Questions on proving and manipulating inequalities.

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-4
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3answers
68 views

How to solve this: $|3-x|\ge2$

How to solve $|3-x|\ge2$ ? I know that if $|x| < y$, then $-y < x < y$. But in this case what to do? Thanks. Here, $|x|$ is the absolute value of $x$.
1
vote
2answers
73 views

$-\varepsilon\log(x)\overset{?}{\geq} -\log(\varepsilon x)$

I'm refering to this proof: http://en.wikipedia.org/wiki/Quantum_relative_entropy#The_result In there it's stated that "Since the matrix $(P_{ij})_{ij}$ is a doubly stochastic matrix and $-\log$ is a ...
2
votes
2answers
381 views

How prove this inequality $f(x)\ge f(0)$

Question: let $a>b>c>0,n\in N^{+},n\ge 2$ be given numbers,show that: ...
3
votes
1answer
62 views

What is the solution to $\frac1{a^2 +2} + \frac1{b^2 +2} + \frac1{c^2 +2} \le \frac{\sqrt2}{2}\frac{\sqrt a+\sqrt b+\sqrt c}{\sqrt{abc}}$

one of my friends asked me if I could solve him a mathematics problem. It looks like this: $$\frac1{a^2 +2} + \frac1{b^2 +2} + \frac1{c^2 +2} \le \frac{\sqrt2}{2}\frac{\sqrt a+\sqrt b+\sqrt ...
0
votes
1answer
522 views

generalized inequalities defined by proper cones

The generalized inequality defined by a proper cone $K$ is that $x \ge_{K} y$ if $x-y \in K$ for $x,y \in K$. Does this means that for any $x \in K$, we have $x \ge_{K} 0$ since $x - 0 = x \in K$ ? ...
3
votes
1answer
37 views

Range of $f(x)=\frac{\sin x -1}{\sqrt{3-2\cos x-2\sin x}}$ for a specified domain

We are asked to find the range of the function $$f(x)=\frac{\sin x -1}{\sqrt{3-2\cos x-2\sin x}}, \;\;\text{for}\;0\le x\le2\pi$$ I tried to find the range of each basic function of cos and sin then ...
0
votes
1answer
41 views

Use mean value theorem on $f(x) = x^{1/5}$, to show that $2< \sqrt[5]{33}<2.0125$

The problem specifically aks us to use mean value theorem on the interval $[32, 33]$ It has always puzzled me that mean value theorem can be used to prove Inequalities. Can anyone show how mean ...
0
votes
2answers
26 views

Spivak Absolute Value Problem (Prologue 9-v)

I'm working on the following problem Express the following with at least one less pair of absolute value signs $$|(| \sqrt2 + \sqrt3| - |\sqrt5 - \sqrt7|)|$$ Now I can see that the ...
0
votes
0answers
37 views

What is the meaning behind this mathematical rebus? [closed]

I believe I saw this 'inequality' in someone's profile description here on Math Stack Exchange. I think it expresses a message or has a meaning. What is it?
3
votes
8answers
161 views

How to show that $(a+b)^p\le 2^p (a^p+b^p)$ [duplicate]

If I may ask, how can we derive that $$(a+b)^p\le 2^p (a^p+b^p)$$ where $a,b,p\ge 0$ is an integer?
1
vote
1answer
27 views

Equality case in the Frobenius rank inequality

In many linear algebra books, the following rank inequalities are found: Frobenius inequality Let $A$, $B$ and $C$ be three matrices such that the product $ABC$ is defined. Then ...
1
vote
1answer
52 views

A simple complex inequality

I feel this is not hard, but no way to prove it $|\sqrt{z^2 -4}-z|\le 2$ Any body can help? Thanks! The total statement should be one of the branchs of square root should satisfy this ...
0
votes
1answer
261 views

Irrational inequalities question: $\sqrt { -3x+1 } + \sqrt {6x+1} < \sqrt {3x+4}$ and $\sqrt { -6x+10 } + \sqrt {-x+2} \gt \sqrt {4x+5}$

Consider the following inequalities: $\sqrt { -3x+1 } + \sqrt {6x+1} \lt \sqrt {3x+4}$ $\sqrt { -6x+10 } + \sqrt {-x+2} \gt \sqrt {4x+5}$ Attempt at a solution; after performing all the ...
1
vote
2answers
73 views

prove ${a_n} = \root n \of {n!} $ is monotonically increasing to $\infty$

Prove ${a_n} = \root n \of {n!} $ is monotonically increasing to $\infty$ I already showed that $a_n$ diverges to infinity like this: I used to the lemma which says that if ...
-8
votes
2answers
60 views

How to solve an irrational inequality?

How to solve the following inequality: $$\sqrt{1-2x} < \sqrt{4 - x}$$ I don't understand why "$(1-2x)$ have to be $\ge 0$". If it was the rule for numbers inside a square root, I was checking ...
2
votes
1answer
80 views

Inconventional Integral inequality

$$\int_a^bw(x)|f(x)||g(x)|\;dx \le \left(\int_a^bw(x)\;dx\right) \max_{a\le x\le b}|f(x)|\cdot \max_{a\le x\le b}|g(x)|$$ I don't really understand this integral inequality. How do I go about ...
4
votes
1answer
307 views

Stochastic integral inequality

Let $W_t$ be a Brownian motion with $m$ independent components on $(\Omega,F,P)$. Let $G(\omega,t)=[g_{ij}(\omega,t)]_{1\leq i\leq n,1\leq j\leq m}$ in $V^{n\times m}[S,T]$ such that ...
8
votes
4answers
173 views

Inequality question­

$$a,b,c,d\ge 0$$ $$a\le 1$$ $$a+b\le 5$$ $$a+b+c\le 14$$ $$a+b+c+d\le 30$$ Prove that $\sqrt{a}+\sqrt{b}+\sqrt{c}+\sqrt{d}\le 10$. We can subtract inequalities to get the answer, but that is ...
2
votes
3answers
111 views

Proving that one of $a(1-b), b(1-c), c(1-a) \le \frac{1}{4}$

how can a prove that at least one of those is less than or equal to 1/4. $$\forall a,b,c\in \mathbb R^+, \ a(1-b)\leq 1/4 \lor b(1-c) \leq 1/4 \lor c(1-a) \leq 1/4.$$ help please!
0
votes
0answers
55 views

Prove the inequality $\sum_{i,j = 1}^n {{A_{i,j}}({x_i}^2 - {x_i}{x_j})} \ge 0 $

A is a square matrix with positive elements and x is a real vector (both of them n>1 dimensional). Prove that for any such matrix and vector $$\sum\limits_{i,j = 1}^n {{A_{i,j}}({x_i}^2 - {x_i}{x_j})} ...
3
votes
0answers
41 views

A Cauchy-Schwartz type inequality

Given positive integers $k<n$ and positive real numbers $x_1$, $x_2, \dots, x_n$. Denote $$ A={x_1\over x_2+x_3+\dots+x_{k+1}}+{x_2\over x_3+x_4+\dots+x_{k+2}}+\ldots+{x_n\over x_1+x_2+\dots+x_k}$$ ...
3
votes
1answer
108 views

inequality $(a+c)(a+b+c)<0$, prove $(b-c)^2>4a(a+b+c)$

If $(a+c)(a+b+c)<0,$ prove $$(b-c)^2>4a(a+b+c)$$ I will use the constructor method that want to know can not directly prove it?
1
vote
2answers
46 views

Questioning a Proof of Khinchine Inequality

[Khinchine Inequality] Let $a_1,\ldots,a_n\in R$, $\varepsilon_1,\ldots,\varepsilon_n$ be i.i.d. Rademacher random variables: $P(\varepsilon_i=1)=P(\varepsilon_i=-1)=0.5$, and $0<p<\infty$. Then ...
0
votes
3answers
25 views

Can the proof for the following 4 cases be simplified to 2 cases?

Let $X$ and $Y$ be finite and disjoint sets. Suppose we are required to prove the following: $|X|\ge 0 \text{ and } |Y|\ge 0 \Rightarrow Q $ where $Q$ is some statement. Therefore, I know I need to ...
0
votes
1answer
70 views

Show that $\int_{\pi/4}^{\pi/2} \frac{\sin x}{x}\,dx\leq \frac{\sqrt{2}}{2}$

Show that $$\int_{\pi/4}^{\pi/2} \dfrac{\sin x}{x}\,dx\leq \dfrac{\sqrt{2}}{2}$$ Any Ideas, how to start ?!
0
votes
1answer
35 views

Geometric Application of Cauchy-Schwarz Inequality Problem

I have been struggling with this problem, and would like to prove the inequality using the Cauchy-Schwarz Inequality: The vertices of a fixed triangle are $A$,$B$ and $C$, and $P$,$Q$ and $R$ lie on ...
0
votes
1answer
65 views

How find this maximum $\sum_{i=1}^{n}(x^3_{i}-x_{i}x_{i+1}x_{i+2})$,$x_{n+1}=x_{1},x_{n+2}=x_{2}$

let $n$ is give postive integer numers,and $x_{i},i=1,2,\cdots,n$ be real numbers,and such $$0\le x_{i}\le i,i=1,2,\cdots,n$$ Find the maximum of the value ...
4
votes
3answers
402 views

An inequality in numbers

Which number is larger? $\underbrace{888\cdots8}_\text{19 digits}\times\underbrace{333\cdots3}_\text{68 digits}$ or $\underbrace{444\cdots4}_\text{19 digits}\times\underbrace{666\cdots67}_\text{68 ...
2
votes
1answer
14 views

Bounds on a recursively defined sequence

I have a sequence defined by $h_0=h_1=1$, $h_2=2$ and $h_{n+1}=(n+1)h_n + \frac{n(n-1)}{2}$. The paper I'm reading claims $n! \le h_n \le 2(n!)$. It is easy to show the first inequality by induction. ...
3
votes
0answers
62 views

Upper bound for the sums of powers of factors

Fix $\alpha \in \,]0,1]$. Is it true that for each sufficiently large positive integer $n$, if $n = x_1 \cdots x_j$, for some integers $x_1, \ldots, x_j \geq 2$, with $j \geq 2$, then $$x_1^\alpha + ...
2
votes
1answer
34 views

Number of integers satsifying inqualities with logarithm

I am trying to solve the problem of finding the integers x satisfying the inequalities: $2\lt log_x45\lt3$ I realize this is a very basic question on logarithms and I have the key with the answers 4, ...
5
votes
4answers
143 views

How find the maximum of the $x^3_{1}+x^3_{2}+x^3_{3}-x_{1}x_{2}x_{3}$

Let $$0\le x_{i}\le i,\, i=1,2,3$$ be real numbers. Find the maximum of the expression $$x^3_{1}+x^3_{2}+x^3_{3}-x_{1}x_{2}x_{3}$$ My idea: I guess $$x^3_{1}+x^3_{2}+x^3_{3}-x_{1}x_{2}x_{3}\le ...
10
votes
1answer
299 views

A unsolved puzzle from Number Theory/ Functional inequalities

The function $g:[0,1]\to[0,1]$ is continuously differentiable and increasing. Also, $g(0)=0$ and $g(1)=1$. Continuity and differentiability of higher orders can be assumed if necessary. The ...
2
votes
1answer
79 views

Homework on basic inequalities.

Let $a_j$ be a sequence of positive reals. Show that (a) $\left(\sum_{j=1}^\infty a_j\right)^\theta \le \sum_{j=1}^\infty a_j^\theta$ for any $0\le\theta\le1$. (b) $\sum_{j=1}^\infty a_j^\theta \le ...
4
votes
1answer
58 views

how to solve this elementary induction proof

this is a seemingly simple induction question that has me confused about perhaps my understanding of how to apply induction the question; $$\frac{1}{1^2}+ \cdots+\frac{1}{n^2}\ \le\ 2-\frac{1}{n},\ ...
3
votes
0answers
72 views

Here is a tough inequality want a concise proof.

I was just encountered an inequality in AoPs, Here it is: http://www.artofproblemsolving.com/Forum/viewtopic.php?f=51&t=4569&view=next, that is, if $a,b,c$ are positive numbers, then we ...
4
votes
2answers
74 views

Binomial expansion inequality

In a paper I am reading, there is a step that seems to come from the following inequality: $$(1+x)^\alpha \le 1+2^\alpha x,$$ where $0<x<1$. (Also, $3\le \alpha \le 9/2$ in the context of the ...
1
vote
1answer
30 views

Help to prove the condition that a right half-open interval is not empty

The right half-open interval is defined as: $[a,b) = \{x \in \mathbb{R}|a \le x \lt b\}$ I need to prove: $[a,b) \ne \emptyset \iff a<b$ My attempt: For $\Rightarrow$: $$\begin{align} ...
7
votes
1answer
1k views

Inequalities in $l_p$ norm

I'm having difficulty with the following problem. Any help would be appreciated. Problem: Consider the sequence spaces $l_p$ with the usual norm. If $1\le p\le q\le \infty$, I want to show the ...
2
votes
3answers
164 views

How can this equality be established by elementary algebraic means?

Let $x \geq 1$. Then is it true that $2x^3 - 3x^2 + 2 \geq 1$? If so, how can I show this using only elementary ideas such as factorisation? Of course, I can demonstrate this using the methods of ...
0
votes
1answer
47 views

Inequality: $2\sqrt{xz}+2\sqrt{yz}+2\sqrt{xy}\geq 3x+3y+3z-3$

Let $x,y,z$ be nonnegative real numbers satisfying $x^2+y^2+z^2=1$. Prove that: $2\sqrt{xz}+2\sqrt{yz}+2\sqrt{xy}\geq 3x+3y+3z-3$ I have tried squaring both sides to get: ...
0
votes
1answer
26 views

Relation between the mean value inequality over an area and over a surface

Suppose that $f$ is a locally integrable function on $\mathbb{R}^{N}$ $(N\geq2)$ such that for all $x$ in $\mathbb{R}^{N}$ and all positive real number $r$ we have \begin{equation} f(x)\leq ...
8
votes
1answer
471 views

How prove this inequality $a+b+c=3$

Let $a,b,c$ be nonnegative real numbers, no two of which are zero such that $a+b+c=3.$ Prove that $$ \dfrac{a}{5b+c^3}+\dfrac{b}{5c+a^3}+\dfrac{c}{5a+b^3} \geq \dfrac{1}{2}$$ I think this inequality ...
3
votes
0answers
30 views

Inequality with three variables

Let $a,b$ and $c$ be three real positive numbers. Prove that $$\sum_{sym}\frac{1}{a^2+4b^2+9c^2}\le\frac{9}{14}(a^2+b^2+c^2)$$ I tried to use Cauchy-Schwartz : ...
13
votes
2answers
306 views

Comparing sums of surds without any aids

Without using a calculator, how would you determine if terms of the form $\sum b_i\sqrt{a_i} $ are positive? (You may assume that $a_i, b_i$ are integers, though that need not be the case) When there ...
1
vote
3answers
41 views

How do I solve: $6(x^2+2)<17x$

How do I solve this kinds of inequality. I can do it if all the 'x' is in one side. However, this one have x at both sides of the equation. And we don't know whether it's a positive or negative value. ...
0
votes
1answer
22 views

How to prove this elementary “ interpolation” inequality?

Suppose $2<p<\infty$ and $0<\theta<1$. Let $n\geq 1$ be an integer. Assume that $$ \frac{1}{p}=\frac{1-\theta}{2^n}+\frac{\theta}{2^{n+1}}. $$ How to prove the following inequality $$ ...
0
votes
1answer
24 views

Prove this statement (inequality)

$x,y,z$ $\in\mathbb R$ then $|x-y| \leq|x-z|+|y-z|$ Prove this statement. I thought it was the triangle inequality, but I can't seem to end up with the correct order.
2
votes
1answer
43 views

Area of a triangle whose each side is less than 2 and greater than1.

What is the area of a triangle if each of its sides is greater than 1 and less than 2? My Try:Let a,b,c be the sides of triangle,then ...
1
vote
1answer
34 views

How find the minimum of the $|w^3+z^3|$,if $|z+w|=1,|z^2+w^2|=14$

let complex $z,w$ such $$|z+w|=1,|z^2+w^2|=14$$ find the minimum of the value $$|w^3+z^3|$$ My idea: let $$z=a+bi,w=c+di\Longrightarrow z+w=(a+c)+(b+d)i,z^2+w^2=(a^2+b^2+c^2+d^2)+2(ab+cd)i$$ then we ...