# All Questions

**-3**

votes

**0**answers

36 views

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

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

302 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

56 views

**6**

votes

**1**answer

338 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

458 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

74 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

562 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

100 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

723 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 [closed]

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

91 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

94 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

47 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

162 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

198 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

38 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

117 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

125 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

193 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

135 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

641 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

529 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 ...

**8**

votes

**2**answers

262 views

### Fano variety of lines on the Segre and the Grassmannian

Does every $\mathbb{P}^{19}\subset \mathbb{P}(\mathbb{C}^5\otimes\mathbb{C}^5)$
intersect the Segre variety of rank one matrices in at least a $\mathbb{P}^1$?
A naive dimension count suggests this is ...

**0**

votes

**2**answers

79 views

### Planar curves identical to their inverses

Is the right strophoid
the only planar curve $C$ whose inverse curve w.r.t. some circle (in this case: centered on the origin)
is identical to $C$?
...

**0**

votes

**1**answer

93 views

### Approximating an integral [closed]

I came across the following integral in my work
$$\int_{-\infty}^{\infty} \frac{\frac{1}{(1- \ \ 2 \pi j s \theta)^{m}}-1}{2\pi j s }\ e^{-2\pi j s\sigma^2}\ ds $$
Assuming $\theta,m,\sigma^2$ are ...

**7**

votes

**2**answers

300 views

### Numbers with all N-digit prefixes divisible by N

In base 10, the number 3816547290 contains every digit exactly once. When I take the first N digits, that substring is divisible by N. For example, 381 is divisible by 3, 38165 is divisible by 5, etc. ...

**0**

votes

**1**answer

110 views

### Short arithmetic progressions in quadratic residues

Let p be a prime number of form 4k+1.
I guess that there are c(d) number of 3-terms arithmetic progressions (AP) in the set of quadratic residues modulo p, where c(d) is an integer constant depending ...

**-3**

votes

**0**answers

50 views

### Chebyshev polynomials and uniformly convergence [on hold]

Let $P_n(x)=\csc(n\alpha)T_{n-1}(x)-\cot(n\alpha)x$ be a sequence of polynomials. Where $T_n(x)$ is the $n-th$ Chebyshev polynomial of the first kind, and $\alpha\in[0,2\pi]$. Is there uniformly ...

**5**

votes

**1**answer

167 views

### Purely noncommutative algebra-Morita equivalence

Morita equivalence of algebras certainly don't preserve commutativity: even if $A$ is commutative there are plenty of noncommutative algebras which are Morita equivalent with $A$---for example all ...

**0**

votes

**0**answers

30 views

### Perturbation method of a boundary value problem

Let $u(x, \epsilon, \theta)$ be the solution of $$u''+(\epsilon \cos(x)+\theta-u)u=0$$ with boundary conditions $u'(0)=0$ and $u'(\pi)=0$. Here $\theta\in [0, 1]$.
I tried to put the solution in ...

**1**

vote

**1**answer

65 views

### Smooth unit vector field on a tetrahedron to interpolate vertex constraints

For a tetrahedron $T\subset \mathbb{R}^3$ with vertices $r_i\in \mathbb{R}^3$ , $i=1,\ldots,4$, and unit vectors $u_i\in \mathbb{S}^2$ at each vertex $i=1,\ldots,4$
consider the (energy) functional
...

**1**

vote

**0**answers

59 views

### Better version of “Monotonicity methods in Hilbert spaces and some applications to nonlinear PDEs..”

I am asking whether any one knows of a better source for the text
Monotonicity methods in Hilbert spaces and some applications to nonlinear partial differential
equations by H. Brezis
which I ...

**10**

votes

**1**answer

158 views

### Free subgroups in algebras of polynomial growth

What is known about free non-abelian subgroups in finitely generated associative algebras of polynomial growth (e.g., over finite fields, to avoid finite-dimensional free subgroups)? For example, are ...

**4**

votes

**1**answer

226 views

### Geometrically connected components of an algebraic group

Suppose that $G$ is an algebraic group over a field $k$. Let $G^o$be the connected component of the identity. Since $G^0$ contains a $k$-rational point (the identity) therefore it is geometrically ...

**2**

votes

**1**answer

67 views

### Measuring the Randomness and Statistics of Convex Polygons

How can I tell, how likely it is, that a given convex polygon with a sufficiently high number of edges is random and, if so, what kind of randomness it is (e.g. white noise)?
What is known about ...

**11**

votes

**2**answers

1k views

### Existential statement without witness

Are there existential theorems of ZFC, or PA say, with no witnesses?
Ie does there exist a formula $\phi$ such that ZFC $\vdash\exists x \phi(x)$, but for all numerals $\underline{n}$, ZFC $\nvdash ...

**1**

vote

**1**answer

102 views

### what are the possible approximations for ideals

(Fix some local ring $(R,\mathfrak{m})$ over a field of zero characteristic.)
Suppose an ideal $J$ is defined by some complicated formula/procedure. And there is no hope of computing it/or writing ...

**4**

votes

**1**answer

238 views

### Endomorphisms and almost all graphs

Is it known what fraction (almost all?) of graphs have a trivial endomorphism monoid? I can't seem to find any reference to the question. Maybe it's related to the question: what fraction of graphs ...

**-1**

votes

**0**answers

51 views

### Length of Paths in Graph [closed]

There is a very large directed graph with n number of vertices, in order of millions. We are given a number p much smaller than n, and two vertices v1 and v2. What is the efficient way of finding a ...