# Recent MathOverflow Questions

### Regarding exponential in a Banach algebra

Math Overflow Recent Questions - Mon, 07/30/2018 - 00:20

Let $A$ be a complex unital Banach algebra. Let exp$(A)$ denote the range of the exponential function on $A$. Now exp$(A)$ lies in the set of all invertible elements of $A$ (denoted by $G(A)$). Can you give an example of an element belonging to $G(A)\setminus$ exp$(A)$?

### How do we compute the even cohomology $H^{2i}(Q)$ of the affine hyperquadric?

Math Overflow Recent Questions - Sun, 07/29/2018 - 18:28

Consider the affine hyperquadric $Q:=\biggl\{(z_1,...,z_{n+1})\in\mathbb{C}^{n+1}\biggl|\sum_{i=1}^{n+1}z_i^2=1\biggr\}\cong TS^n$.

What is a reasonable Kähler metric for $Q$ (induced by the pullback of the metric from the ambient space $\mathbb{C}^{n+1})$? Furthermore, how do we explicitly calculate the curvature form $\Omega$ on $Q$? Hence, compute the Chern classes of $Q$. Given this, how do we find $\chi(Q,\mathcal{O}_Q)$?

If you down-vote, please explain why so I can improve the question. Thanks in advance!

### Tiling with incommensurate triangles

Math Overflow Recent Questions - Sun, 07/29/2018 - 15:35

Say that two triangles are incommensurate if they do not share an edge length or a vertex angle, and their areas differ. Suppose you'd like to tile the plane with pairwise incommensurate triangles. I can think of at least one strategy.

Spiral out from an initial triangle in the pattern depicted below, with the red extensions chosen to avoid length/angle/area coincidences with all previously constructed triangles.           It seems clear that this approach could work, although it might not be straightforward to formalize to guarantee incommensurate triangles. Which brings me to my question:

Q. What is a scheme that details a lattice tiling—all vertices at points of $\mathbb{Z}^2$—composed of pairwise incommensurate triangles?

This requires a more explicit design that effectively describes the triangle corner coordinates in a way that makes it evident that no lengths/angles/areas are duplicated. Without such a clear description, it is not even immediately evident (to me) that it is possible.

The same question may be asked for incommensurate simplex tilings with vertices in $\mathbb{Z}^d$.

### Non-separable metric probability space

Math Overflow Recent Questions - Sun, 07/29/2018 - 14:38

Let us say a metric probability space $(X,\rho,\mu)$ has property (*) if: the support of $\mu$ is contained in a separable subspace of $X$.

Questions: 1. Is there a standard name for this property?

1. Is it true that continuum+choice implies property (*) -- is there a reference?

2. Is it true that if (either? both?) continuum+choice don't hold, (*) can fail? Again, reference please!

### Do the absolute roots restricting to a given root form a Galois orbit?

Math Overflow Recent Questions - Sun, 07/29/2018 - 00:23

Let $S$ be a maximal split torus of a connected, reductive group $G$. Let $P_0$ be a minimal $k$-parabolic containing $S$, $T$ a maximal torus of $P_0$ which is defined over $k$ and contains $S$, and $B$ a Borel subgroup contained in $P_0$ and containing $T$.

The choice of $P_0$ and $B$ determine simple roots $_k\Delta$ and $\Delta$ for $_k\Phi = \Phi(G,S)$ and $\Phi = \Phi(G,T)$.

For each $a \in \space _k\Delta$, the set of $\alpha \in \Delta$ which restrict to $a$ form an orbit under the $\ast$-action of $\operatorname{Gal}(k_s/k)$. If $G$ is quasisplit, then the $\ast$-action is just the usual Galois action on characters. This is explained in section 12 of Brian Conrad's notes on reductive groups over fields.

What if we take an arbitrary $a \in \space _k\Phi$? Do the set of roots in $\Phi$ which restrict to $a$ also form a Galois orbit?

### Find a surface or 3-manifold whose fundamental group is $(\mathbb{Z}/n\mathbb{Z}) \rtimes (\mathbb{Z}/2\mathbb{Z})$

Math Overflow Recent Questions - Sat, 07/28/2018 - 09:08

I know by Van Kampen's Theorem that we can obtain $\pi_1(S_1 \vee S_1) = \mathbb{Z} * \mathbb{Z}$, so I am wondering if we can construct a surface or 3-manifold whose fundamental group is $\mathbb{Z}_n * \mathbb{Z}_2$ or even $\mathbb{Z}_m \rtimes \mathbb{Z}_n$.

This might be an interesting problem because I have written semidirect product $\rtimes$ rather than the free product $*$. A torus knot $K$ is defined in Hatcher as the image of an embedding of a map $f : S^1 \to S^1 \times S^1 \to \mathbb{R}^3 \subset S^3$ given by $z \mapsto (z^m, z^n)$ then the fundamental group $\pi_1(\mathbb{R}^3 - K)$ is $Z_m \ast Z_n$ possibly up to some number-theoretic conditions. Hatcher doesn't quite give you the answer.

I think the semidirect product $\mathbb{Z}_m \rtimes \mathbb{Z}_n$ is unique. We have to specify $\mathbb{Z}_m \lhd G$ and then $G = \mathbb{Z}_m \ltimes \mathbb{Z}_n$.

### Cayley graph properties

Math Overflow Recent Questions - Fri, 07/27/2018 - 14:23

Consider an infinite graph that satisfies the following property: if any finite set of vertices is removed (and all the adjacent edges), then the resulting graph has only one infinite connected component.

So, obviously, the Cayley graph for the group $\mathbb Z \times \mathbb Z$ w.r.t. the standard generating set is an example. Obviously, the Cayley graph for a free group is not an example.

I have a question: what is the name of such a property? Has it been studied?

And the next question: which are the Cayley graphs with this property?

### Is a smooth intersection of hypersurfaces equidimensional?

Math Overflow Recent Questions - Fri, 07/27/2018 - 09:26

Let $X$ be a smooth projective complex algebraic variety. Let $V_i$, for $i=1,\dots, n$, be a collection of (smooth) connected hypersurfaces such that, for all $I\subseteq [n]$, the intersection $\cap_{i \in I} V_i$ is smooth.

Is the intersection $\cap_{i=1,\dots, n} V_i$ equidimensional?

Added later: If we take $V_i$ to be smooth effective (possible non ample) divisors, is the intersection $\cap_{i=1,\dots, n} V_i$ equidimensional?

### Geometric description of the Hilbert-Chow morphism on a subschme of a Hilbert scheme

Math Overflow Recent Questions - Fri, 07/27/2018 - 08:59

Let $S$ be a smooth projective surface, say over $\mathbb{C}$.

We have the Hilbert schemes $S^{[n]}$ and $S^{[n+1]}$ classifying length $n$-subschemes (resp. length $n+1$) together with the flag Hilbert scheme $S^{[n,n+1]}$ classifying pairs $(Z,W)$ with $Z\in S^{[n]}$ and $W\in S^{[n+1]}$ such that $Z\subset W$. There are projections $S^{[n]}\xleftarrow{p}S^{[n,n+1]}\xrightarrow{q} S^{[n+1]}$.

Let $\varphi:S^{[n+1]}\rightarrow S^{(n+1)}$ be the Hilbert-Chow morphism to the symmetric product.

Now pick a reduced $Z_0\in S^{[n]}$, that is $Z_0=\{p_1,\ldots,p_n\}$. Define $X:=p^{-1}(Z_0)$ and $Y:=q(X)$ then $X\cong Y \cong Bl_{Z_0}(S)$.

$\textbf{Question:}$ Can one describe the differences (if there are any) between $Y$ and $\varphi^{-1}(\varphi(Y))$?

From the view of closed points, $X$ is classifying all pairs $(Z,W)$ with $Z\subset W$ and $Z=Z_0$, so $Y$ should classify all length $n+1$ subschemes $W$ in $S$ containing $Z_0$. Such a $W$ is therefore given by adding one point $p\in S$ to $Z_0$. If $p\notin Z_0$ the $W$ is just the reduced point $W=\{p_1,\ldots,p_n,p\}\in S^{[n+1]}$. If $p\in Z_0$ the we get something non-reduced (that's why we blow up $Z_0$ in $X$).

So all $W$ which are reduced should land in the smooth locus of $S^{(n+1)}$ using $\varphi$. The $W$ with non-reducedness (we have $n$ of them) should land in the singular locus, but in the somehow good singular locus, because $W$ still contains $n-1$ simple points and one non-reduced length 2-subscheme.

So if one looks at the preimage, almost all $W$ are in the isomorphism locus of the Hilbert-Chow morphism, and over the $n$ non-reduced $W$ we get a $\mathbb{P}^1$ since we blow up the singularity, as $\varphi$ is a resolution of singularities of $S^{(n+1)}$.

So it seems to me, that we should have $Y\cong \varphi^{-1}(\varphi(Y))$. Is this geometric picture correct? What about the scheme structure in this situation? Do we have $Y\cong \varphi^{-1}(\varphi(Y))$ as schemes? Or is there something more happening, which is not so obvious, since $S^{(n+1)}$ is singular or because of bad properties of $\varphi$?

Can we do the same thing if we start with a more complex $Z_0\in S^{[n]}$?

### Reference for: $p$-primary component of $\pi^S_k$ is $\Bbb Z_p$ when $k=2l(p-1)-1$

Math Overflow Recent Questions - Fri, 07/27/2018 - 08:11

I remember coming across this result some time ago but I am having trouble finding a reference for it. It goes something like this:

Let $p$ be a(n odd?) prime, then the $p$-primary component of $\pi^S_k$ is $\Bbb Z_p$ when $k=2l(p-1)-1$ for $l=1,\dots,p-1$ and is trivial for all other $k<2p(p-1)-2$.

This is what I have written down on the back of an envelope. I checked this with Wikipedia's table and it seems to be true.

What is the reference in which it is proved? And if its simple could you overview it as an answer here?

My guess is that it is proven by Toda but his papers are difficult to search through.

### *Full proof* references for Markov generators with various boundary conditions

Math Overflow Recent Questions - Fri, 07/27/2018 - 08:05

(Note: I've migrated this question from math.stackexchange, as the lack of answers there made me believe it was perhaps too advanced for that forum.)

Consider the one-dimensional heat equation $$\partial_t u(t,x)=\frac12\Delta u(t,x),\qquad t\geq0,~x\in I$$ on some interval $I=(-a,a)$, with some initial condition $u(0,x)=f(x)$ and boundary condition on the interval $I$.

Suppose that we define the following processes

1. $(K_t)_{t\geq0}$, a Brownian motion killed on the boundary of $I$;
2. $(R_t)_{t\geq0}$, a Brownian motion reflected on the boundary of $I$;
3. $(P_t)_{t\geq0}$, a periodic Brownian motion in the interval $I$.

I know from a mix of intuition or folklore that we have the following probabilistic representations the solution $u(t,x)$ for different choices of boundary conditions on $I$: $$u(t,x) =\begin{cases} \mathbb{E}\big[f(x+K_t)\big]&\text{for Dirichlet boundary};\\ \mathbb{E}\big[f(x+R_t)\big]&\text{for Neumann boundary};\\ \mathbb{E}\big[f(x+P_t)\big]&\text{for Periodic boundary}. \end{cases}$$

Question. Are there any references out there with full proofs of these facts?

I've tried extensive researching on this subject to no avail.

### Why do middle roots of the $\chi(p)$ graphs and percolation thresholds vary linearly with diagonal probability $q$ (in large random binary matrices)?

Math Overflow Recent Questions - Fri, 07/27/2018 - 07:21

Consider a large $N\times N$ square lattice, where each cell has a probability $p$ of being "occupied" (let's call denote them as "black") and a probability $1-p$ of being empty (let's denote them as "white"). If any cell, lies in the Von Neumann neighbourhood of a certain cell and also has the same colour as of that central cell, it is said to belong to the same cluster as that central cell. Moreover, if any cell belongs to the Moore neighbourhood of a certain cell, but not its Von Neumann neighbourhood, and is of the same as of that cell, it is considered to belong the cluster as of that central cell with a probability $q$.

I wrote a program to plot the "Euler number" graphs, that is, the $\chi(p) \ [=N_B(p)-N_W(p)]$ vs. $p$ graphs, for different values of $q$, where $N_B(p)$ is the number of black clusters and $N_W(p)$ is the number of white clusters, at a probability $p$.

For a $1000\times 1000$ matrix (averaged over $100$ iterations) the Euler number graph's variation with $q$ is as follows:

When $q=0.5$ the middle root of the curve is extremely close to $0.5$.

I plotted the middle roots ($p_0$'s) in another graph:

For $1000\times 1000$ the middle root $p_0$'s variation with $q$ seems to be almost linear. Also, I plotted the same graph for a few more sizes: $125\times 125$, $250\times 250$ and $500\times 500$. I noticed that as system size increases the "middle root" graph gets more and more smooth and linear.

For what it's worth, I also noticed a similar trend (i.e. "site percolation threshold vs. $q$" graphs getting linear and smoother with increasing size) for the (approximate) site percolation thresholds for these finite size lattices.

Is there any mathematical justification for this trend?

P.S: Answers addressing only the site percolation threshold trend or only the $p_0$ trend are also welcome.

### What English translations are there of work done by the Italian school of algebraic geometry?

Math Overflow Recent Questions - Fri, 07/27/2018 - 06:45

What English translations are there of work done by the Italian school of algebraic geometry?

Perhaps I'm being too spoiled here, given that mathematical French, German, Italian are much easier to pick up on the fly than say, mathematical Russian or Japanese, for a native English speaker.

### Generalization: (The "number" of) smaller sized clusters in large random binary matrices follow a descending order. Why?

Math Overflow Recent Questions - Fri, 07/27/2018 - 06:40

This is a sequel to the question: Why is number of single cell clusters always greatest in a random matrix?

In their answer, @Aaron Meyerowitz came up with a nice strategy to prove why the number of size $1$ clusters in a random matrix are always greater than the number of sizes $2$ clusters in large $N\times N$ matrices (for any $p\in (0,1)$). From my data files, it also seems that the number of sizes $2$ clusters will be greater than the number of size $3$ clusters for all $p\in (0,1)$. That is, at least for the first few natural numbers $n$, the number of clusters of size $n$ is greater than the number of clusters of size $n+1$. Around the site percolation threshold $p=0.407$ there seem to be some fluctuations, however, still, for the first few natural numbers, the cluster sizes continue showing the above trend.

So, my question basically is: Is it possible to generalize the above trend? If yes, up to which natural number $n$ can it be generalized, and why?

P.S:

This sounds a bit like some kind of graphic Benford's law.

I'm not sure if Benford's law is somehow applicable in this situation. However, I'd be interested to hear if someone has any idea regarding this.

### Hypersurface whose "square" level sets intersect all linear subspace of "high" dimension

Math Overflow Recent Questions - Fri, 07/27/2018 - 05:24

Let $k$ be an infinite perfect field (e.g. I'm happy to assume that $k$ has characteristic $0$. On the other hand, the algebraically closed case is not interesting for this question). The question is happening inside the vector space $k^{21}$.

Let $f$ be a homogeneous polynomial of degree 6 in 21 variables with coefficients in $k$. For $\lambda \in k$, let $S_{f=\lambda}$ be the corresponding level set of $f$ in $k^{21}$, i.e. $S_{f=\lambda}:=\lbrace x\in k^{21}~\vert~f(x)=\lambda \rbrace$.

QUESTION: What kind of conditions can one give on $f$ to ensure that for any $8$-dimensional vector subspace $W<k^{21}$, the set $W\cap \big( \bigcup \limits_{\lambda \in (k^{\times})^2} S_{f=\lambda} \big)$ is non-empty?

I hope that the given parameters $21,6$ and $8$ are not really relevant, but this is what I get in my specific situation. Any comment on how to think about this question is welcome!

### Lifting Levi Civita connection

Math Overflow Recent Questions - Fri, 07/27/2018 - 05:10

When in differential geometry one shows , on a riemannian manifold, that a (unique) connection exists, (Levi Civita connection), is it possible to "lift" that notion to the principal bundle of frames over that manifold?In that case, is that lifted connection unique? (if so, the Yang Mills field would precisely be the LV connection)

Does the bundle of frames always admit a one form, even if the manifold has no metric on it?In that case that would provide a lifting to the LC connection...

### A question on Sobolev spaces and PDE, trading regularity for integrability

Math Overflow Recent Questions - Fri, 07/27/2018 - 04:59

Quoting from this blog of Prof Terry

“…as the very useful Sobolev embedding theorem, which allows one to trade regularity for integrability…”

Thats one use of Sobolev spaces. In this context, Fourier and Plancheral methods come very handy, when the corresponding Sobolev space is also a Hilbert space. But that is not always the case…. Only $L^2$ based Sobolev spaces are Hilbert spaces. According to Sobolev emebdding, if the function in $\mathbb{R}^d$ need to be holder continuous, then its gardient needs to be $L^p$ integrable with $p >= d+1$. So for $d >1$, we need $p > 2$, so the associated Sobolev space cannot have a Hilbert space structure. So in this context, we cannot use Fourier Plancheral techniques. “But if” (stress If)… I say, that I can always find a Hilbert space, for any d, (even for cases when d>1), how useful a tool that it would be, in the context of PDE. What would the impact be? Any examples of PDE, on which there would be impact? Appreciate your valuable comment.

### Quotient by a non-free action of a Lie group and manifolds with corners

Math Overflow Recent Questions - Fri, 07/27/2018 - 04:06

The quotient manifold theorem says that

If $G$ is a Lie group acting freely and properly on a smooth manifold $M$ then $M/G$ has a (unique) smooth structure such that the projection $\pi:M\to M/G$ is a submersion.

I was wondering what happens when the action is not free. My intuition suggests that we get corners, I have in mind this example: The action of $\frac{\mathbb{Z}}{2\mathbb{Z}}$ over $\mathbb{S}^2$ induced by the reflection wrt the $zy$-plane. The quotient manifold obtained is $\mathbb{D}^2$ and the boundary $\partial\mathbb{D}^2$ can be identified with the fixed points of the action i.e. $\mathbb{S}^2\cap zy \text{-plane}$.

Does anyone know a theorem that covers the non-free case? Where can I read about it?

### Can we define a normal vector field on the level sets of the distance function?

Math Overflow Recent Questions - Fri, 07/27/2018 - 03:56

Suppose $M$ is a smooth connected complete Riemannian manifold of dimension $n\geq 2$. Let $d:M\times M\rightarrow \mathbb{R}^+$ be the distance induced by the Riemannian metric on $M$. For $p\in M$ we set $d_p:=d(p,\cdot)$. We know that $d_p$ is smooth on $M\setminus (C_p\cup\{p\})$, where $C_p$ is the cut locus of $p$, which is a null set according to the Riemannian measure on $M$. Moreover, $d_p$ is regular in any point $q\in M\setminus (C_p\cup\{p\})$, since its gradient at $q$ is the derivative of the unique minimal geodesic at $d_p(q)$ joining $p$ and $q$.

For $R>0$ consider the level set $d_p^{-1}(R)$. Since $R$ does not need to be a regular value of $d_p$, we may not be able to define a normal vector field globally on $d_p^{-1}(R)$. Is there some characterisation of the intersection $C_p\cap d_p^{-1}(R)$, which states that set is "small" maybe in the sense of some $N-2$-dimensional Hausdorff measure or in some topological sense? Or is there some other way to define a unit normal vector field "almost everywhere" on $d_p^{-1}(R)$?

### A smooth curve and mean value theorem

Math Overflow Recent Questions - Fri, 07/27/2018 - 03:49

Assume that $f$ is smooth function defined in the unit disk $D: x^2+y^2\le 1$, and consider the integral $$I=\int_D f dxdy=\int_0^1r \int_0^{2\pi} f(re^{it})dt.$$

Then it is clear that for $r\in[0,1]$ there is $t_r\in [0,2\pi]$ so that $$I=2\pi \int_0^1 r f(re^{it_r})dr.$$ My question is, can we choose $t_r$ to depend smoothly on $r$.