Norm is a function on a vector space $X$ which generalizes notion of length of vector in general vector spaces.

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Relations between p norms

The p-norm is given by $||x||_{p} = (\sum_{n=1}^{\infty }|x_{n}|^{p})^{1/p}$. For $0 < p < q$, it can be shown that $||x||_{p} \geq ||x||_{q}$ (1, 2). It sppears that in $R^{n}$ a number of ...
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Minimizing maximum absolute column sum norm of the residual between a matrix and its $k$-rank approximation

Let $X \in \mathbb{R}^{m\times n}$ be a matrix with rank $r$. How can we find the optimal $\tilde{X} \in \mathbb{R}^{m\times n}$ whose rank is $k$ where $k\leq r$ and the reconstruction error in ...
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prove that a function is an inner product

I would appreciate some assistance in answering the following problems. We are moving so quickly through our advanced linear algebra material, I can't wrap my head around the key concepts. Thank ...
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95 views

A metric on $\mathbb{R}^n$ such that $d(\lambda x, \lambda y)=|\lambda| d(x,y)$ which is not induced by a norm

Let $V=\mathbb{R}^n$. Let $d:V \times V\rightarrow \mathbb{R}$ a metric on $\mathbb{R}^n$. Assume that for any $x,y\in V$ and $\lambda \in \mathbb{R}$, we have $d(\lambda x, \lambda y) = ...
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Showing inner product comes from a norm defined using Polarization Identity [duplicate]

Possible Duplicate: Norms Induced by Inner Products and the Parallelogram Law Trying to prove if an norm satisfies the Parallelogram Law then a norm arises from an inner product. Let ...
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Norm of integral operator in $L^1$

What is the norm of the operator $$ T\colon L^1[0,1] \to L^1[0,1]: f\mapsto \left(t\mapsto \int_0^t f(s)ds\right) $$ ?
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Discrete fourier transform and Frobenius norm

I have thousands of data files which are essential DFT data of plots such as this: And a DFT of the plot with a "Hard" threshold of 0.9 gives me: This DFT is just the left top corner of the DFT ...
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Upper bound for L1-L2 optimization problem

I am interested in the following convex optimization problem: \begin{align*} \max & ||x||_1 \\ \text{s.t.} & ||x-a||_1 \le K \\ & ||b\circ x||_2 \le 1\\ & x \in R^n \end{align*} where ...
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Why is this true: $\|x\|_1 \le \sqrt n \cdot \|x\|_2$?

Stuck, help, please, I am really new to this. I opened the 2-norm and multiplied by $n$, then I am thinking to square both sides. The problem is that I do not know how to open $(x_1 + x_2 + ... + ...
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My “wrong” comparison between $\ell^2$ and $\ell^1$

For sure, $\ell^2$ is larger than $\ell^1$, because for $|x|<1$, $|x|^2<|x|,$ that is, $||x||_2\leq||x||_1.$ But using Cauchy-Schwartz inequality, I get a "wrong" comparison: ...
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190 views

Equivalence of Schatten and spectral norms

I'd like some help showing the equivalence of these two norms when $p = \log n$. Recall the $p$-th Schatten norm of a linear operator $A$ acting on $\mathbb{R}^{n}$. In the particular case of $p = ...
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243 views

Do Lipschitz-continuous funcions have weak derivatives on bounded open sets?

Let $\Omega\in\mathbb{R}^n$ be open and bounded. I'm wondering if a function $f\in C^{0,1}(\Omega)$ (a Lipschitz-continuous one) is also an element of $W^{1,2}(\Omega)$ (that is the space of weakly ...
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152 views

Different norms for space $C[0,1]$

For the space of all continuous functions we can have the sup norm: $|f|=\sup|f|$ I have also seen the following norm: $|f|=\sup|f(x)|/|x|$ I don't know what this norm is called and therefore can't ...
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Determining a bound on the relative error of a linear system

Suppose that the linear system $A\mathbf{x}=\mathbf{b}$ is perturbed so that $(A+\delta A)\mathbf{x}=\mathbf{b}$. We can calculate the relative error $\frac{\|\mathbf{x-\bar{x}\|}}{\|\mathbf{x}\|}$ ...
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Example of a sequence that converges to two different limits with respect to two complete norms

I've wondered about the following question : Is there an (explicit?) example of a vector space $X$, two complete norms $\|\cdot\|_1$ and $\|\cdot\|_2$ on $X$, and a sequence $(x_n) \subseteq X$ such ...
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Is closure of convex subset of $X$ is again a convex subset of $X$?

Today I was going through my text book exercise and got hold of the following question. Let $X$ be a normed linear space with norm $\lVert\cdot\rVert$ and $A$ is a non empty convex subset of $X$ ...
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Norm in $L^2$ space

I have a brief question regarding the norm of the $L^2$ space defined on an interval $[a,b]$. On various websites I have seen this defined as: $$\|f(x)\| = \int_{a}^{b} f(x)^2 dx$$ However, ...
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Why is $z^T x \le \|x\| \| z \|_*$ for dual norm in $R^n$?

This is probably very obvious, I was looking at this at this link. It looks so much like a Cauchy-Schwarz though. And I would say it is very obvious from the definition if it wasn't for the condition ...
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50 views

Does the norm have a specific name?

Does the norm $$\|f\|=\sup\limits_{t\in[0,T]}\int\limits^t_0|f(\tau)|\ d\tau$$ have a specific name?
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How to implement fuzzy minimum function via fuzzy maximum

I know that I can represent fuzzy max via power function(i need it in neural network) i.e. def max(p:Double)(a:Double,b:Double) = pow(pow(a,p) + pow(b,p) , 1/p) // assumption a >=0 and b >=0 ...
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inequality between entries of the vector and $l_2$ norm of the vector

Let $a=(a_1, \ldots, a_n)$ be a vector in $R^n$. I am wondering for which vectors the following would be true: $$ \|a\|_2^2\geq c \sum_{i\ne j,i,j}a_ia_j, \quad i,j=1, \ldots, n. $$ Here $c>0$ is ...
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$C^1 [0,1]$ with different norm

If the space $C^1 [0,1]$ is equiped with norm $\Vert \cdot\Vert_1$,where $$ \Vert f\Vert_1=|f(0)|+\Vert f'\Vert _{C}=|f(0)|+\sup_{t\in [0,1]}|f'(t)| $$ for any $f\in C^1 [0,1]$, is this space Banach? ...
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Matrix norms and spectral radius

Recently I started exploring convergence of some iterative methods and spotted the equivalent of the spectral radius and a matrix norm. For instance, ...
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Norm and continuous functions

Having the definition: A function $f:\mathbb{R^n}\rightarrow \mathbb{R^n}$ is proper if $\|f(x)\|$ tends to $\infty$ when $\|x\|$ tends to $\infty$. I have to show : a)If $f:\mathbb{R^n}\rightarrow ...
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172 views

Solution for a Frobenius norm inequality

Am trying to find a real scalar $\gamma$ such that for a given pair of real rectangular matrices $X,Y$ the following holds: $\frac{||Y||_{F}^{2}}{5} \leq ||\gamma X||_{F}^{2}\leq ||Y||_{F}^{2}$ ...
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72 views

Scalar multiplication and Frobenius norm

Was wondering on what would be the real number (scalar) $\gamma$ that needs to be multiplied with each entry in a real rectangular matrix $X_{m\times n}$ such that the Frobenius norm of $X$ equals a ...
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Infinity matrix norm example

I have a brief question regarding the infinity matrix norm. The subordinate matrix infinity norm is defined as: $$\|A\|_{\infty} =\max_{1 \leq i \leq n}\sum_{j=1}^{n}|a_{ij}|.$$ This is derived ...
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201 views

inequality between norms of vector

Let $a=(a_1, \ldots, a_n)$ be a vector in $R^n$. It is known that $\|a\|_{\infty}\leq \|a\|_2\leq \sqrt n\|a\|_{\infty}$. Let $k<<n$. For which kind of vectors the following would be true: $$ ...
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Proving norm inequality

There is a brief proof in my textbook that I have one question about. We are supposed to prove that $||x||_{1} \leq n||x||_{\infty}$ for $x \in \mathbb{R}^n$ The book writes the following: ...
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Inequality involving norm and inner product

I am stuck proving this trivial inequality: on a real inner product space, $(||x||+||y||)\frac{\langle x,y\rangle}{||x|| \cdot ||y||}\leq||x+y||$ I have tried to square both sides and use the Cauchy ...
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Norm of a matrix

Suppose $A$ is an $n\times n$ matrix such that $A+A^H-\delta I_n$ is positive-semidefinite, for some $\delta>0$, then can we show a bound on the norm of $A^{-1}$ ? Can we show that this the norm of ...
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Bizarre formula for arc length

I'm reading a book on computer science/math and I found this formula for arc lengths that I've not been able to decipher: $$\left|\int_p^q\left\| {df(x)\over dx} \right\| dx\right|$$ where $\lVert ...
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Equivalence of Norms Question

I have a matrix where the $L_1$ norm on an row is equal to zero. My question is what can I say about the $L_2$ norm on any row of that matrix? Numerically with the example I have, computing the $L_2$ ...
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Where is the square in the Least square regression method?

I'm having a serious doubt in the least square regression problem. I guess its got to do with the notation of norm. Is the least square formulation $||b - \mathbf{A}x||^2$ or is it $||b - ...
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Symmetrically make this matrix orthogonal, but don't you dare use the Frobenius norm…

I have read many of the questions already here in regards to the Frobenius norm, but they do not help me too much. My question is, why is the Frobenius norm not considered a 'proper' norm? In a ...
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equivalence of two definitions of norm equivalence: “$|\cdot|_1=|\cdot|_2^\alpha$” vs. “being a Cauchy sequence is the same for both norms”

Let $|\cdot|_1$ and $|\cdot|_2$ be two norms on a field $\mathbb F$. We call the two norms equivalent if every Cauchy-sequence with respect to $|\cdot|_1$ is also a Cauchy-sequence with respect to ...
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Bound on euclidean norm

Is it possible to find a suitable lower bound on $$\left|\left(tM+\sum_{k=2}^\infty\frac{(tM)^k}{k!}\right)\cdot b\right|$$ for $M$ as $n \times n$ matrix, $b$ as $1 \times n$ vector and for all $t$ ...
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How to prove an inequality for a special structure of strictly triangular matrix

The problem I cause is attached below. I am trying to prove the inequality. By using small $M$, I found that the terms on the left side of the inequality are part of the terms of the expansion on ...
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$l_1$ norm projection with regularization term

I recently encountered an optimization problem and looking for some technical paper for the same.The problem is give as below, $\min f(x)+\lambda*r(x) $ $\ s.t \ x \geq 0, ||x||_1 = 1$. where $x$ ...
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Representing with Hilbert Schmidt Norm

Am trying to see, if the following Trace function can be expressed using a Hilbert Schmidt Norm: $\operatorname{Tr}(X^TAX)$. Here, $X$ is a matrix whose entries take values that are finite and reals ...
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A basic question related with the order of convergence of iterative method

I am working on an iteration problem for computing inverse of a non singular matrix $A$ I have got following relationship between error matrix defined by $E_k = X_k-A^{-1}$. $\|E_{k+1} \|\leq ...
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Looking for an inequality related to the Cauchy-Schwarz inequality

From the Cauchy-Schwarz inequality, we can prove that $$\lVert w(x)\rVert^2_{L^2_{[0,1]}}=\int_0^1 w(x)^2\, dx \leq \sqrt{\int_0^1 w(x) \,dx}\cdot \sqrt{\int_0^1 w(x)^3\, dx}.$$ Is it possible to ...
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Is this a norm? (triangle inequality for weighted maximum norm)

I've been trying to prove that the following is a norm, but wasn't successful. I also cannot find a counterexample. So help is greatly appreciated. Let $x \in \mathbb{R}^N, \ w_i \in \mathbb{R}_+,\ ...
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Vector and matrix norm definitions?

I've questions on these four norms whose definitions I'm memorizing like this: Vector euclidean norm: $(x_1^2+x_2^2+\cdots+x_n^2)^{1/2}$ Vector max norm: $\max\{|x_1|, |x_2|, \ldots, |x_n|\}$ Matrix ...
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$L_{2}$ norm of the gradient of a vector valued function.

I have a vector valued function $U(x,y)=\Big(u_{1}(x,y),u_{2}(x,y)\Big)$. I want to find $\|\nabla U\|_{L_{2}(0,1)}$, but i could not figure how can do it. Do you have any idea?
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Bounding the $l_1$ norm of a vector

Let $x$ be real vector with $\|x\|_1=x_1+\ldots +x_{2n}$. How to bound from above $(x_1+\ldots+x_n)(x_{n+1}+\ldots+x_{2n})$ by $l_2$ norm of the vector $x$. Of course, using $\|x\|\leq\sqrt ...
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What is the connection between strong norms and norms coming from scalar products (in pre-hilbert spaces)?

In the best-approximation problem of seperation theorems in convex analysis, there is the notion of a "strong norm", in the sense that If $\| x^1 + x^2 \| = \| x^1 \| + \|x^2 \| $, $x_1 , x_2 ...
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Equivalents norms in Sobolev Spaces

I know that this is classical but I have never do the calculations to show that the norms in the sobolev space $W^{k,p}(\Omega)$ \begin{equation} \|u\|_{k,p,\Omega}= \Bigl(\int_{\Omega} ...
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Is the problem of calculating the induced norm *difficult*?

Is the problem of calculating the induced norm of a linear operator (in a finite or infinite-dimensional space) generally a difficult one ? And by difficult I mean, that there are no closed formulas ...
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Bound on inverse operator

Define $X = {C^{2, \alpha}}(U \times [0,T])$ and $Y = {C^{0, \alpha}}(U \times [0,T])$ where $U$ is some real interval. Let $F:X \to Y$ be a map. Let $DF(g):X \to Y$ be a bounded linear operator for ...