Smith et al - An Invitation to Algebraic Geometry

Springer UTX, 2000

Algebraic Geometry

"The aim of this book is to describe the underlying principles of algebraic geometry, some of its important developments in the twentieth century, and some of the problems that occupy its practitioners today."

Chapter 1 : Affine Algebraic Varieties

Section 1.4 : Dimension

• http://math.stackexchange.com/questions/95670/surjective-morphism-of-affine-varieties-and-dimension

Chapter 3 : Projective Varieties

Section 3.3 : The Projective Closure of an Algebraic Variety

• The closure $\overline{V}$ of the twisted cubic: $[1:t:t^2:t^3] = \left[\frac{1}{t^3}:\frac{1}{t^2}:\frac{1}{t}:1\right] \rightarrow [0:0:0:1]$ as $t \rightarrow \infty$

Section 3.4 : Morphisms of Projective Varieties

• no single choice of polynomials will work for all points of C: is this supposed to be immediately clear?
• For example, the point at infinity is [0:0:1] when we consider the chart $U_x$: At first it seemed like there should be lots of points at infinity, but then I realized that the points on the "line at infinity" don't count: $\mathbb{P}^2 = U_x \sqcup (\text{the line at infinity}) \sqcup (\text{the point at infinity})$
• A "form" is a homogeneous polynomial (in some number of variables).
• Exercise 3.4.4: $\mathbb{V}(x)$ can be parametrized as [p:0:q:r], and $\mathbb{V}(x-y^4-z^4)$ can be parametrized as $\left[s^4:t^4+u^4:ts^3:us^3\right]$

Section 3.5 : Automorphisms of Projective Space

• Exercise 3.5.1: The 3 is very important here! This does not hold in other dimensions.

Chapter 5 : Classical Constructions

Section 5.4 : Grassmannians

• as the action of g on the determinants is just multiplication by the nonzero constant deg g: clearly k x k subsets must be special somehow... ah - just delete the irrelevant rows of $(a_{ij})$ and work in ${\rm GL}_k(\mathbb{C})$. (The rows are linearly independent by construction.)

Chapter 8 : Maps to Projective Space

Section 8.1 : Embedding a Smooth Curve in Three-Space

• Apparently the intention is that $\phi_x(\lambda) = \varphi_x(\lambda)$ here (both are versions of the lowercase Greek letter phi), which confused me briefly since I assumed one or the other must refer to the parametrization of X.

Section 8.2 : Vector Bundles and Line Bundles

• Definition of vector bundle: the definition says "the diagram commutes," without indicating a diagram. What's going on would be obvious except for the badly-placed page break -- the diagram it's talking about is at the top of the next page.

• They start talking about sections here before they're defined in 8.3.

• I guess the "atlas" functions of a vector bundle are called "local trivializations" because $U_i \times \mathbb{C}^n$ is trivially a vector bundle, which would make the functions $(\varphi_i)|_{\pi^{-1}(U)}$ "local isomorphisms to trivial bundles."

Section 8.4 : Examples of Vector Bundles

• The tautological line bundle: We apparently need to assume that X has positive dimension -- if our projective variety is a single point then we do seem to have a nontrivial section of the tautological bundle.

• The tautological line bundle: This example is defined for a projective variety X (rather than for a quasi-projective variety). This resolves the apparent ambiguity between "tautological line bundles have no nonzero global sections" and the fact that the sheaf of sections isn't just the zero sheaf. The hyperplane bundle on the next page, on the other hand, is defined for a quasi-projective variety X.

• The hyperplane bundle: http://math.stackexchange.com/questions/152298/how-is-the-hyperplane-bundle-cut-out-of-mathbbcn1-ast-times-mathbb