All Questions
2,026 questions with no upvoted or accepted answers
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30
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Signs of difference matrices (sum of submatrices)
Given matrix $A \in \mathbb{R}^{m \times n}$, are there any results related to its difference array
$$A^* \triangleq \left[sign(a_{i,j} + a_{r, s} - a_{r, j} - a_{i, s})\right]_{i<r, j<s}?$$
Or ...
- 75
0
votes
0
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45
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On full rank submatrices of a construction
Take two matrices $T_1$ and $T_2$ in $\mathbb Z^{n\times n}$ with entries uniformly in $[-b,b]\cap\mathbb Z$ at some $b>0$. The matrices will be of rank $n$ each with probability at least $1-\frac1{...
- 1,826
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0
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47
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"Probability" for a partitioned matrix to be singular
Let $A,B\in\mathbb{R}^{n\times n}$ be two nonsingular matrices with $A\ne B$, and consider the following partitioned matrix
$$
M:=\begin{bmatrix}AA^\top + BB^\top & A^\top \Delta_1 A + B^\top \...
- 2,712
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0
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101
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Find occurrences of certain matrix inside a matrix
This problem occurred from my need to find all graphs with a certain topology inside a bigger one. I don't need the subgraphs but the graphs that have the exact topology I am searching.
We know for ...
- 101
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0
answers
92
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A semifield of characteristic zero may have a finite number of elements
A commutative semiring $(S, +, \cdot, 0, 1)$ with unity is said to be a semifield if for all $a, b\in S$, $a+b=0$ implies that $a=0$ and $b=0$, and $a.b=0$ implies that either $a=0$ or, $b=0$.
I ...
- 203
0
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0
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50
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Generalized eigenvectors product
Let's consider a real square matrix $A$ with eigenvalues $\lambda_n$ and eigenvectors $\mathbb x_n$, i.e. $A \mathbb x_n = \lambda_n \mathbb x_n$.
Suppose there are some generalized eigenvectors $\...
- 141
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0
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59
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A system of inequalities involving a skew-symmetric integer matrix
Which skew-symmetric integer matrices $S$ satisfy the following inequalities
$SV_i \ne z_iE_i$ for all $i = 1,\cdots, n$
where
$V_i$ denotes the column with integer entries such that the $i$-th ...
- 356
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0
answers
41
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Orthogonality condition of symmetric matrix pencil
Let $P(\lambda)=\lambda M−L\in \mathbb{R}^{n \times n}$ be a matrix pencil with symmetric nonsingular matrix $M$ and $L$ is a weighted Laplacian matrix of a connected graph. Clearly $(0,1_n)$ is an ...
- 21
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286
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Complexity of pseudo-inverse of random matrix
Assume that $\mathbf{A}_{M\times N}$ is a sparse complex matrix. Then, what is the complexity of computation of its pseudo inverse, i.e.,
$$\mathbf{A}^{\mathrm{H}}(\mathbf{A}\mathbf{A}^{\mathrm{H}})^{-...
- 287
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votes
0
answers
113
views
Special element of a commutative ring
Let $R$ be a commutative ring with $1$ and $S $ be a multiplicative subset of $R $. I am looking for an equivalence condition for the following property in $R $:
Property: There exists a fixed ...
- 1
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0
answers
352
views
Spectral norm of difference of quadratic matrices restricted to a subspace
Say that we have two matrices $X$ and $Y$ of dimensions $(T \times N)$ with $N < T$ and $rank(X)=rank(Y)=N$. Furthermore, define a $(T \times k)$ dimensional matrix $D$ with $k<N$ and $rank(D)=k$...
- 41
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0
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96
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Eigenvalues of the matrix obtained by letting some of the rows vanish, hoping for some inequality
Let $A$ be an $n \times n$ matrix. Let $A_k$ be the matrix obtained by keeping the first $k$ rows of $A$ fixed and substituting $0$ for the rows $k+1$ to $n$. To be precise, we write $A= [R_1...R_k, ...
- 1,465
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400
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Comparison of two similarity matrices
English is not my first language, so please excuse any mistakes.
I'm working with two similarity matrices on the same data set: Suppose I have $n$ items, and I calculated the similarity of each item ...
- 1
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0
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188
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A gap problem in elementary additive combinatorics
Given $a,b\in\mathbb N$ define the set $$\chi(a,b)=\{M\in\{0,1\}^{n^a\times n^b}:\mbox{ every row of }M\mbox{ is distinct}\}.$$
Also given ${\bf{x}}=(x_1,\dots,x_{n^b})\in\mathbb Z^{n^b}$ define the ...
- 1,826
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95
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How to maximum L1 norm problem?
I have met a problem these days.
\begin{equation}
\underset{\omega}{\max} \quad \Vert \text{diag}(\mathbf{h}^H)\mathbf{G}^H\mathbf{\omega}\Vert_1 \\
s.t.\quad\mathbf{\omega}^H\mathbf{G}\mathbf{G}^H\...
- 171
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0
answers
207
views
Sylow subgroups of orthogonal group
According to Wikipedia (current revision) the cardinality of $O(n,q)$ depends on the properties of the field we're working over. These are the results:
We have the following formulas for the order ...
- 109
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0
answers
124
views
What do we know about this ideal of the group algebra?
Let $G$ be a torsion-free amenable group. Consider, $\mathbf M$, the collection of all multiplicative functionals on $\mathbb CG$, the complex group algebra of $G$. So, $\ker\phi$ is an ideal of $\...
- 2,118
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223
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Solving a nonlinear matrix equation
Consider the following nonlinear matrix equation:
$B=PX^{−1}AX$
where $B$ and $P$ are a $1\times n$ row vector and $A$ is a $n\times n$ matrix which are all strictly positive, and $X=diag(x_1,...,...
- 101
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0
answers
142
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Jordan Decomposition of Sparse matrix
Suppose we are given $n \times n$ rational matrix, $A$ with at most $k$ nonzero elements in each row and each column with $k \ll n$.
What is the best algorithm to compute its Jordan decomposition? ...
- 1,503
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51
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Induced grading on free non-associative algebra
Let $A_X$ be a free non-associative algebra over a field $k$ on a set $X$ of free generators, where $X = X_0 \cup X_1$ and $X_0 \cap X_1 = \phi$. We will think of the elements of $X_0$ as even, and ...
- 1,269
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129
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Homomorphisms from $k[x,y]$ to $k[x,x^{-1},y]$
Let $k$ be a field of characteristic zero and let $R_{-1}:=k[x,x^{-1},y]$
be the $k$-algebra of polynomials in $x,y$ containing the inverse of $x$, denoted by $x^{-1}$.
Let $f: k[x,y] \to R_{-1}$ be ...
- 2,837
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0
answers
59
views
Dimension reduction
$A=({B}\otimes{I_{k}})C$ where $B$ is a $N$x$r$ matrix with rank $r$, and $C$ is a $rk$x$rk$ symmetric matrix
$M=DAE$ where $D$ is a $Nk$ x $Nk$ symmetric matrix and $E$ is a $rk$x$rk$ symmetric ...
- 31
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0
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74
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Commutative rings with many nilpotent elements and efficient computation
Let $K$ be commutative ring. Assume that for natural $n$
there are $n$ nilpotent elements $y_i \in K$ satisfying
$y_i^2=0, y_i y_j=y_j y_i \ne 0$ and $\prod_1^ny_i \neq 0$.
Is it possible to compute ...
- 25.4k
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0
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643
views
A new generalization of the dimension?
During my research, I came a cross on these notions :
Definition 1: A structure $S$, is a pair $(X, \mathcal T)$ with $X$ a set and $\mathcal T$ a set of subsets of $X$, stable by arbitrary ...
- 4,074
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0
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330
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Krull dimensions and regular sequences
I am trying to understand if a certain condition on quotient rings is sufficient for a sequence to be regular. Here is the setting:
Let $\mathbb{C}[u_1,...,u_n]$ be the ring of regular functions on $\...
- 1,227
0
votes
0
answers
107
views
Numerical error on the spectrum of a matrix
Let $Q=(q_{j,k})_{1\le j,k\le N}$ be a (Hermitian) $N\times N$ matrix with complex-valued entries. The matrix $Q$ is given numerically and the absolute error on each entry is bounded above by a (small)...
- 16.2k
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0
answers
35
views
Solutions to this equation of the form $A(t_1,t_2)x = b(t_2)$
Given two symmetric positive definite matrices $L_1, L_2\in\mathbb{R}^{n\times n}$ and a vector $w\in\mathbb{R}^n$, under what conditions does the following system of equations of the form $M_1(t_1,...
- 403
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0
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189
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The algebraic connectivity of $P_n$, the path on $n$ vertices does not exceed $\frac{12}{n^2-1}$
The algebraic connectivity of $P_n$, the path on $n$ vertices does not exceed $\frac{12}{n^2-1}$.
Let algebraic connectivity of $P_n$ be denoted by $\mu$. I have proved a result that if $G$ is a ...
- 199
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0
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224
views
Upper bound on matrix perturbation such that all eigenvalues lie within the unit circle
Consider the matrix
$$N=\left[\matrix{\mathbb{I}_n-\epsilon L & X\\ \epsilon Y & Z}\right]$$
where $\epsilon>0$ is a small positive parameter and $Z$ is a square $m\times m$ matrix with ...
- 101
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votes
0
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66
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Tameness of the trivial extension of a finite dimensional algebra
The trivial extension T(A) of a (finite dimensional) algebra A is representation-finite if and only if the algebra A is iterated tilted of Dynkin type.
Questions:
Is there a similar classification ...
- 26.5k
0
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0
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285
views
Gradient of the trace of the logarithm of a product
Suppose $G$ and $A$ are full rank matrices. Is there a closed-form solution for
$$\nabla_G \mbox{Tr} (A \log GG^\top)$$
when $A$ is a PSD matrix?
- 181
0
votes
0
answers
110
views
Hochster-Roberts theorem
I am trying to understand this theorem, and as an example I try this case: Let $R=\mathbb{C}[z_1^{\pm},\ldots,z_n^{\pm}]^{S_n}$ be the algebra of Laurent polynomials. Now $R$ should be somehow ...
- 463
0
votes
0
answers
173
views
A combinatorial 0-1 matrix problem
Let $M \in \{0, 1\}^{n\times n}$.
Given a constant integer $c \ge 2$, let the number of $1$s in each row be equal to $n/c$ (assuming $c$ is a divisor of $n$).
Given a constant $\beta \in (0,1)$, we ...
- 1,677
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0
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147
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A question on the Bass-Papp theorem on injectives
Let $\{Q_{i}: i\in \mathcal{I}\}$ be a family of indecomposible injective modules over a commutative ring $R$ with identity. Is it true that the injective envelope of the direct sum of $Q_{i}$'s ...
- 471
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votes
0
answers
151
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Koethe Conjecture and a Nil Problem of Jacobson Radical
Let $R$ be a ring with identity, and $e^2=e\in R$ such that both $eJe$ and $(1-e)J(1-e)$ are nil, where $J=J(R)$ is the Jacobson radical of $R$. When $R$ is commutative, it is easy to see that $J$ is ...
- 355
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0
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303
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For which monic irreducible $f \in \mathbb{C}[x,y][T]$, $\mathbb{C}[x,y][T]/(f)$ is a UFD?
Let $f=f(T) \in \mathbb{C}[x,y][T]$ be a monic irreducible polynomial:
$f=T^n+a_{n-1}T^{n-1}+\cdots+a_1T+a_0$,
$a_j \in \mathbb{C}[x,y]$, $0 \leq j \leq n-1$.
Denote $B=\mathbb{C}[x,y,T]/(f)=\mathbb{C}...
- 2,837
0
votes
0
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55
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$R/Soc(R_R)$ and a semisimple $R$-module
I am searching for a ring $R$ and a semisimple $R$-module $M$ such that for any $r\in R$, either $r-r^2\in Soc(R_R)-ann_R^l(M)$ or $r+r^2\in Soc(R_R)-ann_R^l(M)$. Thanks for any cooperation!
- 355
0
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0
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89
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Show that a certain ratio of diagonal entries dominates a certain ratio of singular values
Let $D=[d_{ij}]_{i,j=1,\ldots,4}$ be a $4 \times 4$ “density matrix”, that is, a Hermitian (possibly symmetric) positive definite matrix having trace 1—that is, the (nonnegative) diagonal entries sum ...
- 723
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0
answers
101
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Spherical Rings
My question is concerned with filtered rings. It is a classical result that if $R$ is a finitely generated commutative ring graded by a semigroup $S$ then $S$ is also finitely generated.
The reverse ...
- 501
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0
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81
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A quaternion x generates a left ideal of rank 2 if and only if x, ix and jx are linearly dependent?
I am trying to understand the construction of Artin and Mumford of a non-rational unirational threefold in ([1], p.90).
Assume $S$ is a smooth projective surface over $\mathbb{C}$ with a smooth ...
- 1,025
0
votes
0
answers
108
views
Preimage of projection of idèles, and other usual maps
Let $K$ be a quadratic number field.
I am struggling with some "usual" maps in algebraic number theory, but with which I am not used to, confusing a lot of different settings, as idèles, ...
- 5,424
0
votes
0
answers
336
views
Pfaffian minors of skew symmetric matrix under perturbation
Suppose $A$ be a skew-symmetric matrix whose entries are positive numbers. A perturbation of $A$, $A'$, is obtained by adding another skew-symmetric matrix whose entries are positive integers.
My ...
- 493
0
votes
0
answers
202
views
Conical combination of rank 1 matrices with nonnegative entries
Let $A = (a_{ij})$ be an $n \times n$ matrix with entries in the nonnegative real numbers $\mathbb{R}_+$. Suppose that, for each $i = 1,\ldots, n$, the sum $b_i := a_{i1} + \cdots + a_{in}$ of the ...
user94803
0
votes
0
answers
87
views
When does an automorphism extend to a localisation (noncommutative rings)
Let $R$ be a (not necessarily commutative) ring. Let $\tau$ be an automorphism of $R$. Consider the localisation of $R$ at a set of multiplicative elements which satisfy the ore condition, say $X$. ...
- 201
0
votes
0
answers
82
views
The effect of channel error on the determinant of transmitted matrix
Assume the following matrix
$$
E:=\left(
\begin{array}{ccccc}
e_1 & e_2 & \cdots & e_{p-1} & e_{p}\\
e_{p+1} & e_{p+2} & \cdots & e_{2p-1} & e_{2p} \\
\...
- 313
0
votes
0
answers
698
views
Singular Values of Linearly Combined Matrices
I have a question related with singular values of matrix sums.
Let's assume I have matrices $A$, $B$, and $D$ (positive, semi-definite) where $D = A + B$. For singular values of $D$, I know that
$$
...
- 101
0
votes
0
answers
40
views
convex representation of a combinatorial constraint
I have an optimization problem with a weird constraint as follows. Is it possible to express it in some ways that have convex properties:
matrix $\mathbf{X}$ is either
$[1 \ 0 \ 0 \ 0 \ 0\\
\ 0 \ 0 ...
0
votes
0
answers
168
views
In commutativity theorems in ring theory
Suppose that $R$ is a ring such that for any $x\in R$ there exists $1<n(x)\in \mathbb{N}$ such that $x^{n(x)}-x\in Z(R)$. Prove that $R$ is commutative or if it is not commutative, then the ideal ...
- 221
0
votes
0
answers
140
views
Notion of trace in a Jordan algebra
Let A be a Jordan algebra (with identity). If x is in A let A[x] be the subalgebra generated by x and the identity. An element x is regular if the dimension of A[x] is maximal.
For x regular, denote ...
- 586
0
votes
0
answers
154
views
For which matrices deciding permutation similarity is polynomial?
Q1 For which matrices deciding permutation similarity is polynomial?
It is not easier than graph isomorphism (and very likely is equivalent to it).
If necessary, assume the entries are nonnegative ...
- 25.4k