is the infinitesimal study of geometric objects. So it turns out that to deform yourself means to choose a tangent direction on the sphere. More generally, $$\{\textrm{Deformations of }x\textrm{ in } X\}=T_xX=\hom_{k(x)}(\textrm{Spec }k[t]/t^2,X).$$

Let $D=\textrm{Spec }k[t]/t^2$. In general you have this:

Definition. Let $i:Y\hookrightarrow X$ be a closed subscheme. A first order deformation of $Y$ in $X$ (also called a deformation of $i$) is flat morphism $f:\mathfrak X\to D$, where $\mathfrak X$ is a closed subscheme of $X\times D$, $Y$ is the fiber over the closed point of $D$, and $f$ is induced by the projection $X\times D\to D$.

I'll tell you later what nice group describes these objects!

Example. Suppose $X$ is a variety such that $H^1(X,\mathscr O_X)=0$. This cohomology group is the tangent space of any point $[L]\in \textrm{Pic }X$. So if it vanishes, it means that line bundles on $X$ do not deform.

Example. Let $\mathbb P^5_\mathbb C$ be moduli space of plane conics. Let's pick an explicit conic $C\subset\mathbb P^2_\mathbb C$, and let us try to compute its tangent space as a point $p=[C]$ in the moduli space $\mathbb P^5$. So we find: \begin{align} T_{[C]}\mathbb P^5&=\hom_{\mathbb C(p)}(\textrm{Spec }k[t]/t^2,\mathbb P^5) \notag\\ &=\hom_\mathbb C(m_p/m_p^2,\mathbb C)\notag\\ &=H^0(C,N_{C/\mathbb P^2}). \end{align} Is it really $5$-dimensional? Since $$N_{C/\mathbb P^2}=\mathscr O_{\mathbb P^2}(C)|_C=\mathscr O_C(2)=\mathscr O_{\mathbb P^1}(4),$$ yes, it is $5$-dimensional, as expected.

More than finding the expected dimension for the tangent space, it is interesting to observe that, once you define what a first order deformation of $C$ in $\mathbb P^2$ is (as I did above), it turns out that such objects are parameterized by the cohomology group $H^0(C,N_{C/\mathbb P^2})$. So the upshot is: the deformations of the closed embedding $C\subset \mathbb P^2$ are exactly the deformations of $[C]$ as a moduli point in $\mathbb P^5$. There we found another strong link with moduli!

More generally: The first order deformations of a closed subscheme $i:Y\hookrightarrow X$ are parameterized by $H^0(Y,N_{Y/X})$, which is also the tangent space of $[Y]$ as a point in the Hilbert scheme of $X$.

I just realized that my answer is much longer than I thought it was in my mind, so let me finish justifying the ubiquity of local Artinian $k$-algebras: their category is equivalent (under the functor $\textrm{Spec}$) to the category of fat points over $k$. Why on earth should we care about fat points? (A fat point over $k$ is just a $k$-scheme $F$ such that the structural morphism $F_{\textrm{red}}\to \textrm{Spec }k$ is an isomorphism: they are $0$-dimensional schemes having one closed point with some ugly but useful non-reduced structure). Now, $D=\textrm{Spec }k[t]/t^2$ is one such, but it is very special, because it describes the unique scheme structure one can put on a double point. First order deformations are those parameterized by this $D$, and they are flat morphisms (say) over $D$ such that over the closed point there lies the object you want to deform. Considering families over a fatter point, e.g. over $\textrm{Spec }k[t]/t^3$ is the study of higher order deformations. These are very different from the first order one, e.g. you may not have any deformation at all over a certain algebra $A$, whereas over $D$ you always have the trivial deformation (the one corresponding to the element of the cohomology group which is concerned). If you have one, you may want to know if you can extend it further, and this leads to study small extensions of local Artin $k$-algebras.

Good references are online notes by Ravi Vakil, and Sernesi's book Deformations of algebraic schemes.

• There was also an MSRI workshop some years ago; I think the videos are still online and there is a draft of a book written by the organizers floating around the web. Ravi's introductory lecture was probably good, although it has been a long time since I've watched it. – Hoot Jan 29 '15 at 3:02
• @Hoot: I think the workshop you mentioned is the following one: msri.org/summer_schools/419 I didn't know about the "draft" you mentioned. It's not in the link I gave above. Can you give any link for that "draft"? Thanks. – Krish Jan 29 '15 at 9:02
• @Brenin I have now some idea about deformation. Thank you for your elaborate answer. Still many things are vague to me. May be as I read more I will understand it better. I guess in the process of understanding I will come up with more questions. I am not accepting the answer yet as someone might come up with a more illuminating answer. – Babai Jan 29 '15 at 13:28
• @Babai: the topic is immense, and it is impossible to give a complete account of the subject, so I just chose some aspects that capture the philosophy (according to my basic understanding). You are welcome to ask more questions as soon as you read more about it! – Brenin Jan 29 '15 at 15:30
• @Krish Here is one version: math.ucdavis.edu/~osserman/math/Defbookactive.pdf it's very rough! – Hoot Jan 29 '15 at 17:24