# All Questions

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### graph product that commutes with automorphism, and semi direct

Is there a way to construct a "product" of graphs $G\rtimes H$ such that $Aut(G \rtimes H ) \simeq Aut(G) \rtimes Aut(H)$? A related topic is "Semidirect product" of graphs? but not quite ...
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### A few questions about KSVD algorithm (dictionary learning) in a paper

To learn more about dictionary learning, I am currently trying to understand the concept in detail and to do so, I've found the following paper quite informative: KSVD: an algorithm for designing ...
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### Why are diffeomorphism-invariant PDE not elliptic?

In reading geometric analysis papers, I frequently encounter a statement of the form, "The PDE in question is diffeomorphism-invariant, and therefore cannot be elliptic." My vague understanding is ...
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### Dixon's Factorization Method Modulo Question

Looking at Wikipedia's example for Dixon's Factorization Method, it shows the following. We will try to factor N = 84923 using bound B = 7. Our factor base is then P = {2, 3, 5, 7}. We then search ...
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### Without using Heegner-Stark-Baker, $\mathbb{Q}(\sqrt{-11})$ has class number $1$.

Prove that $\mathbb{Q}(\sqrt{-11})$ is of class number $1$. I have found that the ideal $(2)$ of the integer ring $\mathbb{Z}[(1 + \sqrt{-11})/2]$ of $\mathbb{Q}(\sqrt{-11})$ is a prime ideal. ...
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### Overview of nonlinear analysis, differential equations (ODE and PDE), dynamical systems, and mathematical physics, and their relationships

The fields of (i) nonlinear analysis, (ii) ODE and PDE, (iii) dynamical systems, and (iv) mathematical physics are very huge, fertile, and, in a sense, unorganized (see Open problems in ...
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### Characterization of open maps in terms of nets

Here I asked about characterization of closed maps in terms of nets/sequences. I find this view illuminating, so I wanted to ask about open maps. A map $f: X \to Y$ is open if for each $x \in X$ and ...
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### Finf f such that $F \circ F$ is a primitive of f

Find all primitivable functions $f:\mathbb{R} \to \mathbb{R}$ that admits a primitive $F:\mathbb{R} \to \mathbb{R}$ for which $F\circ F$ is a primitive of $f$. From $(F \circ F)'=f$ we get that ...
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### Comparing metric tensors of the Poincare and the Klein disk models of hyperbolic geometry

I was trying to compare the metric tensor at the wikipedia pages of the Beltrami Klein model https://en.wikipedia.org/wiki/Klein_disk_model and the metric tensor of the Poincare disk model at ...
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### Visual proof of $\sum_{n=1}^\infty \frac{1}{n^4} = \frac{\pi^4}{90}$?

In a gorgeous paper "How to compute $\sum \frac{1}{n^2}$ by solving triangles", Mikael Passare offers this idea for proving $\sum_{n=1}^\infty \frac{1}{n^2} = \frac{\pi^2}{6}$: Proof of equality ...
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### Sum of i.i.d. random variables and finding an upper bound

Problem: Suppose that $(X_i)_{i\in\mathbb{N}^+}$ is a sequence of i.i.d. random variables. For some $n\in\mathbb{N}^+$, let $S_n=\sum_{i=1}^n X_i$. Furthermore, let $a$ be a positive constant, and ...
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### The closure of an open set in $\mathbb{R}^n$ is a manifold

I want to solve the following exercise from M. Spivak's Calculus on Manifolds (p. 114): (a) Let $A \subseteq \mathbb{R}^n$ be an open set such that boundary $A$ is an $(n-1)$-dimensional manifold. ...
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At the bottom of page 99 of M. Spivak's Calculus on Manifolds he arrives at the formula $$\partial (\partial c)=\sum_{i=1}^n \sum_{\alpha=0,1} \sum_{j=1}^{n-1} \sum_{\beta=0,1} ... 0answers 61 views +100 ### Behavior of a Solution to Heat equation Compactly Supported in Time We say that a function g\in C^{\infty}(\mathbb{R}^{n}) is of Gevrey class G^{\sigma}(\mathbb{R}^{n}) if for each compact K\subset\mathbb{R}^{n}, there exist C,R>0 such that$$\sup_{x\in ...
I am having trouble understand how to classify what happens to solutions of ODE near singular points. For example, I have a question that is about the ODE; $$(x^2-36)y''+(6-x)y'+(x^2+12x+36)y=0$$ ...