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 Forum: Real Analysis   Topic: Convergence of Series

 Post subject: Re: Convergence of Series
Posted: Thu Jul 07, 2016 1:06 pm 

Replies: 1
Views: 434


We will first prove that \(a_n>0\) for all \(n\in\mathbb{N}\). For \(n=1\), \(a_{1}=\alpha>0\), supposing that \(a_n>0\) for all \(n\in\mathbb{N}\), \(a_{n+1}=a_{n}{\alpha}^{a_{n}}>0\) which proves the induction. If \(\bullet\) \(\alpha=1\), then \(a_n=c\) for all \(n\in\mathbb{N}\) and some constan...

 Forum: Analysis   Topic: Double integral involving the signum function

Posted: Thu Jul 07, 2016 12:54 pm 

Replies: 2
Views: 743


Computation of \(\displaystyle \int_{0}^{\infty}\int_{0}^{\infty}\mathbb{e}^{-\frac{x^2+y^2}{2}}\sin(xy)\;\mathbb{d}x\;\mathbb{d}y\). ---------------------------------------------------------------------------------------------------------------- \[\begin{eqnarray*}\int_{0}^{\infty} \int_{0}^{\infty...

 Forum: Analysis   Topic: Double integral involving the signum function

Posted: Thu Jul 07, 2016 12:52 pm 

Replies: 2
Views: 743


A small comment, with a very brief examination we can easily deduce that $$\int_{-\infty}^{\infty}\int_{-\infty}^{\infty}{\rm{sign}}(x)\,{\rm{sign}}(y)e^{-\frac{x^2+y^2}{2}}\sin(xy) \;dx\;dy=4\int_{0}^{\infty}\int_0^\infty e^{-\frac{x^2+y^2}{2}}\sin(xy)\;dx\;dy\,.$$

 Forum: Analysis   Topic: Identity regarding the euler gamma costant

Posted: Thu Jul 07, 2016 12:42 pm 

Replies: 2
Views: 782


By definition \[\displaystyle \Gamma_{p}(x):=\frac{p^{x}p!}{x(x+1)(x+2)\cdot\ldots\cdot (x+p)}=\frac{p^{x}}{x\left(1+\frac{x}{1}\right)\left(1+\frac{x}{2}\right)\cdot\ldots\cdot\left(1+\frac{x}{p}\right)}\] and \[\Gamma(x):=\displaystyle\lim_{p\to \infty}\Gamma_{p}(x)\,.\] \begin{align*} p^{x}&=...

 Forum: Analysis   Topic: Identity regarding the euler gamma costant

Posted: Thu Jul 07, 2016 12:38 pm 

Replies: 2
Views: 782


Prove that \(\displaystyle \lim_{n\to \infty} \mathcal{H}_{n}-\log(n)=-\int_{0}^{\infty}e^{-t}\log(t)\;dt\).

 Forum: Calculus   Topic: Derivative of a Power Series

Posted: Thu Jul 07, 2016 12:28 pm 

Replies: 2
Views: 506


We have the series \(\displaystyle \sum_{k=1}^{\infty}\frac{(-1)^{k+1}x^{2k-1}}{(2k-1)(2k-1)!}\), now it is obvious that \(\displaystyle \limsup_{n\to\infty}\left|\frac{(-1)^{n+1}}{(2n-1)(2n-1)!}\right|=0\), so the radius of convergence is infinite and the interval we seek is \(\mathbb{R}\). If \(\d...

 Forum: Analysis   Topic: Non periodic function!

 Post subject: Non periodic function!
Posted: Thu Jul 07, 2016 12:16 pm 

Replies: 3
Views: 798


Prove that \(\sin\left(x^3\right)\) is a non-periodic function.

 Forum: Complex Analysis   Topic: Argument Proof

 Post subject: Argument Proof
Posted: Thu Jul 07, 2016 12:11 pm 

Replies: 0
Views: 401


If \(z_1\) and \(z_2\) are the roots of the polynomial \(ax^2+bx+c\) with real coefficients and \(b^2<4ac\) prove that if \(z_1=\overline{z_{2}}\) then $$\arg\left(\frac{z_{1}}{z_{2}}\right)=2\arccos\left(\sqrt{\frac{b^2}{4ac}}\right)\,.$$

 Forum: Probability & Statistics   Topic: Inequality

 Post subject: Re: Inequality
Posted: Sat Mar 05, 2016 11:37 am 

Replies: 2
Views: 1534


$$\begin{eqnarray*}f\left(\mathbb{E}[\mathbb{X}]\right) &=& f\left(\sum_{i}x_{i}\cdot p(x_i)\right)\\&=&f\left(\sum_{i}\left(\frac{1}{2}x_{i}+\left(1-\frac{1}{2}\right)x_i\right)\cdot p(x_i)\right) \\ &\leq& \frac{1}{2}\sum_{i} f(x_i)p(x_i) + \frac{1}{2}\sum_{i} f(x_i)\cdot p...

 Forum: Combinatorics   Topic: Flavor Combinations!

 Post subject: Flavor Combinations!
Posted: Mon Jan 18, 2016 4:12 am 

Replies: 1
Views: 716


An ice cream store has 20 different flavours. In how many ways can we order a dozen different ice cream cones, if each cone has 2 different flavours?
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