3
votes
1answer
36 views

functoriality of $K(G,1)$ spaces in a particular situation involving complex elliptic curves

I apologize if the subject doesn't accurately describe my question. Let $F_2$ denote the free group on two generators. Suppose you have some group homomorphism $A : ...
3
votes
1answer
35 views

Image of Regular Map

Determine the image of the regular map $f: A^2 \to A^2$, $f(x,y)=(x,xy)$ and describe it from the point of view of topology. Would the image of f be $A^2$, because every point of $A^2$ is still in the ...
1
vote
2answers
75 views

Product topology of Affine Varieties

I want to prove that $X \times Y \subseteq \Bbb A^{2n}$ is an affine variety, given that $X,Y \subseteq \Bbb A^n$ are affine varieties. Is this proof correct? Since both $X$ and $Y$ are affine ...
2
votes
1answer
31 views

Open subset in Irreducible Topological Space is dense.

Show that every non-empty open subset of an irreducible topological space is dense. I know a lemma that states that $U \subset$X is dense iff for all $A \in \tau$, $A \cap U \neq \emptyset$. So ...
1
vote
2answers
87 views

Closed sets in Zariski Topology

What are the closed sets on $\mathbb{Z}$, $\mathbb{R}[x]$, $\mathbb{Z}[x]$? I was given this question, but it seems trivial, because aren't the closed sets only the affine varieties? So for each is ...
2
votes
1answer
25 views

Density, Irreducible Topological Space

Let X is a irreducible noetherian topological space, and $U \subseteq X$ is a nonempty open subset. If $B \subseteq U$ is dense, then so is $B \subseteq X$. Is this true or false, and why? Here ...
1
vote
1answer
46 views

Computing the intersections of arbitrary number of planes

Imagine three sets of planes (A, B, and C). The planes in each set are parallel and evenly spaced and so can be defined by a plane equation and a scalar representing the distance between planes. If ...
-1
votes
1answer
54 views

Zariski topology as weak topology [duplicate]

On Wikipedia they say: "Every regular map of varieties is continuous in the Zariski topology. In fact, the Zariski topology is the weakest topology (with the fewest open sets) in which this is true ...
2
votes
1answer
55 views

What topology has $Pic(X)$?

Let $Pic(X):=\{\mbox{Holomorphic line bundles on } X\}/\sim$ be the group of isomorphism classes of line bundles on $X$. It is well-know and easy to prove that, working in Cech cohomology, $Pic(X) ...
1
vote
0answers
30 views

dimension of a subspace of a flag variety

Let $X$ be a topological space. If $X = \bigcup U_\alpha$ is an open covering of $X$ then $$\dim X = \sup_\alpha \dim U_\alpha.$$ Now suppose that $X = \coprod U_\alpha$, i.e., $X$ is the disjoint ...
0
votes
1answer
41 views

The torus as a complex variety

I'm interested in the topological torus, ie. the homeomorphism class of $S^1\times S^1$. Clearly, it can be realized as the real algebraic variety in $\mathbb{R}^4$ as the solution set to ...
0
votes
1answer
70 views

compact open topology on M(X,Y) continuous function space

I'm really confused with the statement. let $X$ be a set endowed with the discrete topology and let $Y$ be any topological space. Show that $M(X,Y)$ with the compact open topology is homeomorphic to ...
6
votes
3answers
128 views

Are the fibers of a flat map homotopy equivalent?

At the end of the Wikipedia article on Deformation Retract, there is the following sentence: Two spaces are homotopy equivalent if and only if they are both deformation retracts of a single larger ...
7
votes
1answer
66 views

Any Real Algebraic Variety Has Finitely Many Path Components?

Consider the following statement: Statement: Any real algebraic variety has only finitely many path components. By 'path component' I mean: Let $X$ be a real algebraic variety. Then $X$ is a subset ...
4
votes
1answer
53 views

Homeomorphism class of GL_n?

For example, it is easy to see that $GL_1(\mathbb{C})$ is a plane minus a point, and $GL_2(\mathbb{R})$ is $\mathbb{R}^4$ with a topological (half-open) cube removed (since the matrices of determinant ...
2
votes
2answers
71 views

Dimension of a $G$-variety $X$ that is a finite union of $G$-orbits

Suppose that $G$ is an algebraic group acting on a variety $X$, and $X$ is a finite disjoint union of $G$-orbits $\mathcal{O}_i$, $i=1,\ldots,n$, under this action. Is it true that the dimension of ...
0
votes
1answer
37 views

intersection of two irreducible spaces not irreducible

Let $X$ be a topological space and let $Y_1,Y_2$ be two distinct irreducible subsets with none containing the other. Then it seems to me that $Y_1 \cap Y_2$ need not be irreducible. Is that right? ...
1
vote
1answer
52 views

Zariski topology and polynomial maps

I've read on my book that Zariski topology is coarser than every topology in which polynomial maps are continous, but no proof of this fact is given. Could someone sketch me the proof of this?
3
votes
2answers
39 views

The two projection maps are different?

I'm reading Vistoli's notes, and I came across something on the bottom of page 31, starting with "The reader might find our definition of sheaf pedantic..." Essentially my problem is the following ...
3
votes
1answer
104 views

How to visualize projective plane

I need to comprehend projective plane as a prerequisite for some other topic. But I can't understand what it really looks like. How can I make this plane natural to me? Moreover I want to understand ...
2
votes
1answer
71 views

$X$ being locally closed is not equivalent to every extension by $0$ of a sheaf $F$ being unique?

In Tennison's book "Sheaf Theory", the author presents a proof that there is a unique extension by $0$ for a sheaf $F$ over $X$ iff $X \subset Y$ is locally closed . However, apparently in the proof ...
5
votes
1answer
93 views

What Information/Advantage do we Gain by Substituting a Continuous Map by a Fibration?

I'm trying to understand the usefulness of "substituting" a continuous map f , by a fibration F. By substituting, I mean there is the result that given a continuous map $f:X \rightarrow Y $ , for ...
1
vote
0answers
40 views

Let $R$ be a conmutavive ring. Prove the following: If $Spec(R)$ is $T_1$ then $Spec(R)$ is Hausdorff.

Let $R$ be a conmutavive ring. Prove the following: If $Spec(R)$ is $T_1$ then $Spec(R)$ is Hausdorff. Here $Spec(R)$ means the Zariski topology over the set of all prime ideals of $R$. To put it ...
0
votes
1answer
43 views

Irreducibility of set

Let $Y$ be a subset of the topological space $X$ and let $\{U_i\}$ be an open cover of $X$. 1.If $Y$ is not contained in any $U_i$ then $Y$ is irreducible? 2.If $\{V_j\}$ is an open cover of $Y$ ...
0
votes
1answer
30 views

Support of form and embedded varieties

I need help with some inclusions. Let $i: S \rightarrow M$ be an embedding between two oriented varieties of dimension k and n respectively. Assume that the $i(S)$ is closed and that $\omega\in ...
-1
votes
1answer
46 views

Irreducible space with infinitely many irreducible components

It would be intuitively satisfying to say the following: A topological space is irreducible if and only if it has exactly one irreducible component. But it is not immediately clear how to prove ...
2
votes
1answer
53 views

Relationship between decompositions of a $G$-variety $V$

Let $V$ be a variety over a field $k$, and let $G$ be an algebraic group over $k$ which acts morphically on $V$. $V$ has three canonical decompositions, and I'm interested in the relationships ...
1
vote
1answer
61 views

If $X$ is a noetherian topological space, then any union connected components of $X$ is clopen.

If $X$ is a noetherian topological space, then any union connected components of $X$ is clopen. This is exercise 3.6P of Vakil. I can see that a union of connected components is closed. This is ...
4
votes
1answer
42 views

Noetherian topological subspaces

I'm trying to prove that any subset of a noetherian topological space is noetherian in its induced topology. MY ATTEMPT OF SOLUTION Let $X$ be a topological space and $Y$ a subspace of $X$. If ...
2
votes
1answer
29 views

About closed map beween schemes

Probabily it's trivial but I've no idea for a proof. Let $f: X \rightarrow Y $ a continuous map between Topological Spaces, with $Im(f)$ closed in $Y$. I know there exist a covering $\{Y_i\}$ of $Y$ ...
1
vote
1answer
66 views

what's wrong with this categorical proof that maps between two covering spaces are unique?

Let $\mathcal{C}$ be the category of finite covers of a fixed base space $S$ (say, connected, locally path connected, locally simply connected. Hell, we can even assume $S$ is a manifold). Morphisms ...
1
vote
2answers
81 views

Why is $\mathbb R^n$ under the Zariski topology not a topological group?

Reasons that $(\mathbb R^n, +, \mathcal Z)$ is not a topological group: Given any two distinct points $\vec{p},\vec q \in \mathbb R ^n$ let $P$ be the unique hyperplane through $\vec p$ which is ...
2
votes
0answers
45 views

Product varieties with the constructible topology

Let $k$ be an algebraically closed field and let $X\subseteq k^n$, $Y\subseteq k^m$ be two affine algebraic varieties. It is not difficult to find examples where the Zariski topology on the product ...
2
votes
2answers
110 views

Genus of a curve: topology vs algebraic geometry

In topology one defines the genus $g$ of a connected orientable topological manifold $X$ as: The maximum number $g$ of cuttings along non-intersecting closed simple curves without rendering the ...
13
votes
1answer
238 views

What is the homotopy type of the affine space in the Zariski topology..?

I'm asking this question out of curiosity, as I was unable to come to a conclusion. Consider the affine space over an algebraically closed field, for concreteness we can work with $\mathbb{C}^{n}$. ...
5
votes
4answers
172 views

Algebraic varieties in $\mathbb{C}^n$ cannot have interior points

I know that the zero-set of a non-zero polynomial in $\mathbb{C}[x_1,...,x_n]$ can not have interior points, but I'm trying to find a proof that doesn't require a knowledge of complex analysis like ...
0
votes
0answers
87 views

Action of the fundamental group on a Universal cover

Let $\pi: \tilde{X} \mapsto X$ an universal cover. I know that $\tilde{X}/Aut(\tilde{X},\pi) \simeq X$. Let $H \subset \pi_1(X,q)$ a subgroup of the fundamental group and consider the orbit space ...
2
votes
1answer
89 views

Lift of a diffeomorphism of the Torus

I'm trying to prove the following formula. Suppose to have $p:\mathbb{R}^{d}\rightarrow\mathbb{T}^{d}$ the canonical projection of the real d- dimensional space in to the d-dimensional torus, and ...
0
votes
0answers
73 views

Guess that Topology

I am interested in a geometry that I'm imagining but don't possess the requisite language to understand completely. I'm searching for an answer which points me in the right direction. The space I'm ...
1
vote
1answer
59 views

The dimension of a subspace is less than dimension of the whole space.

Let $X$ be topological space and $Y\subset X$. The goal is to show that $\dim Y\leq \dim X$. Here we use this definition of dimension. Let $Y_0\subsetneq Y_1\subsetneq \cdots \subsetneq Y_{n}$ be a ...
1
vote
0answers
39 views

Group of covering transformations

The group of automorphisms of a covering $p: E \mapsto X$, to be denoted $Aut(E,p)$, is usually referred to as the group of covering transformations. If $p: E_1 \mapsto E_2$ is an isomorphism of ...
2
votes
1answer
80 views

Why do the Zariski distinguished open subsets form a base?

Let $R$ be commutative ring with unit. I have to prove that the distinguished open sets form a base for the Zariski topology i.e. any non-empty open set is a union of distinguished ones. We have that ...
3
votes
1answer
147 views

The Disk and the Punctured Disk

Can you explane me why $$D = \operatorname{Spec}\mathbb{C}[[t]]$$ is the disk and $$D^{\times} = \operatorname{Spec}\mathbb{C}((t))$$ is the punctured disk? Or give me some links on intelligible ...
0
votes
0answers
65 views

Hartshorne II Prop 2.6

Prop 2.6 constructed a continuous map X to t(X), I cannot verify that it is a homeomorphism. I try to show any open set U is mapped to t(X)\t(X\U). To show it is surjective, let K be an irreducible ...
5
votes
1answer
45 views

Concentrated schemes are closed under finite gluings

Let $X$ be a scheme. Assume that $X = \cup_i X_i$ is finite open covering, such that the $X_i$ as well as their intersections $X_i \cap X_j$ are concentrated, i.e. quasi-compact and quasi-separated ...
4
votes
1answer
86 views

Etale spaces of a presfeaf and the associated sheaf

Given a presheaf $\mathcal{F}$on a topological space $X$, one can construct the etale space $\pi_1 : Y_1\to X$. Let us now look at the associated sheaf $\mathcal{F}^+$ as a presheaf and construct the ...
5
votes
2answers
133 views

Why is a section of a sheaf over closed set defined this way?

Why is a section of a sheaf $F$ over closed set $S \subset X$ is defined as inductive limit $$ \varinjlim_{S\subset U} F(U)\; ?$$ From my point of view, we should define it as a function, which each ...
1
vote
0answers
93 views

Why this is equivalent to proper?

maybe that's an idiot question. Anyway, when I was reading this master thesis http://sma.epfl.ch/~werndli/scripts/mscthesis/elementary_gaga.pdf I could not understand the following equivalence (in ...
3
votes
1answer
94 views

Morphisms between quasiprojective varieties preserve irreducibility

Let $X,Y$ be two quasiprojective varieties and $\phi \colon X \to Y$ a surjective morphism. Let $Z \subset Y$ a closed set such that $\phi^{-1}(Z)$ is irreducible. Prove that $Z$ is irreducible. ...
3
votes
1answer
77 views

$\mathbb A^n$ is not a complete quasiprojective variety

Suppose that $\mathbb K$ is an algebrically closed field. In the course of Algebraic Geometry I've attended we gave the following definition (in what follows every space is to be intended on the field ...