# All Questions

**0**

votes

**0**answers

25 views

### About adjacency matrices of $k-$shift lifts of graphs

I am finding the notation of cyclic lifts of graphs to be very confusing.
Lets say one is looking at a cyclic $k-$lift of a $\vert V \vert$ sized graph.
I would like to understand what is the ...

**3**

votes

**1**answer

86 views

### Is there a standard notation for off-diagonal transpose?

Given a matrix $A=\begin{pmatrix}a&b\\c&d\end{pmatrix}$,
its transpose, obviously, is $A^T=\begin{pmatrix}a&c\\b&d\end{pmatrix}$.
But is there a conventional way of notating the matrix
...

**1**

vote

**1**answer

114 views

### Example of torsion in orientable manifolds?

An orientable manifold can have torsion in its integer homology. But I believe by Poincare duality the manifold must be at least 4-dimensional -- isn't that right? Anyway are there simple examples ...

**1**

vote

**0**answers

74 views

### Generating the sigma algebras on the set of probability measures

I was wondering if somebody could help me see/provide a reference to the following fact: Let $X$ be a metrizable set, $\mathcal{F}$ the corresponding Borel sigma-algebra on $X$, and ...

**4**

votes

**1**answer

180 views

### Bar Construction Model of Ring Spectrum Quotient

Suppose I am given a morphism $f:BG\to BGL_1(R)$ for $R$ some at least $E_1$-ring spectrum and $G$ a loop space. Then This corresponds, I believe, to an action of $G$ on $R$, coming from a morphism ...

**0**

votes

**0**answers

140 views

### For a mathematician that English is not the native language, does he/she think in english or graph or native language? [on hold]

For example, if you are a mathematician with Chinese the native language. During your research you find most of the books or papers are in English, of course when you read them, you probably will ...

**0**

votes

**0**answers

44 views

### Continuous self-information

Let $I(X,Y)$ be the mutual information between two continuous random variables $X$ and $Y$.
We have $I(X,Y) = H(X)-H(X|Y)$, and setting $X=Y$ leads to $I(X,X) = H(X)-H(X|X)$. If $X$ was discrete, ...

**0**

votes

**1**answer

103 views

### Intersection Matrix of a resolution

Probably this is a very easy question. Let $f:X\rightarrow S$ be a resolution of a projective surface such that
$$K_X = f^{*}K_S+\sum_ia_iE_i$$
with $a_i>0$. By Grauert-Mumford theorem the ...

**2**

votes

**1**answer

106 views

### A Krull-Schmidt Theorem for Lie groups?

I wondered whether there is an analogue of the Krull-Schmidt theorem for real Lie-groups. More precisely, what conditions do you have to impose on a connected finite dimensional Lie group $G$ so that ...

**17**

votes

**4**answers

931 views

### Categorical proof subgroups of free groups are free?

This is a crossport of this question from MSE.
Is there a categorical proof that subgroups of free groups are free?
How about the result that subgroups of free abelian groups are free abelian?
...

**-1**

votes

**0**answers

31 views

### Iterative Calculation? [on hold]

Apologies, as I do not know how to phrase this question in the correct terms; however, I will try my best.
I have an equation that looks like this:
D = A - B - C
However, C = ( [A - B] * X )
Is ...

**13**

votes

**2**answers

372 views

### Why is “The Higman Rope Trick” thus named?

I'm studiyng Higman's Embedding Theorem, and a fundamental part of the proof is the following lemma:
If R is a benign normal subgroup of finitely generated group F, then F/R can be embedded in a ...

**2**

votes

**0**answers

75 views

### Besse p134 Riemann tensor in dimension 4

Does someone have a reference for the proof of 4.72 page 134 of Einstein Manifolds? It is said that
$$\check{R}-\vert R\vert^2g/4=S/3 (Ric-S/4) +2\mathring{W}(Ric -S/4) $$
because we are in dimension ...

**-1**

votes

**0**answers

38 views

### Improper integral calculation - limit at infinity [on hold]

Will you please help me prove the following limit is zero ?
$
lim_{x \to \infty} \int_0^{\infty} \frac{1-e^{-u^4}}{u^2} cos(x\cdot u) du
$
Thanks in advance

**7**

votes

**0**answers

97 views

### Alexander polynomial in branched covers

Suppose I am given a homology sphere as a double branched cover over a link (of determinant one). Let a knot in this space be given as a lift of an arc with endpoints on the link. Is there a way to ...

**2**

votes

**0**answers

37 views

### Decidability of first order theory of subclasses of posets

Is the first order theory of finite posets known to be undecidable?
Does anyone know a survey about such results?

**0**

votes

**0**answers

103 views

### p-divisibility of the connected component of the Picard group [on hold]

Let $X$ to be a smooth projective variety over a field of positive characteristic $p>0$, then can one claim $Pic^0(X)$ is p-divisible.

**3**

votes

**3**answers

279 views

### Find an integrable, positive, unbounded, analytic function

Is there a standard example of a function $f \in L^1( \mathbb R)$ which is analytic, positive, integrable but not bounded?
An example which comes immediately to mind is to take the series of narrower ...

**2**

votes

**1**answer

98 views

### Schur covering group for S4

It is well-known that the symmetric group S4 has two Schur covering groups, S4-tilde and S4-hat. There are explicit presentations for both groups, and we know that S4-hat is isomorphic to GL(2,3). ...

**2**

votes

**0**answers

119 views

### Hilbert vs Chow in nice cases

I'm trying to understand the relationship between the Hilbert schemes and Chow varieties in situations where everything is simple. Suppose that $X$ is a smooth projective variety over $\mathbb C$, ...

**0**

votes

**0**answers

29 views

### Question about the stochastic integral of martingales

Let $M=(M_t)_{t\ge 0}$ be a continuous martingale defined on some filtered probability space taking values in $R$. Let $H=(H_t)_{t\ge 0}$ be some bounded progressively measurable process, i.e.
...

**1**

vote

**0**answers

25 views

### Progressive measurability and functional composition

Suppose we have a progressively measurable process $X$ taking values in $\mathbb{R}^d$. What are sufficient conditions on a function $f( x, t, \omega ) \colon \mathbb{R}^d \times [0,\infty) \times ...

**-4**

votes

**0**answers

37 views

### Whats the formula to work out the minimum monthly payment of a loan? [on hold]

I'm a developer, and i'm building a snowball debt calculator.
I want a formula to work out what the minimum monthly repayment would be on a debt with a given interest.
And I really want to get the ...

**3**

votes

**1**answer

155 views

### Is the conditional expectation a contraction in weak $\mathbb L^p$ spaces?

Let $(\Omega,\mathcal F,\mu)$ be a probability space. It is well-known that if $\mathcal A$ is a sub-$\sigma$-algebra of $\mathcal F$, $p\geqslant 1$ and $X$ is an element of $\mathbb L^p$ which takes ...

**0**

votes

**0**answers

29 views

### Integrability - conditions of lax pairs

I'm trying to understand what the conditions are for the Lax pairs for the zero-curvature representation:
$$
\partial_t U - \partial_x V + [U,V]=0
$$
where $U=U(x,t,\lambda)$ and $V=V(x,t,\lambda)$ ...

**0**

votes

**0**answers

28 views

### Upper bound a function

The problem is of finding the maximum of the following function (in terms of i)
$\ f_i = \frac{(2m-i) \cdot i}{2 b} \ln(b) - \tfrac{1}{2} i \ln(i) +O(i) $
providing $\ 0<b \leq m, 0<i \leq m ...

**3**

votes

**1**answer

88 views

### Monte Carlo integration of Gaussian integrals

I was doing a physical problem, and then it comes to this Gaussian integral. The dimension of the integral is very large (dimension = 300~600), and it is difficult to find the maximum of the ...

**1**

vote

**0**answers

96 views

### Help in understanding “Local well-posedness for the Maxwell-Schrodinger system”

Is there someone who knows the following paper
"Local well-posedness for the Maxwell-Schrodinger system" by M.Nakamura and T.Wada.
I'm trying to study it but I've some doubts. In particular I'm not ...

**12**

votes

**1**answer

336 views

### Can ZFC prove it cannot derive an inconsistency in $n$ steps?

Let $Con(\mathtt{ZFC}, n)$ denote the statement "$\mathtt{ZFC}$ cannot prove the contradiction within $n$ steps (or better within $n$ symbols) within a given proof system (say a natural deduction to ...

**9**

votes

**0**answers

238 views

### Who first talked about “holes” in homology?

The question Why do the homology groups capture holes in a space better than the homotopy groups? and many others here use the idea that homology counts the ``holes'' in a space. The comments on this ...

**0**

votes

**1**answer

26 views

### distances-based dispersion measuring approach

Is there any known approach or method to measure the dispersion of a set depending on the distances between its points (i.e.: without calculating the average or the mean) ?
thanks.

**4**

votes

**0**answers

67 views

### Is the space $S'(\mathbb{N})$ of slowly increasing sequences the projective limit of Hilbert sequence spaces?

Let $S(\mathbb{N})$ be the space of rapidly decreasing sequences and $S'(\mathbb{N})$ its topological dual, the space of sequences bounded by a polynomial.
For $m\in \mathbb{Z}$, we also define ...

**0**

votes

**0**answers

75 views

### What is the spectrum of $L^1(G:H)$?

Let $H$ be a compact subgroup of a locally compact topological group $G$ and
$$ L^1(G:H)=\{f\in L^1(G): R_h f=f\;(a.e)\; \forall h \in H\}$$ and $\widehat{(G:H)}=\{\xi\in \hat{G}:\xi|_H=1\}$($\hat{G}$ ...

**1**

vote

**1**answer

85 views

### Isomorphisms of Positive and Negative Spinor Bundles

Here is an extract of the doctoral thesis of C. Lewis under the supervision of D. Joyce (https://people.maths.ox.ac.uk/joyce/theses/LewisDPhil.pdf, 1998):
2.6 Spin Bundles and the Dirac Operator
...

**1**

vote

**2**answers

51 views

### Order-preserving image of a complete lattice

If $L$ is a complete lattice and $P$ is a poset and $f: L\to P$ is an order preserving map, does this imply that $P$ is a (complete) lattice?

**7**

votes

**3**answers

182 views

### Set with small internal radius, small perimeter and prescribed area

Given a regular set $E\subset \mathbb R^2$ define
$$
R(E) = \sup\{r\colon \exists x,\ B(x,r)\subseteq E\}
$$
to be the radius of the largest circle contained in $E$ and let $|\partial E|$ be the ...

**1**

vote

**1**answer

40 views

### positively invariant set respec to fractional system

In my research I need to show that the set
$$M := \{X \in \mathbb{R}^4,X≥0\}$$
where
$$X(t)=(x_1(t),x_2(t),x_3(t),x_4(t))^T$$
is positively invariant with respect the following system of ...

**3**

votes

**0**answers

33 views

### Signatures of latin squares: what about the extremal cases?

For a latin square (LS) of order $n$, we will define a cut (or maybe general transversal, I don't know whether there is an entrenched name for this) as a collection of $n$ cells such that no two share ...

**2**

votes

**1**answer

59 views

### Notion of strongness in cut rule

I've read somewhere that the cut rule in sequent calculus
$$\frac{A \vdash \mathbf{C}, B \qquad A',\mathbf{C} \vdash B'}{A,A' \vdash B,B'} (\text{cut})$$
states that the $\mathbf{C}$ on the right is ...

**2**

votes

**0**answers

86 views

### Number of common solutions of polynomial system

Let $ \mathbb{F}_p$ be a finite field and $\{f_j\}_{j=1}^{j=r} \subseteq \mathbb{F}_p[X_1,...,X_n]$ be a set of polynomials.
Let consider the system of equations:
$f_j(x_1,...,x_n)=0$ for $j = ...

**1**

vote

**0**answers

51 views

### Bound the expectation of trace norm of random Hermitian matrix

Suppose $H_i$ are traceless $d\times d$ Hermitians, $X_i$ are Standard normal distribution for $1\leq i\leq d^2$.
We would like to bound the following expectation on the trace norm
...

**3**

votes

**1**answer

168 views

### line bundle descends?

Let the permutation group $S_4$ act on $\mathbb C^4$ by permuting the coordinates. Consider the categorical quotient $\mathbb P(\mathbb C^4)/S_4$. It is a projective variety by a theorem of Mumford. ...

**2**

votes

**1**answer

58 views

### Order-preserving images of $(\mathcal{P}(\kappa),\subseteq)$

Is there a cardinal $\kappa \neq \emptyset$ and a connected poset $P$ of cardinality $\leq \kappa$ such that there is no surjective order-preserving map from $(\mathcal{P}(\kappa),\subseteq)$ onto ...

**-5**

votes

**0**answers

54 views

### reducible measure functions [on hold]

Let $f:\Bbb R^n \rightarrow \Bbb R^n$ be a nonexpansive map, that is $\|f(x) - f(y)\| \leq \|x-y\|.$
I want to know that if $f$ reduces measure, i.e. if $A,f(A) \subseteq \Bbb R^n $ are measurable, ...

**-3**

votes

**0**answers

73 views

### A Classification problem in measure theory [on hold]

Let $m$ be the Lebesgue measure on $\Bbb R$. I want to find some measurable functions $f:[0,1] \rightarrow [0,1]$ with the following property.
$$\forall A\subseteq [0,1]~ (\text{Lebesgue ...

**1**

vote

**1**answer

33 views

### What's the most efficient way to solve this euclidean projection on non-negative affine space constraint?

I've come across this convex optimization problem in my research where I need to project a matrix $X_0$ onto a non-negative affine space constraint and box constraints. Concretely,
$X \in ...

**0**

votes

**0**answers

64 views

### Gateaux but not Frechet differentiable functional [migrated]

For functional between Banach spaces X,Y:
By Gateaux differentiable at $u\in X$ I mean that there exists bounded linear operator $dF(u)$ s.t. $F(u+t\xi)-F(u)=dF(u)\xi+o(t)$ for all $\xi\in X$.
For ...

**1**

vote

**1**answer

170 views

### Explicit examples of Hasse--Weil zeta-function calculations for curves

The problem of calculating Hasse--Weil zeta-function for a given curve $C/\mathbb{F}_p$ over a finite field is far from being easy, especially for large genus (as discussed by Wouter Castryck at ...

**0**

votes

**0**answers

83 views

### Solve PDE system almost equal to the Ernst equation of general relativity

I am trying to find a solution to the following elliptic quasilinear system of PDEs:$$\Delta G+\nabla G\cdot\nabla H=0$$ $$2\Delta H-\nabla H\cdot\nabla H-e^{2H}(\nabla G\cdot\nabla G)=0$$with the ...

**0**

votes

**1**answer

80 views

### holomorphic continuation

consider the function given by $f(t):=\sum\limits_{n=0}^{\infty}e^{-\left(n+\frac{1}{2}\right)^2t}$ for $t\in (0,\infty)$.
This function can be continued holomorphically for all complex numbers with ...