Questions tagged [ca.classical-analysis-and-odes]
Special functions, orthogonal polynomials, harmonic analysis, ordinary differential equations (ODE's), differential relations, calculus of variations, approximations, expansions, asymptotics.
3,445
questions
17
votes
1
answer
930
views
Catalan's constant fast convergent series
NOTE. UPDATE 2 introduces proven series for Catalan's constant that is possibly the fastest currently known.
Working with some conjectured continued fractions that were published here, I have found ...
0
votes
0
answers
151
views
How to prove negativity of a $3\times3$ determinant whose elements involve trigamma, tetragamma, and pentagamma functions?
The classical Euler gamma function can be defined by the integral
\begin{equation*}
\Gamma(z)=\int_0^{\infty}t^{z-1}\operatorname{e}^{-t}\operatorname{d}t, \quad \Re(z)>0.
\end{equation*}
Its ...
1
vote
0
answers
74
views
Analytical solution to coupled ODEs arising in molecular evolution
The following system of coupled ODEs arises in the study of DNA sequence evolution:
\begin{eqnarray*}
\frac{da}{dt} & = & \frac{\mu (1-y) b u}{S - y(S-b-v)} - (\lambda +\mu ) a \\
\frac{db}...
5
votes
5
answers
3k
views
Generalized Gauss-Green theorem
I am looking for a generalized version of the Gauss-Green theorem also known as the divergence theorem:
Divergence theorem - Wikipedia
A quick search on MathSciNet suggests that there are ...
96
votes
28
answers
14k
views
Probabilistic proofs of analytic facts
What are some interesting examples of probabilistic reasoning to establish results that would traditionally be considered analysis? What I mean by "probabilistic reasoning" is that the approach should ...
3
votes
2
answers
621
views
Simultaneous extensions of strongly convex functions
21/03/2017: I have decided to accept Denis Serre's answer, even though it does not exactly answer my question, however I like its simplicity and I'd say it is close enough to the desired claim. Of ...
0
votes
0
answers
96
views
Find an explicit solution to $ -u''= u^2\log |u|$
It is known that $u=e^{-|x|^2+\frac{n}{2}}$ is a solution to the equation $-\Delta u= u\log |u|~~ \text{in}~ \mathbb{R}^n$, which can be obtain by a limiting process ($p\to 1^+$) of
$-\Delta u=\frac{u|...
0
votes
2
answers
334
views
How can I derive functional properties of (the solutions of) this simple functional differential equation?
I've not yet finished a course in functional analysis so I'm unsure how to go about this, but I've always been fascinated by a simple functional differential equation I concocted for almost no reason.
...
10
votes
2
answers
596
views
Bi-Lipschitz extension
Given a bi-Lipschitz homeomorphism
$\Phi:\mathbb{B}^n(0,1)\to\mathbb{R}^n$, (that is a bi-Lipschitz map onto the image), can one find a bi-Lipschitz homeomorphism $\Psi:\mathbb{R}^n\to\mathbb{R}^n$ ...
1
vote
1
answer
144
views
On the compact embedding of Sobolev space
In dimension three, we know that the Sobolev space $W^{\frac{13}{11},11}(D)$ is compactly embedded into $W^{1,11}(D)$, where $D$ is a bounded domain in $R^3$ with smooth boundary. My question is: Does ...
4
votes
0
answers
167
views
References for derivative w.r.t. initial condition of an ODE
Let $b:\mathbb R_+ \times \mathbb R^d \to \mathbb R^d$ be measurable such that for all $n \in \mathbb N$ we have
$$
\sup_{t \ge 0} |b(t, 0)| + \sup_{t \ge 0} \sup_{x \in \mathbb R^d} |\nabla^n_x b (t, ...
2
votes
2
answers
232
views
What can be said about the roots of the polynomial $x^{n+1} - (1 - x)^n ( n + x )$?
I have encountered the polynomial equation
$$x^{n+1} = (1 - x)^n ( n + x )$$
where $n \geq 0$, and I am interested in its real roots.
The number $n$ can be an integer or, more generally, any positive ...
2
votes
1
answer
147
views
Hölderness of the inverse to a $W^{1,p}$-homeomorphism (with additional conditions) of a Lipschitz domain
Let $\Omega_1 \subset \mathbb R^2$ be a bounded simply-connected Lipschitz domain, and $f: \bar \Omega_1 \rightarrow \bar \Omega_2$ be a homeomorphism, which is a diffeomorphism on $\Omega_1$ such ...
85
votes
12
answers
87k
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Why is the gradient normal? [closed]
This is a somewhat long discussion so please bear with me. There is a theorem that I have always been curious about from an intuitive standpoint and that has been glossed over in most textbooks I ...
5
votes
1
answer
239
views
Does convolution of a compactly supported function with Gaussian need to have fraction of the $L_1$ mass in the original interval?
Let $f \in L_1(\mathbb{R})$ be such that $\operatorname{supp} f \subset [0,1]$, and let $K$ be the gaussian kernel $K(t) := \frac{1}{\sigma \sqrt{2 \pi}} \exp(-t^2/2\sigma^2)$, with some small $\sigma ...
1
vote
0
answers
150
views
A generalization of polynomials in one variable
Let us consider the space of polynomials $P^N$ of degree $\le N$. If $f\in P^N$ vanishes in $>N$ points, then $f\equiv 0$, but for any $N$ points, or fewer, there exists $f\neq 0$ vanishing at ...
3
votes
1
answer
617
views
What is the Fourier series of $\sin(1/x)$ in $[-\pi,\pi]$?
What is the Fourier series of $\sin(1/x)$ (or $x^k\sin(1/x)$, where $k$ is a positive integer) in $[-\pi,\pi]$? This function evidently does not satisfy Dirichlet's conditions. However, Dirichlet's ...
54
votes
8
answers
9k
views
Does the formal power series solution to $f(f(x))= \sin( x) $ converge?
I have spent some time using gp-pari. There is, of course, a formal power series solution to
$ f(f(x)) = \sin x.$ It is displayed below, identified by the symbol $g$ because I am not entirely sure ...
1
vote
1
answer
128
views
Time varying domain in Chen Wenxiong and Li Congming 's study on $-\Delta u=\exp u$ in $\mathbb R^2$ and $\int_{\mathbb R^2}\exp u(x) \, dx< +\infty$
I'm considering a problem about time varying domain in Chen Wenxiong and Li Congming 's study on $-\Delta u=\exp u$ in $\mathbb R^2$ and $\int_{\mathbb R^2} \exp u(x) \, d x< +\infty$.
LEMMA 1.1 (...
3
votes
0
answers
103
views
Regularity of solutions to an integral ODE
I'm trying to figure out the regularity for solutions to the following integral equation:
\begin{align}
\begin{cases}
\displaystyle{\frac{d}{dt}}u(t,x)&=\displaystyle{\frac{1}{\sigma_d\epsilon^{d+...
2
votes
1
answer
489
views
Radius of the ball where the inverse of Lipschitz maps exists
I am aware of the inverse function theorem for Lipschitz maps, which uses the notion of generalised derivative $\delta_{x_0} f$ of a Lipschitz map $f$, due to F.H.Clarke in On the inverse function ...
6
votes
1
answer
732
views
Twisted Riemann sums
Let $f(x)$ be a real-valued Riemann integrable function supported in $[0,1]$ with range in $[0,1]$. Let $\alpha$ be irrational. Consider the weighted Riemann sum
$$S_N:=\frac{1}{N}\sum_{k=1}^Nf\left(\...
8
votes
3
answers
1k
views
Are all positive eigenfunctions principal eigenfunctions?
In a given domain $\Omega$, we have: $\Delta u=-\lambda u$ with $u>0$. Does this mean that $u$ is a principal eigenfunction for $\Delta$ in $\Omega$?
Also, more generally, does this also apply for $...
3
votes
1
answer
134
views
Recover an $L^1$ integrand by partial differentiation
Denote by $m$ the 2-dimensional Lebesgue measure on $\mathbb{R}^2$. Let $f$ be an element of $L^1(m)$ that takes only nonnegative values. Define $F : \mathbb{R}^2 \rightarrow [0,\infty)$ by
$$F(x,y) = ...
1
vote
0
answers
58
views
$L^p$ norm of Fourier transform of function composed with a diffeomorphism
Suppose $f$ is a compactly supported smooth function from $\mathbb{R}^n$ to $\mathbb{C}$ and $A$ is a diffeomorphism on $\mathbb{R}^n$, do we have any theorems relating the $L^p$ norm of $\hat{f}$ and ...
3
votes
0
answers
148
views
Number of positive roots for an exponential sum
Given $n\geq 3$ distinct constants $c_1, c_2, ..., c_n \in\mathbb{C}$, I want to bound/estimate the number of positive real roots for the equation
$$f(x):=\sum_{i=1}^{n}\dfrac{c_i^n}{\prod_{j\neq i}(...
2
votes
1
answer
194
views
Asymptotics for oscillatory integral
Consider the following integral for $f \in C_c^{\infty}(\mathbb R^n)$, $x_0$ fixed (possibly zero), and $n \ge 3$
$$F(\lambda) = \int_{\mathbb R^n} e^{i\lambda \vert x-x_0 \vert^2} \frac{f(x)}{\vert x ...
5
votes
1
answer
980
views
Analytic functions where all derivatives vanish at infinity and which are bounded
Let $C_0(\mathbb{R})$ denote the analytic functions $f : \mathbb{R} \rightarrow \mathbb{R}$.
I wonder whether there a functions $f \in C_0(\mathbb{R})$ with $f \neq 0$, such that there is a constant $...
-4
votes
1
answer
103
views
Uncountable Cantor's diagonal argument on $S^2$ [closed]
Let $F: S^2 \rightarrow \mathbb{R}^2$ be a continuous function. Does there exist a unit vector $v \in \mathbb{R}^2$ and a continuous function $f(x):S^2\rightarrow \mathbb{R}$ such that $f(x)>0$ on $...
1
vote
0
answers
66
views
Estimating commutator of Fourier integral
Let $f(x)= \log(\vert x\vert)$ on $\mathbb R^2$ and define $s_n:H^2 \to L^2$ where $H^2$ is the second Sobolev space by
$$ s_n(g)(x) = \frac{nf(x)}{4\pi i} \int_{\mathbb R^2} e^{\frac{in\vert x-y\...
2
votes
0
answers
73
views
Verify the explicit solution formula for a degenerate Fokker-Planck equation $\partial_t u = \nabla\cdot(D\,\nabla u + Cxu)$
Consider the following degenerate Fokker-Planck equation in $\mathbb{R}^d$
$$\partial_t u = \nabla\cdot(D\,\nabla u + Cxu),\quad u(t=0) = u_0 \label{1}\tag{1}$$
where $D \in \mathbb{R}^{d \times d}$ ...
0
votes
2
answers
236
views
Fractional Laplacian of $(a-x)_+^\alpha$ in $(0,1)$
How can I compute the spectral fractional Laplacian of $(a-x)_+^\alpha$ on $\Omega = (0,1)$?
Here the operator is defined as $$(-\Delta)^s u = c_{N,s} \int_0^\infty (e^{t\Delta_N}u(x) - u(x)) t^{-1 - ...
4
votes
1
answer
154
views
Uniqueness of critical points for Lipschitz perturbations of uniformly convex Hamiltonians
Consider a macroscopic free energy functional of the form
$$\mathcal{F}_\beta(\mu):= \frac{1}{\beta}\int_{\mathbb{R}^d}\log(\mu)\mu dx + \int_{\mathbb{R}^d}V(x)\mu(x)dx + \iint_{(\mathbb{R}^d)^2}g(x-y)...
2
votes
1
answer
2k
views
Uniqueness of the logarithm function
In my Analysis class, we started to prove a theorem that said:
Let a > 1. So there is a unique increasing function $f:(0,\infty)\to\mathbb{R}$ so that:
$f(a) = 1$
$f(xy) = f(x) + f(y)\quad\forall ...
3
votes
1
answer
211
views
Reference request: analysis of a nonlinear Fokker-Planck type equation
It is well-known that the linear Fokker-Planck equation (written in one space dimension for simplicity) $$\partial_t \rho = \partial_x \left(\rho_\infty \partial_x\left(\frac{\rho}{\rho_\infty}\right)\...
33
votes
6
answers
2k
views
Is there a topology on growth rates of functions?
I've often idly wondered one can say about the collection of "growth rates". By growth rate, let's say we mean an equivalence class of functions $(0,\infty) \to (0,\infty)$, where two ...
1
vote
0
answers
94
views
Regularity of solution to heat equation
If $u$ is solution of $u_{t}=\Delta u$ in a bounded domain $\Omega$, is it true that:
$\sum_{\lvert \alpha\rvert=2k}\|D^{\alpha}u\|_{L^{2}(\Omega)}^{2}\leq |\Delta^{k}(u^{(l))}\|_{L^{2}(\Omega)}^{2}=\...
3
votes
1
answer
227
views
Computing global maximum
For $\lambda\in\mathbb{R}$, I want to find the expression of $f(\lambda)$:
$$f(\lambda)=\max_{E\in\mathbb{C}}arccosh{\frac{|E^2+i\lambda E|+|E^2+i\lambda E-4|}{4}}-arccosh{\frac{|E^2-i\lambda E|+|E^2-...
7
votes
5
answers
1k
views
Generalizations of the Euler–Maclaurin Summation Formula
I'm using the Euler–Maclaurin formula in a research project I'm working on. While brilliant, the elementary proof found in Apostol - An Elementary View of Euler's Summation Formula does not give me ...
0
votes
0
answers
36
views
Conditions for an ODE with convolution term to have a probability distribution solution
Suppose we have a simple ODE like:
$$
y''(x) + 2ay'(x) + by(x) = 0
$$
with the condition $y(0)=0$ for $x\leq 0$. Then the solution on $(0,\infty)$ will be of the form $Axe^{-ax}$ when $a^2=b$, $Ae^{-...
0
votes
0
answers
47
views
When is the derivative of an ODE solution wrt the initial condition a simple exponential?
Suppose we are given an autonomous ODE $y' = f(y)$ such that $y$ is smooth with respect to the initial condition.
We then have
$$(\nabla y)' = \nabla f(y) \nabla y.$$
If $\nabla f(y(t)) $ commutes ...
5
votes
2
answers
286
views
Euler–Maclaurin formula in $\mathbb{Z}^d$
I was wondering whether there is a Euler–Maclaurin formula of sorts for expressions such as
$$
\sum_{x \in [a,b]^d\cap \mathbb{Z}^d} f(x) - \int_{[a,b]^d}f(x)
$$
where $d\ge 2$ is an integer, $a,b \...
11
votes
2
answers
1k
views
Is it a coincidence that the universal parabolic constant shows up in the solution to square point picking?
The expected distance $d$ of randomly selected points within a unit square to the square's center is
$d = \frac{1}{6} P$
where P is the universal parabolic constant
$P = \sqrt{2} + \ln{\left(1+\...
142
votes
7
answers
14k
views
Source and context of $\frac{22}{7} - \pi = \int_0^1 (x-x^2)^4 dx/(1+x^2)$?
Possibly the most striking proof of Archimedes's inequality $\pi < 22/7$ is an integral formula for the difference:
$$
\frac{22}{7} - \pi = \int_0^1 (x-x^2)^4 \frac{dx}{1+x^2},
$$
where the ...
121
votes
12
answers
27k
views
How to solve $f(f(x)) = \cos(x)$?
I found the following equation on some web page I cannot remember, and found it interesting:
$$f(f(x))=\cos(x)$$
Out of curiosity I tried to solve it, but realized that I do not have a clue how to ...
0
votes
1
answer
83
views
On the validity of a certain Grönwall-type inequality
Assume that $u~ \colon \mathbb{R}_+ \to [-M,M]$ is a bounded continuously differentiable function such that $u(0) = 0$ and $$u(t) \leq \int_0^t \lambda(s)~u(s)~\mathrm{d}s + C \label{1}\tag{1}$$ where ...
21
votes
4
answers
6k
views
When I can safely assume that a function is a Laplace transform of other function?
If I have a function and I want to represent it as being the Laplace transform of another, that is, I want to be sure that there is $\hat{f}(s)$ such that my function $f(x)$ can be written as:
$$f(x) =...
2
votes
1
answer
326
views
What are the best definitions for smoothness of a 2D curve (real-valued function)?
Sounds like a trivial question, but could not find any answer other than the fact that there are many ways to define it. My problem is this: I look at different elevation maps,
some with sharp ...
10
votes
0
answers
199
views
Bi-Lipschitz mappings
Assume that we have a bi-Lipschitz mapping $f:\bar{\mathbb{B}}^n(0,1)\to\mathbb{R}^n$. The mapping need not be smooth anywhere and it may happen that it cannot be extended to a homeomorphism of a ...
0
votes
1
answer
135
views
How are the Legendre Polynomials of second kind for negative degrees defined?
For a script I have to evaluate the associated Legendre polynomial of second kind $Q^0_{n}(z)$.
Until now, I was using an implementation that is based on the definition in equation 8.702 in the Book &...