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12:13 AM
2
Q: Matrix involving reciprocal factorials

Hui ZhangLet $m$ and $n$ be two integers and $m \le n$. There are a matrix $A$ of $m$-by-$m$ with $A(i,j) = 1/(2n+2j-2i)!$ and a vector $r$ of $m$ entries with $r(i) = 2/(2n+2i)!$. Is there a formula for the inner product of $r$ and the first column of the inverse of $A$? Or, can it be shown that the inn...

 
12:35 AM
1
Q: Particular solution of this second order differential equation

A_alkI am a bit stuck on the following second order differential equation, using the method of undertermined coefficients. $$y'' + 3y' + 2y = xe^{-x}$$ The homogenous solution is easy to find, but I run into some issues with the particular solution. $y = Axe^{-x}$ doesn't seem to be a good enough gues...

 
 
2 hours later…
2:52 AM
1
Q: Unambiguous derivative notation in Spivak's "Calculus on Manifolds"

Hank I don't understand Spivak's comment at the end that $f$ means something different on the two sides of the equation. Don't they both refer to the same function? Also, the expression $D_1(f \circ (g, h))$ isn't clear about which variable should be first. The first var of $f$ is $u$, but the first ...

 
 
5 hours later…
7:46 AM
1
Q: Write sets involving random variables

StarConsider these two sets: $$ A\equiv \{x\in X: \forall \xi>0 \text{ }\exists N_{\xi, x} \text{ s.t. } \forall N\geq N_{\xi, x} \text{ } d(p_N, I(x))\leq \xi)\}, $$ $$ B\equiv \bigcap_{\xi>0} \bigcup_{N=1}^\infty\bigcap_{K=N}^\infty \{x\in X: d(p_K, I(x))\leq \xi\}, $$ where $A$ and $B$ are non-em...

 
 
3 hours later…
10:27 AM
0
Q: General formula for reversing double integral bounds

LightninBolt74The double integral over the region: $$ R = \left\{ \left( x,\: y \right) : a \leqslant x \leqslant b,\: g\left( x \right) \leqslant y \leqslant h\left( x \right) \right\} $$ is expressed as $$ \iint_R f\left( x,\: y \right) \mathrm{d}A = \int_a^b \left[ \int_{g\left( x \right)}^{h\left( x \right...

 
11:12 AM
2
Q: Density of squares using large sieves

BatrachotoxinI am reading Serre's Lectures on the Mordell-Weil Theorem, where he specifically talks about a Large Sieve inequality and proceeds to give an example. Theorem. (Section 12.1) Let $K$ be a number field, $\Lambda$ be a free $O_K$ -module of rank $n$. Let $\|\cdot \|$ be a norm over $\Lambda_{\math...

 
 
1 hour later…
12:31 PM
1
Q: Existence of Nash Equilibrium in a Game with Mixed Strategy Spaces

graphtheory123I am considering formulating an applied research problem as a simultaneous zero-sum game with two players. The first player's set of actions is an infinite and compact subset of $\mathbb{R}^n$, while the second player has a finite number of actions. For a specific action $s_2$ chosen by Player 2,...

 
 
2 hours later…
2:36 PM
2
Q: what does p capacity of set actually saying

Document123Fix $1\leq p<n$. Define, \begin{equation} K^p\equiv\{f:\mathbb{R^n}\rightarrow\mathbb{R}\ \vert\ f\geq 0, f\in L^{p^{\ast}}(\mathbb{R}^n), Df\in L^{p}(\mathbb{R}^n;\mathbb{R}^n)\}. \end{equation} If $A\subset\mathbb{R}^n$ we define the quantity \begin{equation} \text{Cap}_p(A) \equiv \inf\le...

 
2:58 PM
1
Q: What is the 3d average of a right angle?

KlausKImagine to have a right angle in 3d space, like a large capital "L" floating in space. Averaged from all observation directions, what is the average projected/visible angle of the "L" shape? (Of course, the legs of the "L" are supposed to be infinitely thin and long.) Usual disclaimer: this is no...

 
3:21 PM
2
Q: First Isomorphism theorem for rings - what am I missing

DibidusI am writing some strict algebra notes and am stuck with the most basic stuff, as is the First Isomorphism theorem for Rings. I am working over commutative rings with unity. The problematic part of the formulation of the First Isomorphism theorem for Rings says that if $f: R \to S$ is a ring homo...

 
 
2 hours later…
4:52 PM
0
Q: Definition of "interval of continuity" for function defined on sets

Greg MartinAt the beginning of Chapter 8 of Kubilius's Probabilistic Methods in the Theory of Numbers, the author defines $Q=Q(E)$ to be a completely additive nonnegative function defined for all Borel subsets $E$ of $\Bbb R^n$ (that is, some kind of measure defined on the Borel subsets of $\Bbb R^n$). The ...

 
 
4 hours later…
9:01 PM
5
Q: How to apply Ito's Formula to show that this is a martingale?

DovahkiinIn the book Brownian Motion, Martingales and Stochastic Calculus by J.F. Le Gall, in order to give an alternatice derivation of the distribution of $L_{U_{a}}^{0}(B)$ where $L^{0}_{t}(B)$ is the Local-Time at $0$ of a Standard Brownian Motion and $U_{a}=\inf\{t:|B_{t}|\geq a\}$, he states as a re...

4
Q: What problems will arise if we define matrix multiplication this way?

pie$a\in \mathbb{R}^n$ $a=(a_1,a_2,\dots,a_n)$ lets define this to be equivalent to $(a_1,a_2,\dots,a_n,0,0,0 \dots,0)$ (finite many zeros) by this I think we can make an $n \times m$ matrix $A$ equivalent to any $ p \times p$ $B$ matrix with $p\ge \max\{n,m\}$ by $B_{ij}= A_{ij}$ if $1\le i\le n$...

 
9:51 PM
5
Q: Stuck on Differential Geometry proof

querrymanMy concrete questions are (for context see below): Is is true that $i$ as below embeds $M$ in $T^*\mathbb{R}^n$? Is it true that $M$ is Lagrangian in $\mathbb{C}^n$ if and only if $i(M)$ is Lagrangian in $T^*\mathbb{R}^n$? Is it true that Theorem 4.3 implies that $$dh=\text{proj}_{d\psi}y\iff dh...

 
 
2 hours later…
11:50 PM
1
Q: Probability distribution of stochastic process with independent increments asymptotically approaching Gaussian

BulworthI am independently working through van Kampen's Stochastic Processes in Physics and Chemistry. It's a rewarding book to work through, but I am having trouble with question 46 on pg. 89. Below is the question (screenshot also attached). Recommended reading on the subject also appreciated. A Marko...

 

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