# All Questions

**1**

vote

**1**answer

157 views

### Vector fields whose divergence are proper maps

Let $X$ be a polynomial vector field of degree $2$ on $\mathbb{R}^{2}$. Does there exist a nonvanishing smooth function $g$ such that $Div(gX)$ is a proper map?Or at least the zero locus of ...

**7**

votes

**2**answers

305 views

### Least supersingular prime

Given an elliptic curve over the rationals, what can one say about the size of the smallest supersingular prime?

**1**

vote

**1**answer

79 views

### A subgroup of outer automorphisms group of a free product

I would like to ask a question about automorphisms of free products of groups.
More specifically, let $G = G_1 \ast ... \ G_n \ast F_r$ where $F_r$ is free group on r generators. We can define the ...

**0**

votes

**0**answers

49 views

### Schoenberg correspondence on $L^p$

Schoenberg correspondence states that $\psi: \mathbb R^d\longrightarrow \mathbb C$ is conditionally positive definite and hermitian if and only if $e^{t\psi}$ is positive definite for each $t>0$. ...

**-6**

votes

**0**answers

30 views

### convergence and divergence using a root test [on hold]

why does
infinity
sigma n^7 / 7^n
n=1
converge using a root test?
I'm a bit confused on this series..

**1**

vote

**0**answers

32 views

### Moser's iteration for non homogeneous quasilinear elliptic PDE

I want to know some reference of Moser's iteration for non homogeneous quasilinear uniformly elliptic PDE, in particular, $u_{ij}(\delta_{ij}+u_iu_j|\nabla u|^2)=0$. I want to know how to deal with ...

**2**

votes

**1**answer

82 views

### System of linear first order PDE with constant coefficients

recently in my researches I've come across the following operator
$$L\left(\begin{array}{c}
a_1\\
\vdots\\
a_n
\end{array}\right)=M_1\left(\begin{array}{c}
...

**1**

vote

**0**answers

124 views

### On Prüfer domains

Is there any Prüfer domain $R$ that has a prime ideal $P$ that is not finitely generated but $xP$ is subset of a finitely generated ideal $I$,for some $x$ in $R-P$ and $I$⊂$P$?

**-9**

votes

**0**answers

122 views

### How can I calculate the significance of an acquisition test campaign on Facebook by using chi-squared test? [on hold]

Should I use number of installs and spending or should I use impression and install numbers in Chi-Squared Test (Evan's Awesome A/B Tools)? Here is the link of the ...

**2**

votes

**0**answers

100 views

### Surjectivity of multiplication maps with respect to pullback

Given a morphism $f: Y \to X$ and a globally generated line bundle $\mathcal L$ on $X$ such that $H^0(X,\mathcal L) \otimes H^0(X,\mathcal L) \to H^0(X,\mathcal L \otimes \mathcal L)$ is surjective.
...

**-3**

votes

**0**answers

26 views

### Should simulation from a student-t copula distribution yield the input correlation matrix [on hold]

I am using mathematica to simulate random variates from a student-t copula distribution. Assuming that I input in the correlation matrix R, after generating a certain number of random variates, should ...

**11**

votes

**2**answers

282 views

### Entire function bounded at every line

I would like to ask about, does there exists an entire function which is bounded on every line parallel to $x$ - axis , but unbounded on the $x$ - axis.

**-4**

votes

**0**answers

157 views

### How to calculate math expectation [on hold]

How to calculate math expectation of maximum of difference between two lognormal random variable:
E[max(X-Y,0)] =?
How to proceed?

**-4**

votes

**0**answers

51 views

### Derangement,recursion and circular permutation [on hold]

N people are invited to a dinner party and they are sitting on a round table.
Each person is sitting on a chair there are exactly N chairs.
So each person has exactly two neighboring chairs, one on ...

**2**

votes

**0**answers

91 views

### Strange invocation of Shapiro's lemma

I'm having trouble understanding a claim in a paper I'm reading. To avoid having to explain a lot of notation, I'll abstract the claim a bit. Assume that $G$ is a group with a subgroup $H$. Also, ...

**6**

votes

**2**answers

318 views

### A proposition on cyclic group

$G$ is a cyclic group iff
$$ \forall H < G, \ \exists k, \ H = \{a^k : a \in G\}. $$
Is it right?

**0**

votes

**0**answers

30 views

### Canonical forms of symmetric/skewsymmetric quaternionic matrix

$A$ belongs to $n$-dimensional quaternion symmetric matrix, in the sense that $A=A^T$, where $T$ means transpose. Under transformation $U$, $A\rightarrow U\cdot A\cdot U^T$, where $U$ is $n$-dim ...

**-3**

votes

**0**answers

35 views

### Find examples non compact surface satisfy properties every point is hyperbolic for Gaussian curvature [on hold]

Find examples non compact surface satisfy the following properties for Gaussian curvature:
(a) every point is hyperbolic.
(b) every point is elliptic.
(c) every point is parabolic.
(d) that the point ...

**2**

votes

**2**answers

257 views

### Polynomials of low degree that clone polynomials of higher degree

Let $f(x_1,\dots,x_{16})=(x_1+x_2+x_3+x_4)(x_5+x_6+x_7+x_8)(x_9+x_{10}+x_{11}+x_{12})(x_{13}+x_{14}+x_{15}+x_{16})\in\Bbb R[x]$.
Let $\mathcal{Z}$ be the zero set of $f$ in ...

**-6**

votes

**0**answers

54 views

**6**

votes

**1**answer

322 views

+50

### What is the universal property of quotienting a normaliser of the subgroup?

Let $G$ be a group, $H$ a subgroup and $X$ a $G$-set. By taking orbits $X/H = X \times_H 1$ or fixed points $X^H = \mathrm{Hom}_H(1,X)$ we obtain a set on which $H$ acts trivially, and we've destroyed ...

**0**

votes

**1**answer

166 views

### Coaction of a group

Suppose $G$ is a finite group which acts on a $C^*-$algebra which we denote by $A$. I was wondering if there is a naturally induced coaction on $A\otimes C(G)$, here $C(G)$ denotes functions on $G$.
I ...

**9**

votes

**1**answer

449 views

### Is forcing computable?

By results similar to Tennenbaum's theorem we know that there exist no computable models of $ZF$. But suppose we are given, as a sort of oracle, access to some model of $ZF$ (e.g. we can make oracle ...

**3**

votes

**0**answers

71 views

### Local time of Brownian motion + Lipschitz continuous function

Let $\mathrm{ Lip} (M)$ denote the space of all functions on $[0,T]$ with Lipschitz constant and $L^\infty$ norm bounded by $M$. Let $(B_t)_t$ be a Brownian motion defined on the probability space ...

**-1**

votes

**1**answer

43 views

### Integrating factors and integrability of an ODE system

The following argument is from a paper about the Bendixson-Dulac Theorem.
Consider a smooth differential equation on the plane
$$
x'=g(x,y),\quad y'=h(x,y).
$$
Suppose there exists a function ...

**1**

vote

**0**answers

95 views

### Ext and cup products and subvarieties

I am trying to understand Remark 11.3 in Huybrechts's amazing book on derived categories (FM transforms in AG).
He starts with smooth projective varieties $j\colon Y \subset X$ and aims to describe ...

**11**

votes

**4**answers

553 views

### Zeros of the derivative of Riemann's $\xi$-function

The Riemann xi function $\xi(s)$ is defined as
$$
\xi(s)=\frac12 s(s-1)\pi^{-s/2}\Gamma(s/2)\zeta(s).
$$
It is an entire function whose zeros are precisely those of $\zeta(s)$.
Since $\xi$ is real ...

**1**

vote

**0**answers

98 views

### Criterion for normality of a schematic image

Consider a projective flat morphism
$$
f\colon X\to Y
$$
between normal varieties. Let's say over the complex numbers. The geometric fibers of $f$ are all irreducible.
I would like a criterion to ...

**5**

votes

**1**answer

712 views

### textbooks on modern algebraic geometry for 21st-century starters

As for learners in algebraic geometry in 21st century, is there a textbook, lecture note or anything like that to introduce algebraic geometry utilizing the language of derived categories and stacks?
...

**0**

votes

**0**answers

20 views

### Combining Pearson correlations [on hold]

I have variables a, b, c, x
I know sample values A, B, C, but not X
I know Pearson correlations pairwise: ax, bx, cx (and ab, ac, bc too if it helps)
Now what is the most likely value for X?
...

**-3**

votes

**0**answers

34 views

### Odd-cycle inequality [closed]

Consider the stable set problem. An odd hole is a cycle with an odd number if nodes and no edges between nonadjacent nodes of the cycle. Show that if H is the node set of an odd hole, the following ...

**1**

vote

**1**answer

90 views

### Rademacher type of a Banach space is always less than or equal to 2

Before I ask my question I will provide a brief introduction.
I came across the notion of Rademacher type while reading Assaf Naor's article An introduction to the Ribe program, which can be found ...

**2**

votes

**1**answer

90 views

### A surface on which all regular curves have nowhere vanishing curvature

Let $S$ be a surface in $\mathbb{R}^{3}$ such that every regular curve $\gamma\subset S$ has nowhere vanishing curvature, that is $\kappa(z)\neq 0$ for all $z\in \gamma$. Does this imply that ...

**1**

vote

**1**answer

46 views

### Groups arising as direct limits of a stationary system of primitive matrices over the integers

I am interested in the kinds of groups of the form $\displaystyle\lim_{\longrightarrow}(\mathbf{Z}^k,M)$ where $M$ is a primitive (some power of $M$ has strictly positive components) $k\times k$ ...

**-8**

votes

**0**answers

155 views

### Maths to take a user chosen number to a predictable number [closed]

As part of simple card trick, I want to allow a user to choose a number between 1 and 100 and then ask them to do various maths to lead them to the same number so their choice becomes irrelevant.
One ...

**4**

votes

**0**answers

82 views

### Contractibility of a poset-indexed colimit

Let $(X,\leq)$ be a poset with distinguished element $p$, and let $P'$ be the poset of "finite chains which weakly descend to $p$" given by all $\sigma = (x_0 \geq x_1 \geq \cdots \geq x_k \geq p)$ ...

**-6**

votes

**0**answers

30 views

### Boolean algebra 1´=0 ; 0´=1 ; x+1=1 [closed]

Hi I have a problem to solve, in Boolean algebra. I have to prove that
1´=0 ; 0´=1 ; x+1=1
I solve the first problem
x*0=0 -> x*0=x*0+0=x*0+x* x´=x*(0+x´)=x*x´=0
previous 3 problems ...

**5**

votes

**1**answer

193 views

### Is there a left orderable profinite group?

Is there a profinite group $G$ with a binary transitive relation $<$ such that for any different $x,y \in G$ either $x < y$ or $y < x$ and such that for any $x,y,z \in G$ we have that $x < ...

**-1**

votes

**0**answers

37 views

### Metrics mappings which are metrics [closed]

A function f: Z x Z => R is a metric iff
forall a,b in Z. f(a,b) >= 0.
forall a,b in Z. f(a,b) = f(b,a).
forall a,b in Z. f(a,b) = 0 iff a = b.
forall a,b,c in Z. f(a,b) + f(b,c) >= f(a,c).
Given ...

**2**

votes

**1**answer

61 views

### Universal and left-factoring order-preserving maps

Trying to get a different angle for the question Fixed points and universal maps for posets, I want to compare universal maps to a different kind of functions.
First recall that for posets $P,Q$ an ...

**4**

votes

**0**answers

115 views

### The cohomology of an $S_{3}$ cover of an elliptic curve ramified in one point

Let $E/\mathbb{C}$ be an elliptic curve. Let $C \to E$ be a Galois cover with group $G = S_{3}$ (symmetric group on $3$ elements), ramified in one point. (To clarify: there is a unique point in $E$ ...

**7**

votes

**1**answer

124 views

### Intersection Cohomology and $L^2$ cohomology

In the study of singular spaces, topological methods like intersection cohomology have played an important role. They have led to the development of technology like perverse sheaves and these find ...

**2**

votes

**1**answer

219 views

### A perfect domain that is not integrally closed?

Does there exist an integral domain $R$ of characteristic $p > 0$ that is perfect (i.e., $x \mapsto x^p$ is bijective on $R$) but not integrally closed in its field of fractions?

**2**

votes

**0**answers

56 views

### Topological/numerical constraints for the existence of more than one pencil

A famous theorem of Castelnuovo and de Franchis tells us that for $S$ a smooth projective complex algebraic surface that for $b \geq 2$, pencils $f : S \to B$ of genus $b :=g(B)$ are in bijective ...

**5**

votes

**1**answer

188 views

### Existence of internal toposes/inner models in a topos

It has been known for some time that one can define a topos as a model of a (finitary) essentially algebraic theory (or in other words, can be defined internal to any category with finite limits). In ...

**2**

votes

**1**answer

132 views

### A Category-ish Structure with Morphism Domains containing Multiple Objects?

I am working on formalizing software design using category theory.
However the most natural way for me to express what I want is with a Category where multiple morphisms can join into a single ...

**27**

votes

**1**answer

634 views

### Producing finite objects by forcing!

It is a trivial fact that forcing can not produce finite sets of ground model objects. However there are situations,
where we can use forcing to prove the existence of finite objects with some ...

**-3**

votes

**0**answers

76 views

### What is a discrete shape [closed]

I saw this term in a paper on tiling using shapes.
Can you give the definition for discrete shape?
I googled on the web, and could not find any explanation for this concept...

**0**

votes

**0**answers

27 views

### What is the relation between linear subgraph and matching polynomial? [closed]

I am confused about these following three concepts,
An edge-cycle subgraph of a graph $G$ (also called a linear subgraph of $G$) is a subgraph of $G$ whose components are cycles and edges.
A set of ...

**26**

votes

**2**answers

525 views

### Why do sporadic simple groups have so few conjugacy classes?

In finite group theory, there's a general intuition that the further away a group is from abelian, the fewer conjugacy classes it will have. So it is to be expected that non-abelian finite simple ...