All Questions
12,823 questions
12
votes
5
answers
2k
views
Examples of badly behaved derivatives
Consider a real valued function $g$ on an open interval $(a,b)$ which is the derivative of a function continuous on $[a,b]$ at each point of $(a,b)$. The function $g$ has the intermediate value ...
- 549
8
votes
2
answers
2k
views
The dual group of $\mathbb Q$
What is the dual group of the additive group of rational numbers equipped with the standard topology inherited from $\mathbb R$? As a group, this dual group is isomorphic to $\mathbb R$ (see the ...
- 162
12
votes
3
answers
1k
views
Distribution of fractional parts of n^{3/2}
What can be said about the limiting distribution of the sequence of fractional parts of $\{n^{a},n>0\}$ for $a\in(1,2)$. I ran a computer experiment for $n\sqrt{n}$ and it looks like uniformly ...
- 1,414
7
votes
4
answers
827
views
$n$th root of $(a,b) \mapsto (\operatorname{gm}, \operatorname{am})$
Suppose $0 < a < b$, and let GM and AM be respectively the geometric and arithmetic means of $a$ and $b$. Does the mapping $(a,b) \mapsto (\operatorname{GM}, \operatorname{AM})$ have a well-...
4
votes
2
answers
707
views
Selecting basic sequences
Suppose $(x_\alpha)_\alpha$ is an uncountable, linearly independent family of norm one vectors in a Banach space. Can one always select a basic sequence (or at least a minimal system) from this family?...
- 355
5
votes
1
answer
666
views
Question regarding divergence
Let $E$ be a closed and convex set of distributions on a finite set $A$. Let $P',Q'\notin E$ and let $P^{\star},Q^{\star}$ be their respective estimates in $E$ with respect to the KL-divergence, i.e.,...
- 779
1
vote
1
answer
635
views
Closed range for a continuous linear transformation
I have a Banach space $B$ and a continuous linear transformation $F:B \rightarrow B\times B$. One of the induced transformations $F(1):B \rightarrow B$ and $F(2):B \rightarrow B$ into the factors of ...
- 549
4
votes
2
answers
2k
views
Arctangents and the golden ratio
Why is the golden ratio lurking in $(d/dx)\arctan\left( x + \frac{1}{x} \right)$
$$
= \frac{\left(\frac{1+\sqrt{5}}{2}\right)}{x^2 + \left(\frac{1+\sqrt{5}}{2}\right)^2} + \frac{\left(\frac{1-\sqrt{5}}...
6
votes
1
answer
369
views
Denominators in the solution to Hilbert's XVII
Hilbert's seventeenth problem asks to prove that every positive semidefinite form can be written as the sum of squares of rational functions. Currently we don't seem to have a good understanding of ...
- 85.6k
3
votes
2
answers
391
views
Distribution of zeros
Suppose $f(z) = P(z)e^{Q(z)}$ where $P,Q$ are real polynomials. What is the number of non-real zeros of $f^{(k)}$ as $k$ increases?
We know that $f''$ has $\geq m$ zeros where $m$ depends on $Q(z)$.
- 61
2
votes
3
answers
5k
views
Justification for the matching condition for the wave function at potential jumps. Why is it both restrictive enough and sufficiently general?
Consider Schrödinger's time-independent equation
$$
-\frac{\hbar^2}{2m}\nabla^2\psi+V\psi=E\psi.
$$
In typical examples, the potential $V(x)$ has discontinuities, called potential jumps.
Outside ...
- 3,184
9
votes
1
answer
1k
views
Partial sums of the Chu--Vandermonde identity
I am interested in finding a lower bound of the sum:
$$\sum_{i=0}^d \left(\genfrac{}{}{0pt}{}{n}{i}\right)
\left(\genfrac{}{}{0pt}{}{m}{k-i}\right)$$
when $d < k$ (and assuming both $n\geq k$, $m\...
- 91
7
votes
1
answer
737
views
Question about projections on a Hilbert space
Sorry for the vague title, I can't think of a better one that isn't overly long.
Suppose that $S$ is a commuting set of projection operators on a Hilbert space. I'll introduce the following notation: ...
- 391
7
votes
2
answers
790
views
Question about von Neumann algebra generated by a complete algebra of projections
Hi all, sorry if this is a dumb question, I don't know much about von Neumann algebras except the definition and a few relevant facts I've managed to prove by myself so I expect the answer will turn ...
- 391
0
votes
2
answers
503
views
A Jordan arc in the unit disk
Let $D$ be the open unit disk, and $J$ a Jordan arc (that is, a homeomorphic copy of $[0, 1]$) that lies in $D$, except $J(0)$ lies on the boundary of $D$, say $J(0)=1$. I would like to see that $D\...
- 95
4
votes
2
answers
1k
views
Product over the primes
I'm trying to estimate the product
$$\prod_{p\lt q\lt r\lt s}1-\frac{24}{(pqrs)^2}$$
where $p,q,r,s$ are primes.
This is for the purpose of calculating the density of Sloane's A070284 [1]. The idea ...
- 9,114
1
vote
1
answer
717
views
Double dual space of a C* algebra A
We know that $A$ embeds into $A$** (the double dual space of $A$ ). Is the following true? If $\Psi$ is in $A$** and weak* continuous, is there an element $a \in A$ such that $ \Psi$ is the ...
- 831
8
votes
3
answers
2k
views
Multiplicative integral of $\Gamma(x)$
A recent question on the notion and notation of multiplicative integrals
( What is the standard notation for a multiplicative integral? ) induced me to play with the Riemann products of the Gamma ...
- 60.5k
3
votes
2
answers
360
views
Convergence of a series of orthonormal gaussian variables
Does anyone have an idea how to prove the following? It is a step in the proof of some theorem in a book about gaussian processes.
Let $f_n$ be an orthonormal sequence of gaussian variables. Consider ...
- 170
8
votes
1
answer
920
views
Looking for references talking about category of topological vector spaces
It's known that category of topological vector spaces is not abelian but quasi-abelian or exact category. I am looking for the references playing with this category(category theory). All the related ...
- 5,515
5
votes
2
answers
579
views
Improved versions of discontinuous functions
Given a set X (such as the set of points in an interval), the space ℝX of all real-valued functions on X is not usually the function space we work with -- it is "too large" in some sense. Thus, ...
- 8,903
7
votes
2
answers
521
views
How large (small) can be the measure of a set where a polynomial takes small values ?
A $n$-th degree polynomial has precisely $n$ roots. So it is natural to ask the question
how large ( and small) can be the measure of a set where a polynomial takes small values ?
This, and other ...
- 1,795
21
votes
0
answers
876
views
Are the eigenvalues of the Laplacian of a generic Kähler metric simple?
It is a theorem of Uhlenbeck that for a generic Riemannian metric, the Laplacian acting on functions has simple eigenvalues, i.e., all the eigenspaces are 1-dimensional. (Here "generic" means the set ...
- 6,247
5
votes
3
answers
5k
views
Spherical Harmonics - a bunch of questions about them
Hi there,
Please tell me if I should divide these into individual questions next time.
Short intro:
Spherical Harmonics are a nice collection of functions. They are orthogonal and allow you to take ...
- 151
7
votes
3
answers
1k
views
If *Y* is weakly dense in *X*, is the unit ball in *Y* necessarily dense in the unit ball in *X*?
Let X be a normed space and denote by X* the space of all bounded linear functionals on X. Take a linear subspace G ≤ X* which separates the elements of X, i.e., for each x ∈ X, there is an f &...
- 73
3
votes
1
answer
1k
views
Cyl(E) = Borel(E) for E non-reflexive Grothendieck Banach space
This is sort of a follow-up to Borel(X) = \sigma(X') for X non-separable
PROBLEM: Given a Banach space $E$ over $\mathbb{K} \in \{\mathbb{C}, \mathbb{R}\}$ that has the Grothendieck property. ...
- 1,338
40
votes
1
answer
2k
views
Curious $q$-analogues
Consider the Fibonacci polynomials
$$F_n (x) = \sum_{j = 0}^{\left\lfloor {n/2} \right\rfloor }\binom{n-j}{j} x^{n - 2j} $$
and the Lucas polynomials
$$L_n (x) = \sum_{j = 0}^{\left\lfloor {n/2} \...
- 5,622
1
vote
1
answer
443
views
The comparison between the square of the functional value and the sum of squares of the $L^2$ norms of function and its Laplacian
I was reading a paper where I came across the following argument :
For any $x$ in $M$ and for a geodesic ball $B(x; \varepsilon)$ in a compact Riemannian
manifold $M$ with injectivity radius bigger ...
- 1,471
4
votes
1
answer
1k
views
Limit of a discrete time dynamical system
I have the following discrete time dynamical system
$$ y(t+1) = y(t) + \frac{1}{1+\exp(z+ u f y(t))} ,\quad y(0)=0,$$
where $z$ is a real number $f$ and $u$ are non-negative reals. I know I have ...
- 53
39
votes
3
answers
7k
views
What is the standard notation for a multiplicative integral?
If $f: [a,b] \to V$ is a (nice) function taking values in a vector space, one can define the definite integral $\int_a^b f(t)\ dt \in V$ as the limit of Riemann sums $\sum_{i=1}^n f(t_i^*) dt_i$, or ...
- 114k
17
votes
4
answers
3k
views
Is Conway's base-13 function measurable?
Robin Chapman introduced me to Conway's Base 13 Function. Now, my real analysis is a tiny bit rusty, so maybe my question has a really simple and quick answer, but here it goes:
Consider the support ...
- 39k
4
votes
3
answers
644
views
disjoint translates of a dense uncountable set
If {c(n)} is an arbitrary sequence of irrational numbers converging to 0 then Q + c(n), the set obtained by adding c(n) to the set of rational numbers Q, is clearly disjoint from Q for each n.
Is ...
- 549
2
votes
1
answer
2k
views
Cesaro convergence implies weak convergence of a subsequence
Suppose a bounded sequence $(x_n)$ converges to $x$ in the Cesaro sense (i.e., $\frac{1}{n}(x_1 + x_2 + \dots + x_n)\rightarrow x$) in a separable Hilbert space $H$. How to prove that some subsequence ...
- 5,019
0
votes
1
answer
1k
views
series Sum[(-1)^n/(x+n)]
I need the following sum ( in the sense of principal value):
$$\sum_{s=-\infty}^{\infty}\frac{(-1)^{s}e^{-2\pi isy}}{x+s}$$
It is possible to show that
$$\sum_{s=-\infty}^{\infty}\frac{e^{-2\pi isy}}{...
- 267
1
vote
2
answers
3k
views
Weak-* compactness in L^1
Hey I'm really stuck on what I think is an interesting 'paradox'. Consider the sequence of functions $f_n = 1_{[n,n+1]}$ (indicator functions of the interval $[n,n+1]$.
These are uniformly bounded ...
- 19
3
votes
1
answer
615
views
When is a fixed point of f^n a fixed point of f?
Let $E$ be a Banach space and $f:E\to E$ be a continuous map. By $f^n$ we denote the $n$-th iterate of $f$, i.e. $f^n:=\underbrace{f\circ f\circ\cdots \circ f}_{\text{n times}}$. Let $x_0$ denote a ...
- 41
21
votes
3
answers
6k
views
Why pi-systems and Dynkin/lambda systems? On the relative merits of approaches in measure theory.
What is the point of $\pi$-systems and
$\mathcal{D}$ / Dynkin /
$\lambda$-systems?
I am an analyst in the process of consolidating my measure theory knowledge before moving on to harder/newer ...
- 1,771
25
votes
9
answers
6k
views
Function with range equal to whole reals on every open set
There is an example of a function that is unbounded on every open set. Just take $f(n/m) = m$ for coprime $n$ and $m$ and $f(irrational) = 0$.
I want to generalize this in a way to get a function ...
- 2,821
13
votes
3
answers
2k
views
Set of real numbers with positive measure containing no midpoints
Does there exists a subset E of R with positive measure and without containing any midpoints (i.e. x,y distinct in E, (x+y)/2 not in E)?
- 133
12
votes
2
answers
1k
views
Positivity of sequences via generating series
There are different ways of showing that a given sequence $a_0,a_1,a_2,\dots$
of integers, say, is nonnegative. For example, one can show that $a_n$ count
something, or express $a_n$ as a (multiple) ...
- 13.5k
12
votes
1
answer
5k
views
Points of continuity of Baire class one functions
This is an idle question motivated by two comments I made to a previous MO question (which I just searched for, unsuccessfully). That question asked if the characteristic function of the rationals is ...
- 65.4k
4
votes
2
answers
1k
views
Factoring and solving trinomials
Has the problem of factoring (over the rationals) the general trinomial $ax^n+bx^k+c$ with $a,b,c\in\mathbb{Z}$, $n,k\in\mathbb{N}, n>k>1$ been solved? By solved I mean a classification theorem ...
- 11.8k
14
votes
3
answers
1k
views
Finite dimensional Feynman integrals
In a sense this is a follow up question to The mathematical theory of Feynman integrals although by all rights it should precede that question.
Let $S$ be a polynomial with real coefficients in $n$ ...
- 23.5k
14
votes
5
answers
13k
views
The Fundamental Theorem of Calculus in Lebesgue Theory
I am interested to what extent the famous identity
$$
\int_a^b f'(x) \ dx=f(b)-f(a)
$$
is true for a function $f:[a,b]\to \mathbb C$ continuous on $[a,b]$ and differentiable on $(a,b)$. One famous ...
- 359
11
votes
1
answer
603
views
Reference for a particular Radon transform on non-positively curved spaces
Let me first recall that the classical Radon transform takes a (smooth compactly supported, say) function $f$ defined on $\mathbb{R}^n$ as an input, and gives as output the map $H\mapsto \int_H f$ for ...
- 14.4k
3
votes
2
answers
594
views
A question about the Kakeya problem
Besicovich proved a long time ago that a straight line segment of fixed length could be rotated 360
degrees within a subset S of the Euclidean plane such that $M(S)$ is arbitrarily small-where M is ...
- 6,215
14
votes
1
answer
1k
views
Any further applications of Freudenthal's 1936 Spectral Theorem?
Seemingly completely forgotten, back in 1936, the Dutch mathematician Freudenthal, quite well known at the time, proved his so called Spectral Theorem, see chapter 6 in Luxemburg & Zaanen : Riesz ...
5
votes
2
answers
1k
views
Applications of minmax theorem(s)
Intro We suppose $X$ and $Y$ are nonempty sets and f: $X\times Y \rightarrow \mathbb{R}$. A minimax theorem is a theorem that asserts that, under certain conditions,
$$ \inf_Y \sup_X f = \sup_X \...
6
votes
1
answer
444
views
When does a matrix define a convolution operator on a hypergroup?
Let $H$ be a discrete hypergroup. Suppose I have a matrix $A=(A_{x,y})$ indexed over $H$ with nonnegative entries which defines a bounded operator on $\ell^2(H)$. When does there exist $f\in\ell^1(H)$ ...
- 5,425
39
votes
3
answers
6k
views
On linear independence of exponentials
Problem.
Let $\{\lambda_n\}_{n\in\mathbb N}$ be a sequence of complex numbers . Let's call a family of exponential functions $\{\exp (\lambda_n s)\}_{n\in\mathbb N}$ $F$-independent (where $F$ is ...
- 22.3k