Mathematical analysis. Consider a more specific tag instead: (real-analysis), (complex-analysis), (functional-analysis), (fourier-analysis), (measure-theory), (calculus-of-variations), etc. For data analysis, use (data-analysis).

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14 views

statistical analysis

Two independent random samples of annual starting salaries for individuals with masters and bachelors degrees in business were taken and the results are shown below ...
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2answers
15 views

Proving Lipshitz continuous over a convex set with Projection Operator

Suppose a problem $$\min_{x \in \mathbb{R}^{n}} f(x)$$ subject to $x \in \Omega$ which is a closed and convex set. If $\nabla f(x)$ is Lipschitz continuous in $\Omega$, then prove that $$e(x) = x - ...
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1answer
29 views

An equation with multiple solutions: finding the maximum of the function of the solutions.

Possibly, this is a bad (stupid) question, but sometimes some discussion helps. I have a fixed point equation (involving $\tanh$). I would like to derive the dependency of some function of the fixed ...
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1answer
44 views

Generating functions for $\log^3(1-x)$ of $\log^3(x)$

I am trying to find generating functions which will give me a power logarithm. I am trying to find generating sums in the form $$\sum_{n=1}^{\infty} a_n\,x^n = -\frac{\log^2(1-x)}{1-x}$$ or ...
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1answer
87 views

Given $|f'(x)|\leq r<1$ show that $f(x)=x$ is unique solution

Suppose that $|f'(x)|\leq r<1, \forall x\in R$. How do I show that the equation $f(x)=x$ has a unique solution?
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1answer
21 views

Composition of a Dirac delta and a function in higher dimensions

Coming from a physics background, I was taught the formula for the composition of a Dirac delta and a function. Indeed, if we consider a nice function $ f : \mathbb{R} \to \mathbb{R} $, one can write ...
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19 views

Differential equation: $A(x)y''(x)+A'(x)y'(x)+y(x)/A(x)=0$

So give the differential equation $$A(x)y''(x)+A'(x)y'(x)+\frac{y(x)}{A(x)}=0,$$ with $A(x)$ a known function and $y(x)$ te be determined. What is the solution for this differential equation ? I've ...
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3answers
99 views

How to compute the integral $\int^{\pi/2}_0\ln(1+\tan\theta)d\theta$? [duplicate]

How to compute the integral $\int^{\pi/2}_0\ln(1+\tan\theta)d\theta$. If we let $t=\tan\theta$, then the integral becomes to ...
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0answers
16 views

Density and Fredholmness

Let $X$ be a Banach Space and $Y$ a dense subset of $X$. An operator $T:X \to X$ is said to be Fredholm if it has closed range, $\dim \ker(T)<\infty$ and $\dim coker(T) < \infty$. Here is my ...
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24 views

Inhomogeneous ODE (2nd order) - question to Laplace-transformation?

I've the following inhomogeneous second order ODE: $$a_1\cdot u(t) + a_2\cdot u'(t) + a_3\cdot u''(t) = b_1\cdot y(t) + b_2\cdot y'(t) + b_3\cdot y''(t)$$ The parameters $a_i$ and $b_i$ are ...
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54 views

Showing unstablity of differential equation.

Assume differential equation $$ x'=2x+y+x \cos t-y \sin t $$ $$ y'=-x+2y-x\cos t+y \sin t $$ Show that solution $(x(t),y(t))=(0,0)$ is unstable. Is there a non-trival solution such that ...
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22 views

Weak convergence in Lp [on hold]

got a little problem with this ex. I could use some help. Let $U := \Pi_{i=1}^d(a_i, b_i) \subset \mathbb{R}$ ($a_i < b_i$ for each $i$) and let $f \in L^p(U)$ for some $1<p<+\infty$. Let us ...
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1answer
43 views

Is the polynomial a zero polynomial?

Let $p(x)$ be a polynomial over $\mathbb{R}$ with $deg[p(x)]\leqslant n$. If $p(1)=p(2)=\cdots = p(n+1)=0$, then will the polynomial be necessarily a zero polynomial? i.e., if a polynomial of degree ...
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2answers
80 views

Integral $\int_{1}^{2011} \frac{\sqrt{x}}{\sqrt{2012 - x} + \sqrt{x}}dx$

Evaluate: $$\int_{1}^{2011} \frac{\sqrt{x}}{\sqrt{2012 - x} + \sqrt{x}}dx$$ Using real methods only. I am not sure what to do. I tried finding a power series, which was too ugly. I just need some ...
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2answers
62 views

Does $\int_0^\infty |f'(x)| dx < \infty$ conclude $\lim_{x\to \infty} f(x)<\infty $

$f:[0,\infty) \to \mathbb R $ is $C^1$ and $$\int_0^\infty |f'(x)| dx < \infty$$ then can we prove that $\lim_{x\to \infty} f(x)$ exists and $$\lim_{x\to \infty} f(x)<\infty $$ My attempt: ...
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50 views

How prove there exsit $\xi\in (0,1)$ such $|f(\xi)|\le|f'(\xi)|$

let $f:[0,1]\to \mathbb{R}$ be a differentiable function such that $f(1)=0$, Prove that there is $\xi\in(0,1)$, such that $$|f(\xi)|\le|f'(\xi)|$$ My idea: I think we can prove there exsit ...
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2answers
25 views

Problem 8, Section 2.7 in Erwine Kryszeg's Introductory Functional Analysis With Applications

Here is Problem 8 in the Problem Set following Section 2.7 in Erwine Kryszeg's Introductory Functional Analysis With Applications: Show that the inverse $T^{-1} \colon R(T) \to X$ of a bounded ...
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1answer
8 views

Dirichlet problem, solvability

Let $\Omega\subset \mathbb R^n$ and $f\in L^2(\Omega)$. We consider $\{u\in C^1(G):||u||_{1,2,G}<\infty \}$,where $||u||_{1,2,G}$ is the "sobolev-norm" with parameter $1,2$ on $G$. Then $H^1(G)$ ...
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33 views

Knowing a function while only knowing its partial derivatives?

So again we study a physics course without studying mathematics course We are in the work energy chapter , and I'd like to know if you can know the function $f(x,y,z)$ if you know all of its partial ...
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3answers
20 views

Prove of a Landau-equalities

I have to prove or disprove the following Landau-equalities: $$ O(f+g) = O(max(f,g))$$ and $$O(f-g) = O(min(f,g))$$ with $f,g: \mathbb N \to \mathbb R^+$ . To show equality of two sets, one has to ...
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1answer
25 views

A question about continuous functions

Let $f:B \longrightarrow \mathbb{R}^n$ a continuous and injective function from closed ball in $\mathbb{R^n}$ to $\mathbb{R}^{n}$. I'd like to know $f$ has to maps the boundary $\partial B$ in the ...
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1answer
36 views

Interval of converge of $\sum_{n=1}^{\infty} \frac{n!(x+1)^n}{(2n-1)!}$

Find the interval of converge of: $$\sum_{n=1}^{\infty} \frac{n!(x+1)^n}{(2n-1)!}$$ I will use the ratio test. Let $\displaystyle a_n = \frac{n!(x+1)^n}{(2n-1)!}$ $\displaystyle a_{n+1} = ...
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19 views

key contributions of Augustin Louis Cauchy to analysis

I am wondering if any one could organize some key contributions of Augustin Louis Cauchy to analysis properly ?
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45 views

Prove that a continuous inverse-transformation of $f: [0,1) \cup \{ 2 \} \to [0,1]$ exists

I am having this transformation $f: [0,1) \cup \{ 2 \} \to [0,1]$ $$f(x) = \begin{cases} x & x \neq 2 \\1 & x = 2 \end{cases}$$ I've already proved that it is continuous. Question: Is ...
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22 views

Prove that a function lies in $L^1$ and in $W^{(1,1)}$ for some parameter

I want to do the following tasks Let $G:=B_1(0)\subset \mathbb R^2$ be the open ball around $0$ with radius 2 in the norm $||\cdot||$ and $u_{\rho}(x)=||x||^{\rho}_2$, $x\in G$. Show the following ...
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1answer
26 views

Partial derivative is bounded

Let $f(t,z)$ be a bounded (say by a constant $M$) continuous function on $\mathbb{R}_t \times \mathcal{U}$ where $\mathcal{U}$ is an open neighborhood of $0 \in \mathbb{C}_z$. Moreover, for each fixed ...
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2answers
57 views

If f is differentiable with a continuous derivative function, then the set of critical points of f is closed.

If f is differentiable with a continuous derivative function, then the set of critical points of f is closed. Is this a true statement? I'm kinda lost.
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1answer
58 views

Is sinus an unique function?

On $\mathbb{R}$, is sinus the unique $C^{\infty}$ function f with all is derivate and itself between -1 and 1 and also $ \frac{df}{dx}(0)=1 $ ?
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1answer
27 views

If a function has asymptote and the derivative does not, then its second derivative is not bounded

Let $f\colon\mathbb{R} \rightarrow \mathbb{R}$ be a function with second derivative everywhere in is domain. Prove that if $\lim_{x\rightarrow\infty}f(x)=b \in \mathbb{R}$ and ...
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1answer
57 views

2048 Tournament Word Problem [on hold]

Problem: There was a 2048 tournament. And after the results were counted and were announced, winners got candy. The 1st place got 2 less than a third of candy, 2nd place got 4 less than half of the ...
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2answers
41 views

Prove ${x:d(x,p) < d(x,q)}$ is open in metric space $X$

$X$ is a metric space and $p \neq q$ $\in X$. I want to prove that $E=$ $\{x:d(x,p) < d(x,q) \}$ is open in metric space $X$. I think I can directly prove this by showing every point $x \in E$ ...
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1answer
22 views

$\left\Vert J(x)^{-1}\right\Vert<2\left\Vert J(x^*)^{-1}\right\Vert. $?

Could you please help me to prove this theorem: Suppose $J:{\bf {\rm R}}^m\rightarrow{\bf {\rm R}}^{n\times n}$ is a continuous matrix-valued function. If J(x*) is nonsingular, then there exists ...
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1answer
44 views

How to proove that a bijective transformation is NOT continous

I am having this transformation $f: \mathbb R \to \mathbb R$ $$f(x) = \begin{cases} x & x \in \mathbb R \setminus \mathbb Q \\x+1 & x \in \mathbb Q \end{cases}$$ I've already prooved ...
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2answers
34 views

Convex function and expectation

I was wondering: if f is a convex function and X a random variable, what does E(f(X)) = f(E(X)) implies? Thanks a lot, David
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9 views

$0$ is an unstable equilibrium if $f$ is Lipschitz with certain conditions

Consider the following system: $$x'=-x^3-xy^2+2x^2y^2$$ $$y'=-2y+x^2y-3x^3y$$ There are two questions: The first one is to show that $(0,0)$ is uniformly asymptotically stable. The second question ...
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28 views

Resolvent operator

Let's consider the following operator on $L^2(\mathbb{R}^3)$ $$A(t)=\Delta+b(t,x)\cdot\nabla$$ where $\Delta$ is the Laplace operator and $b(\cdot,\cdot)$ a smooth vector field. How to compute the ...
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1answer
40 views

A compact set, which is not closed.

I'm looking for a compact set, which is not closed. I read somewhere that $Z^+$ are compact and not closed, but I don't understand why. Are there any other examples of compact sets that are not ...
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2answers
66 views

Suppose that f is integrable on $[a,b]$. Prove there is a number $x$ in $[a,b]$ such that $\int_a^x f = \int_x^b f$

Also, show by example that it is not always possible to choose $x$ in $(a,b)$ I've proven the first part (in the title), but I can't seem to think of a scenario for the second part. Perhaps my brain ...
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analyses exam bad translation [on hold]

where is the function droves $\frac {x^3} {8} + \frac {3k}{8}x^2$, $k>0$ , for every $k>0$, the graph of the function $f(k)$ in the $1$st quadrant adjacent to the turning point has also a high ...
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1answer
41 views

Prove the energy is constant in a PDE?

I calculated the $$ \begin{align} \frac{dE(t)}{2\,dt} & = \int_\Omega u_tu_{tt}+DuDu_t+u^3u_t\,dx \\ & =\int_\Omega [u_t(u_{tt}-\Delta u)+u^3u_t] \, dx+\int_{\partial \Omega} u_t ...
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48 views

Covariant derivative for a covector field

In the lecture we had the immersion $f: U \subset \mathbb{R}^2 \rightarrow \mathbb{R}^3$. Now we said that a map $\lambda : U \rightarrow T^*f$ is a covector field. Okay, that's fine. Also, we ...
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1answer
28 views

Completeness, Compactness, Sequentially Compactness for $X = [0,1] \cap \mathbb{Q}$

$X = [0,1] \cap \mathbb{Q} \subset \mathbb{R}$ a metric space with the metric of $\mathbb{R}$. Show $X$ is not complete, is totally bounded, and is not sequentially compact. For completeness. I ...
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1answer
18 views

something about diffeomorphism

Suppose $A$ and $B$ are both open sets, and there is a diffeomorphism $g$ between them. My book says that the chain rule implies that $Dg$ is non-singular. I don't understand. Can anyone tell my why?
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1answer
32 views

calculate weak derivate of $|x-2|^2$

Let $u$ be a function with $u(x):=|x-2|^2$ on $I:=(-1,1)$. I want to test whether $u \in H^2(I) \backslash H^3(I)$. Let $\phi$ be in $C_0^\infty(I)$. Then: $T_u(\phi '') = \int_{-1}^1 |x-2|^2 ...
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1answer
32 views

A problem with equality in a inequality for convex function

Let $f:\rightarrow \mathbb R$ be a convex function on a convex subset $D$ of linear space $X$. Assume that for some pairwise disjoit $x_1,x_2,x_3\in D$ and some $t_1,t_2,t_3\in (0,1)$ such that ...
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20 views

Reference about $p$-homogeneous functions

I'm looking for a book about $p$-homogeneous functions. I am particularly interested in the associated (nonlinear) eigenvalue problems. However, a reference containing most of the known properties of ...
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27 views

L2 norm and L1 norm inequality

In the vector space, we have the following inequality $$ ||x||_2 \leq ||x||_1 $$ where x is a vector. I am wondering that we have similar inequality for function's norm. L1 norm of function f is ...
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16 views

Inverse transformation of continous transformation is bounded

I am having a continous transformation: $f: \mathbb C \to \mathbb C $ with $B \subseteq \mathbb C $ bounded. Now I want to proove that $ A = f^{-1} (B)$ is bounded! How can I proove that this ...
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0answers
12 views

positive integrable part implies downside integrable

Let $A: M\rightarrow GL(d)$ measurable where $(M, \mathcal{B},\mu)$ is a probability space, then are equivalent: $$\log^+\Vert A^{\pm1}(x)\Vert\in L^1(\mu)\Leftrightarrow \log^-\Vert ...
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0answers
24 views

$\|L\|^1$-functions: Is this condition valid?

I tried to prove that a function is absolutely integrable, and it was very hard to integrate. I thought, maybe one could prove that a function is in $L^1$ also this way, which would be quite easier in ...