Tagged Questions

13
votes
13answers
451 views

objects which can’t be defined without making choices but which end up independent of the choice

It happens a lot of times that when one defines a new object (ring, module, space, group, algebra, morphism, whatever) out of given data one first chooses some additional structure …
0
votes
1answer
17 views

solve non-convex quadratic constrained quadratic programming

$\min_{\beta}\beta^{T} A \beta$ $s.t. \ \beta^{T} C \beta=1\ and\ \beta\geqslant 0$ Here $A,C\in \mathbb{R}^{M\times M}$, $\beta \in \mathbb{R}^{M}$ I saw in one paper saying t …
10
votes
3answers
2k views

When is $L^2(X)$ separable?

I have never studied any measure theory, so apologise in advance, if my question is easy: Let $X$ be a measure space. How can I decide whether $L^2(X)$ is separable? In reality, …
1
vote
1answer
196 views

When does a $W^*$-algebra have a standard Borel spectrum?

EDIT: André Henriques has commented below that the correct separability condition is not weak-* separability as I have written below, but separability of the predual. This post c …
0
votes
0answers
43 views

What’s the missing number of this antiprimes sequence?

Composite numbers $n$ such that $A179382((n+1)/2)=(n-1)/(2^c)$ for some $c > 0$. I named this numbers "antiprimes". $a(1-5):92673, 143713, 3579553, 4110529, 28688897$ $a(6) > 68 …
0
votes
0answers
33 views

Recognize this matrix norm?

I stumbled on the following simple matrix norm, which I haven't seen elsewhere. I wonder if it is well known, has a name, and has been studied elsewhere. The definition of this nor …
18
votes
2answers
740 views

Hahn’s Embedding Theorem and the oldest open question in set theory

Hans Hahn is often credited with creating the modern theory of ordered algebraic systems with the publication of his paper Über die nichtarchimedischen Grössensysteme (Sitzungsber …
0
votes
2answers
67 views

Can group solvability be dected by identities among the generators

For "$n=1$" the answer is "yes." A group is abelian iff its generators commute. Let $G_0=G$ be a group and let it be generated by $X_0=X$. For each $n>0$ let $G_n=[G_{n-1},G_{n- …
1
vote
1answer
89 views

analysis question related to $L^p$ type inequalities

Dear mathoverflowers. Just wondering if the following inequality is true. For all $ p >1$ there is some $C$ such that $ | |x+1|^p-|y+1|^p -p(x-y)| \le C ( |x|+|y| + |x|^{p-1} …
0
votes
0answers
4 views

Solving systems of integral equations using Volterra series

I came across this problem when trying to solve the following integral equations arising in direct scattering: $$ \begin{align} n_{11}(x,z)=1+\int_{-\infty}^xe^{-izy}u(y)n_{21}(y,z …
4
votes
3answers
74 views

How many Perfect Matchings in a regular bipartite Graph

Hi Guys, We have a d-regular bipartite Graph $G = (X,Y,E)$ with $|X| = |Y| = n$ and $|E| = nd$. i want to know a Upper Bound of the number of Matching Thankx
15
votes
0answers
428 views
+150

Orders in number fields

Let $K$ be a degree $n$ extension of ${\mathbb Q}$ with ring of integers $R$. An order in $K$ is a subring with identity of $R$ which is a ${\mathbb Z}$-module of rank $n$. Quest …
3
votes
4answers
154 views

Surfaces ruled over elliptic curves

Ground field $\Bbb{C}$. Algebraic category. Elliptic surfaces are those surfaces endowed with a morphism onto some smooth curve, with generic fiber an elliptic curve. Suppose $E$ …
1
vote
0answers
26 views

What is the meaning of the cospecialization map?

This question comes from the same place as my other one. In reading SGA 4 1/2, but not SGA4 itself (at least, not the obvious sections xv + xvi), one can learn about the "cospecia …
1
vote
0answers
46 views

Is a Lie group equivariantly formal under conjugation by a maximal torus?

Given an action of a group $G$ on a topological space $X$, the associated homotopy quotient is $$X_G := (EG \times X)/G,$$ where $EG$ is the total space of a universal principal $G …

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