Questions tagged [functional-inequalities]
For questions about proving and manipulating functional inequalities.
1
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1answer
34 views
Bound for $e^{-\alpha x}$
For a part of my proof I need to establish that $e^{-\alpha x} \lt h(x)$, where $\alpha,x \gt 0, $ and $x,\alpha \in\mathbb{R}$. I thought for a while and couldn't find a function independent of $\...
1
vote
1answer
24 views
Alternative form for Liapunov inequality
Let $1<p<q<\infty$, and $r\in [p,q]$ whith $\frac{1}{r}= \frac{\alpha}{p}+ \frac{1-\alpha}{q}$. If $f\in L_p\cap L_q$ then
$$\|f\|_r \leq \|f\|_p^\alpha\|f\|_q^{(1-\alpha)}$$
My teacher ...
1
vote
0answers
15 views
Finding discrete solutions to inequality involving Exponential Integral
I want to identify the least natural number $n$ (of course, it suffices to solve this problem for the reals, and then take the floor) such that
$$-c \text{Ei}\left(-e^{\frac{a-d}{c}} (n+1)\right)+a-...
1
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0answers
37 views
How to find which of two events -drawn from a normal distribution- is more likely?
I know the probability of event A is given by:
$$\Phi(f(x)+g(y)) - \Phi(f(x)-g(y)), $$
and the probability of event B is
$$\Phi(m(y)+n(x)) - \Phi(m(y)-n(x)).$$
where $\Phi$ is the cumulative ...
2
votes
2answers
68 views
Inequality for $\sin(20°)$
Prove that $$\frac{1}{3} < \sin{20°} < \frac{7}{20}$$
Attempt
$$\sin60°=3\sin20°-4\sin^{3}(20°)$$
Taking $\sin20°$=x
I got the the equation as
$$8x^3-6x+\sqrt{3} =0$$
But from here I am not ...
4
votes
1answer
35 views
Understanding the proof of an inequality
Basically the method applied is the following, we fix $a=\frac{a_1+a_2+...+a_n}{n}$, if: $$f(x)\ge f(a)+f'(a)(x-a) $$This inequality holds for all x, then summing up the inequality will give us the ...
0
votes
0answers
24 views
Injectivity of integral operators
Let $K:L^2[0,1]^{d_1}\to L^2[0,1]^{d_2}$ be integral operator
$$(Kf)(y) = \int f(x)k(x,y)d x.$$
If $d_1>d_2$ is it possible for $K$ to be injective?, e.g. let's take $d_1=2,d_2=1$.
More generally, ...
0
votes
0answers
12 views
Estimate of a general integral involving laplacians
I have two real functions $u,\eta$ defined on $R^n$ and with compact support so we can do all integrations by parts we want, and we need to estimate $\int | \nabla \eta \cdot \nabla u|^2$ by $\int | \...
5
votes
2answers
71 views
Determine all functions $f : \mathbb{N} \rightarrow \mathbb{N}$ such that, for every positive integer $n$, we have: $2n+2001≤f(f(n))+f(n)≤2n+2002$.
Determine all functions $f : \mathbb{N} \rightarrow \mathbb{N}$ such that, for every positive integer $n$, we have: $$2n+2001≤f(f(n))+f(n)≤2n+2002\,.$$
I don't know where to start as in is there a ...
-1
votes
1answer
36 views
Matries inequality with norms
Let $P$ and $C \neq0$ a $q \times q$ matrices. I want to prove that there exists a positive constants $\alpha$ such under some assumptions under $P$ we have the inequality
$${\left\| {P\left( {I - C} ...
0
votes
0answers
31 views
Functional is weak lower semicontinuous but not weak continuous
I want to show that the functional
$L(u)=\int_0^1 \sqrt{1+(u'(x))^2} dx$ is lower semicontinuous in terms of weak convergence in $W^{1,p}(0,1), p\in(1,\infty)$ but not continuous. Our definition of ...
1
vote
2answers
44 views
Showing that $f(x)$ is convex in $(0,3)$
I've got the following function: $$f(x)=\frac{1}{16x}-\frac{1}{(x+3)^2} $$
And I wish to show that it is convex in the open interval $(0,3)$, took the second derivative, i.e.$$f''(x)=\frac{1}{8x^3}-\...
1
vote
1answer
44 views
A definite integral inequality
Suppose $f(x)$ has continuous derivative on $[-\pi, \pi]$, $\,f(-\pi)=f(\pi)\,$ and $\,\int_{-\pi}^{\pi}\, f(x)\, dx=0$. Then prove that:
$$
\int_{-\pi}^{\pi} [\,f'(x)]^2\, dx \ge \int_{-\pi}^{\pi} f^...
1
vote
0answers
25 views
Inequality involving Weibull distribution
define R(x) as the weibull survival function
$R_1(t)=e^{-\alpha_1 t^{\beta_1}}$
$R_2(t)=e^{-\alpha_2 t^{\beta_2}}$
with $\alpha_1, \alpha_2 > 0 $ and $\beta_1, \beta_2 >1 $
$\phi_1(t)=\int_{...
0
votes
0answers
29 views
Remark 16 in Chapter 2 from Brezis - Functional Analysis, Sobolev Spaces, and Partial Differential Equations.
Let $E,F$ be two Banach Spaces and $A : D(A) \subset E \to F$ be a linear unbounded operator which is densely defined. Now, we would like to define $A^{*}$ as the adjoint of $A$. Let $A^{*} : D(A^{*}) ...
1
vote
1answer
15 views
Proving that $|x^*(x)| \leq \rho(x,Y)$
Exercise :
Let $(X,\|\cdot\|)$ be a normed space, $Y$ a subspace of $X$ and $x^* \in X$ with $\|x^*\| \leq 1$ such that $x^*|_Y = 0$. Show that $\forall x \in X \setminus Y$, it is : $|x^*(x)| \leq ...
0
votes
0answers
20 views
Proving complicated transcendental inequality
Suppose we have a function $f$ of four posirive real numbers $a,b,c$ and $d$ in a domain that, for a given real number $0<r<1$ they satisfy
$$rc<b<a,$$
$$rc<rd<a.$$
We then have
$$...
0
votes
1answer
24 views
Inequality of derivatives
I'm developing a mathematical model for a physical system and have come across the following logical quandary (at least, it is for me). I'm having trouble proving whether the following proposition is ...
0
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0answers
19 views
Jensen type inequality for a non-convex function.
I suppose that a function $f$ is $\geq 0$ on $[-1,1]$, decreasing and $f(t)(1+t)$ is concave.
Moreover for every $a,b \in [-1,1]$, $a<b$, we have a (simple positive) measure $\mu_{a,b}$ such that
$...
1
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0answers
25 views
Prove upper bound inequality for the dimension of the space of symmetric tensors
I want to check that the dimension of the space of symmetric tensors $N(n,m) := dim(Sym^m(\mathbb{R}^n))$ satisfies $N(n,m) \leq \frac{n^m}{m!}(1+\frac{2m^2}{n})$.
Thus I need prove inequality.
If $...
2
votes
1answer
49 views
Poincare type inequalities
I want to prove if following inequality holds:
$$\int_0^1(f')^2\ dx\geq f^2(1)-f^2(0)$$
where $f$ is a function in $H^1([0,1])$ satisfying $\int_0^1f \ dx=0$. It is actually a one dimensional case of ...
1
vote
1answer
118 views
Does the following inequality always hold true?
$$ 0\lt \frac{\sum_{i=n}^{n + P_n - 1} P_i}{P_n \cdot P _{P_n}} \leq 1 $$
Or is there a lower bound bigger than zero? Which I believe not to be the case.
Some basic examples are as follows:
$(1)$
...
2
votes
1answer
59 views
RMO practice problem inequality
Let $a_n$ & $b_n$ be two sequences such that $a_0$ , $b_0$ > 0 and $a_{n+1}$ = $a_n$ + $\frac{1}{2b_n}$ & $b_{n+1}$ = $b_n$ + $\frac{1}{2a_n}$ $\forall$ n $\geq$ 0. Then prove that $$max(a_{...
1
vote
1answer
22 views
Equivalence of norm in Banach Space
Let $A$ be a densely defined closed linear operator in a Banach space $X$ and $\sigma(A)$ be its spectrum. We define its spectral radius $r_{A} := \sup\limits_{\lambda \in \sigma(A)}|\lambda|$. Now, ...
0
votes
3answers
26 views
Determine the set of all complex number z satisfying following conditions
I’m having some troubles of calculating complex numbers where I need to deal with absolute values and inequalities.
Here is an example I’ve been working on but I get stuck
Re(2/z)+Im(4/z)<1
I use ...
3
votes
5answers
101 views
Why does proving inductively $n < 2^n$ for $n \geq 1$ imply it is true for real values of $n$?
If we prove by induction that $2^n > n$ for $n \geq 1$ where $n \in N^+$, how can one know this inequality holds for real values of n like $2^{2.5} > 2.5$?
Maybe a bit silly question but I can'...
0
votes
1answer
66 views
How is this proposition true?
I have a function of $n$, where $n$ is an integer, defined as follows $$D(n)=\frac{nK}{K+\bigg\lceil\frac{n}{M}\bigg\rceil(n-1)},$$ where $K$ are $M$ are some constant positive integers. For this ...
0
votes
0answers
36 views
How to prove the following inequality for a function?
I have function $$D(M\gamma)=\frac{M\gamma K}{K+\gamma(M\gamma-1)}\tag{1}$$ where $K$ is some positive constant. In this case, how to show that $$D(M(\gamma-1))<D(M\gamma),~~\text{for } \gamma<\...
0
votes
1answer
21 views
Best way to check to see if conditions are (or can be) satisfied?
Sorry to bother you guys, but a project I'm working on necessitates checking to see whether somewhat obscure conditions can all hold, given some bounds on possible values for each variable of interest....
3
votes
1answer
103 views
Interpolation inequalities
Let $\Omega$ be a regular domain of $\mathbb{R}^d$, $d=2,3$. Let $\mathcal{T}_h$ be a triangulation of $\Omega$ of size $h>0$.
Assume we can prove
\begin{equation*}
\begin{aligned}
\|v\|_{L^2(\...
2
votes
0answers
47 views
A generalisation of a Hardy inequality
I found the following result in Brezis' book.
Let $\Omega$ be a bounded open set of class $C^1$. Let
$d(x):=\operatorname{dist}(x,\partial\Omega)$. Then there exists $C>0$
such that $$ \|\...
1
vote
1answer
51 views
Equality of functions on real numbers implies equality on rationals?
Define a pair of functions $p, h: \mathbb{Q} \to [0,1]$ over the rational numbers, with the properties
\begin{align}
0 \le p(x) \le 1 && 0 \le h(x) \le 1 && (x\in\mathbb{Q}), \\
\end{...
2
votes
0answers
80 views
Problem in proving fixed point theorem
Let $ f(x)=x^r, 1<r<\infty, x\in \mathbb{R^+}=[0,\infty) ~and ~n\in \mathbb{N}.$ Define
$$\pi(x)= 1 ~~if~~ x\leq n ~and =0 ~if~ x< (n+1).$$ Then for any $x,y\in\mathbb{R},~~ $ I want to ...
0
votes
0answers
13 views
Showing boundedness below by sobolev norm
Suppose I know that $h(f)$ is bounded by $h(f) \geq (1 -2ab)\Vert f' \Vert^2_{L^2} - \frac{4a}{b} \Vert f \Vert^2_{L^2}$ with $a,b > 0$ and $f \in H^1$. Then for any $b \leq \frac{1}{2a}$ we can ...
2
votes
0answers
59 views
Unclear inequality of L2 norms (Poisson equation for modeling flow)
I encountered a problem working through a paper about modeling flow with the use of the Poisson equation (source given below). There appears an inequality of L2 norms I don't understand so far. Your ...
2
votes
0answers
82 views
Prove or disprove that integral term is log-concave
Consider the function
$h(x_i, x_j) := \int_{x_i}^{\overline{z}}f(z)F(z-x_i+x_j)dz$,
where $f(z)$ is a twice continuously differentiable and strictly positive probability density function defined on $...
1
vote
2answers
59 views
Prove that $\inf\limits_{f\in X}\,I_a(f)=1-a$.
Let $X:=\big\{f\in \mathcal{C}^1[0,1]\,\big|\,f(0)=0\text{ and } f(1)=1\big\}$, $0<a<1$, and $I_a(f):=\displaystyle\int _0 ^1 x^a \left(\frac{\text{d}}{\text{d}x}\,f(x)\right)^2 dx$.
Then prove ...
1
vote
0answers
25 views
Semi-bounded operator implies semi-bounded sesquilinear form
So my question is quite simple:
If an operator (which corresponds to a sesquilinear form) is semi-bounded (from below) then the sesquilinear form is also semi-bounded (from below)?
I'm pretty sure ...
-3
votes
2answers
181 views
Inequality : $\sqrt {x} - 6 - \sqrt{10} -x \geqslant1$ [closed]
I have solved it by squaring both sides and got inequality $x \geqslant 17/2$ but after that, the solution part have concluded on the equation
$4x^2 + 289 - 68x \geqslant4(10 - x)$
How this ...
1
vote
0answers
21 views
Tricks for finding good “close enough” solutions to multivariate recursive relations
As part of an undergraduate project I am attempting to find a solution to this set of recursive inequalities:
$$g(n,l,k) \leq g\left(\frac{n-1}{2},l,k-1\right)$$
$$k \cdot g(n,k,l)+\log_2 (n-2^l +1) ...
1
vote
1answer
125 views
Find min of $P = \dfrac{1}{(a-b)^2} + \dfrac{1}{(b-c)^2} + \dfrac{1}{(c-a)^2}$
Let $a, b, c \in \mathbb{R}^+$ such that $a^2 + b^2 + c^2 = 3$. Find the minimum value of $P = \dfrac{1}{(a-b)^2} + \dfrac{1}{(b-c)^2} + \dfrac{1}{(c-a)^2}$?
In fact, we have that $P \geq \dfrac{4}{...
0
votes
1answer
53 views
Is this inequality true? If true what is the simplest proof?
Suppose we have $$\frac{a_i^1}{b_i^1+c}>\frac{a_i^2}{b_i^2+c},~\forall ~i\in\{1,2,\cdots L\}$$ and $a_i^1<a_i^2$, $b_i^1<b_i^2$ $\forall ~i\in\{1,2,\cdots L\}$. Then can we say that following ...
2
votes
5answers
62 views
Trig function bounded on interval (without calculus), prove that $x^{3/2}\sin x + \sqrt{9-x^3}\cos x \leq 3$.
If $0 \lt x \lt \dfrac{\pi}{2}$, prove that
$$x^{3/2}\sin x + \sqrt{9-x^3}\cos x \leq 3$$
This question must be done without calculus. First, I tried splitting it into the intervals $(0,\pi/4)$ ...
0
votes
1answer
17 views
Range of values in inequalities
The function f and g are defined by
$$ f(x) = (x-2)(4-x), 0 \le x \le 4 $$ $$ g(x) = |x-2|, 0 \lt x \le A $$
(i) Find the range of values of A for which the composite function $fg$
is ...
0
votes
0answers
15 views
Condition on functions satisfying certain inequality
I came across the following problem:
Given two functions u,v mapping $[0.5,1]\to (0,1]$ where u(x) is the identity function, $u(x)=x$. Suppose that $\frac{u(x)}{u(1-x)}> \frac{v(x)}{v(1-x)}$ holds....
0
votes
0answers
64 views
Can inequalities be proven by taking all variables to be equal?
For example-
$$ \frac{a}{c+a-b} + \frac{b}{a+b-c} + \frac{c}{b+c-a} \geq 3. $$
This can be proven if take a=b=c
Is there any theorem regarding this?
0
votes
0answers
48 views
What inequalities can be obtained for summation of piecewise linear convex functions?
Let $f_i(x):\mathbb{R}^n\to\mathbb{R}$ be a piece-wise linear convex function with $i=1,\dots,m$. We know that $F(x)=\sum_{i=1}^m f_i(x)$ is also a piece-wise linear convex function (see this post). I ...
0
votes
0answers
12 views
Embedding of gauge map
Let $X$ be a compact $n$-manifold, and $p\geq n/2$.
Q How to show that there is a real polynomial $P_{m,p}(x)$ of degree $m+1$ with trivial zero-degree term, such that for any $u:X\to S^1$, we have
$...
1
vote
1answer
57 views
Estimate on smooth functional
Let $f:H\to H' $ be a smooth map, between two Hilbert spaces.
Suppose that $f$ is a local diffeomorphism, i.e. $f: B_\epsilon\to B'_{\epsilon'}$ is differentiable, and there exists $g:B'_{\epsilon'} \...
1
vote
1answer
70 views
Finding the operator norm in $L^p$ spaces
There is an operator given:
$K: X \to Y$,
$(Kf)(x) = \int \limits_0^1 k(x, y)f(y) \mbox{d}y$.
Let's define: $X = L^P([0, 1]), Y = L^q([0, 1])$, where $\frac{1}{p}+\frac{1}{q} = 1, k \in L^q([0, 1]^2)$....
