All Questions
8,725 questions
12
votes
0
answers
482
views
What is the infinity category of subspaces of $\mathbb{R}^n$?
Let $\mathcal{J}$ denote the topological category of finite-dimensional real inner product spaces with linear isometric embeddings. The space of morphisms $\mathcal{J}(\mathbb{R}^k, \mathbb{R}^n)$ is ...
- 901
1
vote
0
answers
64
views
Energy minimization and boundary homotopy types of compact manifolds
Let $X$ be a connected, compact, smooth manifold of dimension $n$ with a non-empty boundary $\partial X$. Define the boundary homotopy type of $X$ as the homotopy type of the pair $(X, \partial X)$.
...
4
votes
1
answer
496
views
Homotopy groups of the space of diffeomorphisms
Let $M$ be a smooth, closed, and connected manifold of dimension $n \geq 5$. Let $\operatorname{Diff}(M)$ denote the space of diffeomorphisms of $M$ with the $C^\infty$-topology.
Is there a general ...
- 111
3
votes
0
answers
134
views
When do quotients of $G$-vector bundles exist?
Let's work in the category of smooth (paracompact, Hausdorff) manifolds. Let $M$ be a manifold and $G$ a Lie group acting on $M$. Suppose $E$ is a $G$-vector bundle on $M$ (that is, $G$ acts on $E$ by ...
- 51
4
votes
0
answers
249
views
Bounds for torsion in Betti cohomology
Let $X\subset \mathbb{P}^{N}_{\mathbb{C}}$ be a smooth, projective variety of dimension $n$ and degree $D$. Is there an upper bound on the torsion in the Betti cohomology groups $H^{i}(X, \mathbb{Z})$ ...
- 41
11
votes
1
answer
690
views
$\zeta(-n)=2^{r_1}\frac{|K_{2n}(O)|}{|K_{2n+1}(O)|} R_K$ and replace $K$-theory with $\mathbb{S}$
There seems to be an agreement among experts that the formulas by Lichtenbaum in the 70's $$\zeta(-n)=2^{r_1}\frac{|K_{2n}(O_K)|}{|K_{2n+1}(O_K)|} R_K$$
follow from the resolution of the Bloch-Kato ...
- 705
7
votes
1
answer
435
views
What are the covering spaces of $\mathbb{Q}$?
Let $X = \mathbb{Q}$, topologized as a subset of the real line. Is there a reasonable description of the covering spaces of $X$?
Here is something more precise. One way of constructing covers $p: \...
- 395
4
votes
0
answers
112
views
Differentials on free algebras over operads
I am currently reading Cyclic Operads and Cyclic Homology by Getzler-Jones and have some confusions.
I am under the impression that given an (associative, say) algebra $A$ that an almost-free ...
- 195
7
votes
1
answer
464
views
Direct limits in homotopy category
It is known that the homotopy category $\mathrm{HoTop}$ is not complete nor cocomplete. Moreover, it also fails to have filtered colimits in general. I am wondering if the universal property of the ...
- 173
5
votes
1
answer
208
views
Reference for homotopy groups of filtered homotopy colimits
It seems to be well known that for a filtered category $I$
and a functor to the category of pointed spaces $X:I \to \mathcal{S}_*$
the homotopy groups of the filtered homotopy colimit are colimits of ...
- 1,484
13
votes
1
answer
560
views
Intuitive reason for periods of 2 and 8 in Bott periodicity?
Is there a reasonably simple explanation for why Bott periodicity for $U$ and $O$ have periods 2 and 8, respectively? For example, in the $h$-cobordism theorem the requirement that $n \geq 5$ has the ...
- 131
8
votes
0
answers
827
views
Can you glue the Betti $X_\text B$ and de Rham stacks $X_\text{dR}$ together?
Let $X$ be a complex algebraic variety. Is there a (derived prestack) over a base
$$\pi\ :\ X_\text{dR,B}\ \to\ S$$
where $S=\mathbf{R},\mathbf{R}/\mathbf{R}^\times,\mathbf{A}^1_\mathbf{C}/\mathbf{G}...
- 5,711
6
votes
1
answer
360
views
On connected sum of compact manifolds along a submanifold
Let $M_1$ and $M_2$ be two compact manifolds of dimension $n\ge 3$. Let us have embeddings $i_1: K \to M_1$ and $i_2: K \to M_2$ for a closed manifold $K$ of dimension at most $n-1$ such that the ...
- 506
5
votes
0
answers
92
views
For spaces $U$ and discrete sets $I,J$, are maps $f\colon U \times I \rightarrow U \times J$ commuting with the projection to $U$ covering spaces?
Let $U$ be a topological space, let $I$ and $J$ be discrete sets, and let $f\colon U \times I \rightarrow U \times J$ be a continuous map that commutes with the projection onto the first factor. In ...
- 395
6
votes
1
answer
206
views
A stable splitting of linear surjections
Some computations I've been doing in Weiss calculus predict the following stable splitting of the space of linear surjections: $\Sigma^\infty_+ \mathrm{Sur}(\mathbb{R}^n,\mathbb{R}^{m_1+m_2})$
as the ...
- 5,849
2
votes
2
answers
185
views
Relative, local coefficient Poincaré duality
Let $X$ be a connected, oriented and aspherical (meaning $\pi_i(X) = 0$ for $i\geq 2$) manifold of dimension $n$, and $S$ a local coefficient system (i.e., a $\pi_1(X)$-module). If $X$ is closed, we ...
- 985
6
votes
0
answers
223
views
Under what generality are the compactly supported singular and sheaf cohomologies equal?
Edit: I have since resolved my question.
If X is locally compact Hausdorff in addition to being cohomologically locally contractible with coefficients in $A$ - eg it is a manifold or an open subset of ...
- 1,020
3
votes
0
answers
85
views
de Rham cohomology relative to a closed subset
I am interested whether there exists a versions of de Rham relative cohomology $H^\bullet(M, N)$ in which $N$ does not need to be a manifold. I know there are two main definitions in literature as ...
23
votes
0
answers
720
views
Which proofs of the fundamental theorem of algebra are "essentially the same" vs. "essentially different"?
The classic MO thread Ways to prove the fundamental theorem of algebra contains $60$ proofs of FTA, and I'm sure there are many more in the literature. It would be nice to have some way to organize ...
- 118k
1
vote
2
answers
408
views
Using topology for proving periodicity
Let $f\in\mathcal{C}\big(\mathbb{R},\mathbb{C}^\ast\big)$ a continuous function with modulus $r$ satisfying: $f(t)=r(t)e^{it}$. Assume that the image of $f$ is homeomorphic to the unit circle.
...
- 449
9
votes
0
answers
212
views
Left adjoint functor between categories of polygons?
EDIT: Based on very helpful comments from Alec Rhea and Qiaochu Yuan I am adding some specification on objects and morphisms, hoping that this clarifies the idea behind these categories. I have also ...
- 6,937
5
votes
0
answers
159
views
Topologies on the infinite join
Let $G$ be a topological group. Following Milnor one way of defining the total space of the universal bundle $EG$ of $G$ is to form the infinite join
$$
EG = G^{\ast \infty} = G \ast G \ast \dots
$$
...
- 7,637
5
votes
1
answer
380
views
Proving the Cork Theorem
I am reading Kirby's paper paper "Akbulut's corks and h-cobordisms of smooth simply connected 4-manifolds" and I have a question about how to actually prove the cork theorem from the results ...
4
votes
1
answer
218
views
Topological interpretation of the canonical cover of a logarithmic Enriques surface
A normal projective surface $Z$ with at worst quotient singularities is called a logarithmic (log) Enriques surface if its canonical Weil divisor $K_Z$ is numerically equivalent to zero, and $H^1(Z,\...
- 213
1
vote
1
answer
133
views
Is the product of torus and sphere a cover of the symmetric square of torus?
Let $T$ denote the $2$-dimensional torus and $T^{(2)}$ denote its symmetric square (which is the orbit space of the canonical $\mathbb{Z}_2$ action on the $4$-torus $T \times T$).
One can see $T^{(2)}$...
- 31
2
votes
0
answers
100
views
A question about Milnor space
Let $G$ be a compact group and $H$ be a closed subgroup of $G$. I know that
if $G\rightarrow G/H$ is a principal $H$-bundle, then we can choose $E_{H}$ as $%
E_{G}$, where $E_{G}$ is the Milnor space ...
- 1,367
1
vote
1
answer
385
views
How does homotopy theory simplify topology but allow for complexity in higher category theory?
I'm trying to understand the dual nature of homotopy theory, which seems to play different roles in algebraic topology and higher category theory.
In algebraic topology:
Homotopy theory is often seen ...
- 441
2
votes
0
answers
44
views
Link invariants on a thickened surface
Let $\Sigma$ be an oriented surface. I want to know about link invariants in $\Sigma\times [0,1]$. I already know the Ozawa polynomial introduced in this paper, but I couldn’t find any other than that....
- 21
2
votes
0
answers
84
views
Infinity-morphisms for operadic algebras
Is there an already studied notion of $\infty$-morphism between algebras over a quasi-free operad $P = (T(E), \partial)$?
If the operad $P$ is Koszul, or of the form $\Omega C$ for $C$ a cooperad, ...
- 215
3
votes
2
answers
305
views
Are $H^3(A,U(1))$ and $\operatorname{Ext}^1(A,A^\vee)$ isomorphic for $A$ finite Abelian?
Motivated by three-dimensional Dijkgraaf-Witten TQFTs for finite Abelian groups $A$, that are classified by $H^3(A,\mathbb{R}/\mathbb{Z})$, it seems natural that this group is (naturally) isomorphic ...
- 813
3
votes
1
answer
118
views
Characterization of self-conjugate spin$^c$ structures
Let $M$ be an oriented Riemannian $n$-manifold. Then we can choose a trivializing open cover $M=\bigcup_\alpha U_\alpha$ for $TM$ and corresponding transition functions $g_{\alpha \beta}:U_\alpha \...
- 335
1
vote
0
answers
61
views
Map from simplex to itself that preserves sub-simplices: revisited
Here it is proved that, if $f$ is a continuous map from an $n$-simplex $\Delta$ to itself, that maps each sub-simplex of $\Delta$ to itself, then $f$ must be onto $\Delta$ (surjective).
I would like ...
- 3,559
6
votes
1
answer
348
views
Detailed exposition of construction of Steenrod squares from Haynes Miller's book
$\DeclareMathOperator\Sq{Sq}$Chapter 75 of Haynes Miller's book on algebraic topology contains a beautiful construction of the Steenrod squares $\Sq^i$.
Roughly speaking, it goes as follows. All ...
- 63
3
votes
2
answers
141
views
Accessible literature on fractional dimensions of subsets of $\mathbb R^n$
I am currently wondering whether it is realistically possible to choose the topic "Fractals and fractal dimensions" for a seminar aimed at undergraduate students in the 2nd semester, with ...
- 1,942
4
votes
1
answer
183
views
When can a generalized connected sum be aspherical
Let $M$ and $N$ be compact $n$-dimensional manifolds with a common (nicely embedded) compact submanifold $S$ (we may assume that the inclusion of $S$ in $M$ and $N$ is $\pi_1$-injective). Let $M\#_S N$...
- 311
3
votes
0
answers
79
views
Rational model for composition of linear isometries
There is a composition map on spaces of linear isometries (over $\mathbb{C}$ say)
$$
\mathcal{L}(\mathbb{C}^k, \mathbb{C}^\ell) \times \mathcal{L}(\mathbb{C}^\ell, \mathbb{C}^m) \longrightarrow \...
- 901
27
votes
8
answers
3k
views
Object of proven finiteness, yet with no algorithm discovered?
I explain my title by two examples in number theory:
The rational points on elliptic curve over number fields forms a finitely generated abelian group, so its rank is an integer, but so far we do not ...
- 1,053
7
votes
0
answers
192
views
Eulerian posets and order complexes
To every poset $P$ it is possible to associate its order complex $\Delta(P)$. The faces of $\Delta(P)$ correspond to chains of elements in $P$. An Eulerian poset is a graded poset such that all of its ...
- 1,889
7
votes
0
answers
219
views
Twisting cochain intuition
I'm currently reading through Ed Brown's paper "Twisted tensor products, I", (MR105687, Zbl 0199.58201) and I couldn't find any simple examples of twisting cochains. I understand all ...
- 171
4
votes
0
answers
121
views
$E_k$-operads and actions on objects inside $k$-tuply monoidal $n$-category
I understood more or less the claim that $k$-tuply monoidal $n$-categories can be seen as $n$-categories equipped with an action of the $E_k$-operad.
For $k=2$, we have a homotopy equivalence $E_2(r) \...
- 1,813
2
votes
0
answers
75
views
Action of $V$ on the homology of a subposet of the poset of affine subspaces of $V$
Let $(V,Q)$ be a pair, with $V=\mathbb{F}_2^{2n}$ ($n\geq 2$) and $Q$ a nondegenerate quadratic form on $V.$ We consider the poset $\mathcal{P}_n$ of affine totally isotropic (with respect to $Q$) ...
- 245
4
votes
1
answer
255
views
Intersection pairing on non-compact surface
Let $S$ be a smooth oriented connected $2$-manifold. We have an algebraic intersection pairing $\omega\colon H_1(S) \times H_1(S) \rightarrow \mathbb{Z}$. If $S$ is compact, then this is ...
- 43
1
vote
0
answers
58
views
Which sheaves are good for calculating extraordinary restriction?
Let $X$ be a sufficiently nice locally compact Hausdorff space and let $i:Y\subset X$ be the inclusion map of a sufficiently nice closed subspace. For example, one could take $X$ to be a locally ...
- 23.5k
5
votes
0
answers
160
views
$\infty$-category of spectra and cofibrancy
I have two options for the $\infty$-category of spectra. I would like to know they are equivalent as $\infty$-categories.
Premise: by work of Dwyer and Kan, if we have a simplicial model category, the ...
- 410
4
votes
1
answer
164
views
Are monomorphisms in an $\infty$-topos preserved by $0$-truncation?
Let $\mathfrak{X}$ be an $\infty$-topos and let $f\colon X\to Y$ be a morphism of $\mathfrak{X}$. We say that $f$ is a monomorphism if it is $(-1)$-truncated which means that for every $Z\in\mathfrak{...
- 10.5k
8
votes
1
answer
232
views
Product structure in Milnor exact sequence
Let $h^*$ be a (multiplicative) generalized cohomology theory. Let $X$ be a CW complex which is a union of an increasing sequence $X_0 \subset X_1 \subset X_2 \subset \cdots$ of subcomplexes. Then ...
- 959
7
votes
1
answer
845
views
Algebraic K-theory and Witt groups
Let $S$ be a ring with involution (with 2 invertible). Suppose that the non connective algebraic K-theory $K(S)$ is 0 (i.e. $K_{n}(S)=0$, for all $n$).
Can we say something about the (higher) Witt ...
- 855
6
votes
1
answer
426
views
Nilpotency of generalized cohomology
$\newcommand\pt{\mathrm{pt}}$Let $(X,\pt)$ be a connected, pointed, finite CW complex and let $h$ be a generalized cohomology theory. Let $\smash{\tilde{h}}^*(X)$ denote the kernel of restriction $h^*(...
- 959
0
votes
0
answers
138
views
Shub Conjecture and polynomial entropy
The Shub conjecture on topological entropy $h(f)$ of self map f on manifold M says that the topological entropy is greater (or equal) than (to) the log of maximum absolute values of the ...
- 366
6
votes
1
answer
245
views
Fundamental group of the homeomorphism group of a compact manifold
Let $X$ be a compact connected manifold and $\mathcal H(X)$ be the group of all homeomorphisms of $X$, equipped with the compact-open topology. Is the fundamental group of $\mathcal H(X)$ countable? ...
