# Questions tagged [multilinear-algebra]

Tensors, multivectors, wedge products, multilinear maps, exterior (Grassmann) algebras.

113
questions

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votes

**1**answer

172 views

### Irreducible representations of $\mathrm{SL}_n(K)$, $K$ finite

Let $\mathrm{SL}_n/K$ ($K$ finite) be given with its natural action on an $n$-dimensional vector space $V/K$. Consider the action of $\mathrm{SL}_n$ on the $m$-fold tensor product $V\otimes \dotsc \...

**1**

vote

**1**answer

36 views

### Expectation value of multilinear forms over independent Gaussian vectors

Let $A$ be a symmetric multilinear form on $\left(\mathbb{R}^d\right)^{\otimes n}\times \left(\mathbb{R}^d\right)^{\otimes n}$ and consider the random variable:
\begin{align*}
X=A(g_1,\ldots,g_n,g_1,\...

**13**

votes

**1**answer

2k views

### Why does this matrix have zero determinant?

This curious identity arose from studying reductions of the maximal ideal in certain monomial algebra. It can be proved "by hand", (i.e, using Macaulay 2), but I am seeking a more conceptual ...

**1**

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**0**answers

100 views

### Transformations of the cubic forms [closed]

Is there a way to understand whether there exist linear transformation that brings one cubic form of n variables to another form? In particular one of the examples I am interested in are two cubic ...

**2**

votes

**0**answers

40 views

### Rank-1 decomposability of symmetric tensors

My question is about rank-1 decomposability of symmetric tensors over the reals.
Let $v_1,\dots,v_n\in\mathbb{R}^d$ be vectors. Construct the object:
$$
V=\sum_{j=1}^n \underbrace{v_j\otimes v_j\...

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34 views

### On symmetric tensors with same rank, different orders

Let $A,B$ be two symmetric tensors of same rank $m$; and orders $k$ and $\ell$, respectively. In particular, assume that $A,B$ admits the following structure: There exists $v_1,\dots,v_m\in\mathbb{R}^...

**2**

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47 views

### Is there a way of separating symmetric multilinear forms?

I have a very simple question which seemingly falls into the category of multilinear algebra. I need this concept in one of my research papers, but unfortunately did not learn it before. Let me state ...

**0**

votes

**1**answer

160 views

### Sketching Frobenius norm of a tensor with a rank-1 random tensor

Let $A\in\mathbb{R}^{n^k}$ be a $k$-dimensional tensor with $n$ elements along each dimension. Moreover suppose $u_1,u_2,\dots,u_k\sim\text{Unif}(\pm1)^n$ are $n$ dimensional vectors with each of ...

**2**

votes

**1**answer

181 views

### Exterior derivative independence from coordinate systems

In the book Mathematical Methods of Classical Mechanics by V.I. Arnold, the author introduces (p.189) the concept of exterior derivative as "the principal linear part of the increment" of the function ...

**2**

votes

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145 views

### Normal multivariate orthant probabilities

(Previously I posted a similar question on math.SE, hoping that this question would have an easy answer. As the question appears hard, I am hoping I can perhaps get more feedback here.)
Let $\mathbf{...

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52 views

### Function for unique volume element

This is an issue that I'm am trying to solve for a fine-tuning measure in particle physics, but it is purely mathematical. Consider three vectors $\{v_1, v_2, v_3\}$ in $\mathbb{R}^3$. I would like a ...

**7**

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**1**answer

277 views

### Finite dimensional commutative algebras containing infinitely many nilpotents whose $d$-way products are nonzero

I'm interested in the following strange question: for some $d > 1$, what is the minimum dimension of a commutative $\mathbb{C}$-algebra containing infinitely many elements that square to zero, but ...

**7**

votes

**1**answer

254 views

### Exterior powers and choice

Under the assumption that any vector space has a basis (so under the assumption of the axiom of choice), we can prove the following algebraic statements :
1) If $\varphi:V\to W$ is an injective ...

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39 views

### Integrating over some domain of the Stiefel manifold to analyze its support

Define the square $d$ dimensional Stiefel manifold as
$$V_{d} = \{ R \in \mathbb{R}^{d \times d} : R ^\top R = I_d \} .$$
How does one integrate on this manifold over a domain defined as $\{ R \in V_{...

**19**

votes

**1**answer

1k views

### Why is the standard definition of a $(p, q)$-tensor so bizarre?

At time of writing the first definition of a $ (p, q) $-tensor on the Wikipedia page is as follows.
Definition. A $ (p, q) $-tensor is an assignment of a multidimensional array $$ T^{i_1\dots i_p}_{...

**5**

votes

**1**answer

233 views

### Is every basis for $\bigwedge^kV$ satisfying a “complementary” property a rescaling of a “standard” basis?

This is a cross-post.
Let $V$ be a $4$-dimensional real vector space. Let $\omega_{i_1,i_2}$
be a basis for $\bigwedge^2V$, where each $\omega_{i_1,i_2}$ is decomposable. Suppose that for every $\...

**1**

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**0**answers

49 views

### Efficient way to express a symmetric tensor in terms of rank one elements

Let
$$P(\mathbf{x})=P(x_1, \ldots, x_n)=x_1^{k_1}x_2^{k_2}\cdots x_n^{k_n}$$
be a homogenous polynomial of degree $k=k_1+\cdots+k_n$.
It follows from a standard polarization identity (see for ...

**8**

votes

**1**answer

196 views

### A symmetric bilinear form and a Plücker identity

It turns out that a special case of something I'm working on gives, as a corollary, a rather 19th-century-looking elementary statement about the rank of a certain symmetric matrix. I thought I would ...

**7**

votes

**1**answer

170 views

### Is there any sort of higher-order SVD (quadratic and above) for dimensionality reduction?

(Posted this on math.stackexchange and cross.correlated over more than a week ago, but didn't get an answer, and this is a question in my research so this seems like it might have been the better ...

**2**

votes

**1**answer

342 views

### Is there a generalization of eigenvalues and eigenvectors to tensors?

Two perhaps ill-posed or just silly questions:
Let $n>0$, $T$ be an $(n+2)$-tensor, and $\otimes$ denote the Kronecker product of tensors. Is there a tensor generalization for the fundamental ...

**2**

votes

**1**answer

115 views

### Non-negativity condition for special quartic

I know that a necessary and sufficient condition for the positivity of a quartic polynomial of many variables is in general difficult. I have a somewhat special case, maybe here more can be said. Let $...

**11**

votes

**2**answers

616 views

### Clifford algebras as deformations of exterior algebras

$\def\Cl{\mathcal C\ell}
\def\CL{\boldsymbol{\mathscr{C\kern-.1eml}}(\mathbb R)}$
I'm not an expert in neither of the fields I'm touching, so don't be too rude with me :-) here's my question.
A well ...

**2**

votes

**2**answers

451 views

### A kind of “Curvature tensor” for higher dimensional tensors

I begin my question with a multilinear question then I will consider two local smooth analogies:
Assume that $\alpha$ is a real valued symmetric $k$-tensor, that is a $k$-linear map $\alpha:\...

**5**

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**1**answer

91 views

### Looking for a tractable algorithm or formula for the determinant of a tensor

It is possible to define the determinant of a tensor.
We think of a tensor as a collection of numbers but this collection easily extends to a proper multilinear map.
If $T:\{1,....,n\}^m\to \mathbb C$ ...

**2**

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**0**answers

62 views

### Points on Sphere whose image, under symmetric positive definite matrix, is contained in cube

Let $\Sigma \in \mathbb{R}^{n \times n}$ be a symmetric, positive definite matrix and let $\mu_r$ denote surface measure on the sphere in $\mathbb{R}^n$ with radius $r$. Let
$$
R = \{x \in \mathbb{R}^...

**1**

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**1**answer

219 views

### Number of Symmetric matrices of fix rank over finite fields

This might be a question that shouldn't be asked here. But I need some help.
I want to count the number of $n\times n$ symmetric matrices over the finite field $\mathbb{F}_q$ and rank $r$. I found the ...

**0**

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114 views

### Number of Symmetric matrices

Let $S_m(q)$ denote the space of all $m\times m$ symmetric matrices over the finite field $\mathbb{F}_q$ of size $q$. What is the number of matrices $A=(a_{ij})\in S_m(q)$ of rank at most $3$ and $a_{...

**1**

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68 views

### Maximum Number of Skew-Symmetric matrices

I want to count the maximum number of rank 2 matrices in a space of certain dimension but I am stuck at some point. Any help/ suggestions are appreciated. Here is the question.
Let $\mathbb{M}_m$ be ...

**1**

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**1**answer

157 views

### Is the Waring rank homogeneous polynomials sub-multiplicative?

For a homogeneous degree $d$ polynomial $P$, the symmetric or Waring rank $W(P)$ is the minimum $r$ such that $P = \sum_{j=1}^r l_j^d$, where $l_j$s are linear forms. Now, is the Waring rank sub-...

**3**

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85 views

### Number of vectors such that the projection is decomposable

Let $V$ be a vector space of dimension $n\geq 6$ over the finite field $\mathbb{F}_q$. Let $\omega\in\bigwedge^{n-3}V$ be a nonzero element. Define the annihilator subspace of $\omega$ by $V_\omega=\{...

**0**

votes

**1**answer

129 views

### An upper bound for skew symmetric rank 2 matrices

Earlier, I had asked a similar question but that was not the correct problem where I got stuck. After a few quick answer, I realized that and I apologize for that.
Let $B_m$ be the space of all skew-...

**20**

votes

**3**answers

1k views

### Simultaneous “orthonormalization” in $\mathbb{C}^4$

Let $A$ be a positive, invertible $4 \times 4$ hermitian complex matrix.
So we have a positive sesquilinear form $\langle Av,w\rangle$. Say that a pair $(v,w)$ of vectors in $\mathbb{C}^4$ is good ...

**4**

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213 views

### Formal multidimensional Taylor series expansion over commutative rings

If $F:V\to W$ is a smooth at $a\in V$ function between finite-dimensional vector spaces over $\mathbb{R}$, then we have
$$
F(x) = \sum_{k=0}^N\frac{1}{k!}(D^kF)(a)[(x-a)^{\otimes k}]+\text{remainder},
...

**1**

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**1**answer

93 views

### Sum of certain decomposable elements

Let $V$ be be a vector space of dimension $m$ over any field and $\ell\leq m$ be a positive integer. Let $\omega_1,\ldots,\omega_r \in\bigwedge^\ell V$ are linearly independent, completely ...

**17**

votes

**1**answer

727 views

### Tracing the word “form”

Today the word form can refer to (at least) three different kinds of mathematical object:
A homogeneous polynomial. This was apparently started by Gauss (1801), renaming what others had called ...

**5**

votes

**1**answer

438 views

### Waring rank vs tensor rank of symmetric tensors?

Suppose we work in an algebraically closed field. Then, do the Waring rank (symmetric tensor rank) and tensor rank of a symmetric tensor coincide in general? Recall that tensor rank is rank with ...

**3**

votes

**1**answer

110 views

### Non alternative $k$-linear maps vanishing on $\sum x_i=0$

Assume that $V$ is a finite dimensional real vector space of dimension $n$.
Is there a $\mathbb{R} -$ valued $k$- linear map $T$ on $V$ which is not an alternative form but it vanish on all $k$- tuple ...

**1**

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**0**answers

51 views

### Maximum number of matrices satisfying given rank conditions

Assume that we have $2k$ matrices $S_1,\ldots,S_k$ and $\Phi_1,\ldots,\Phi_k$ over some finite field $F$ such that
(i) $S_i\in F^{l/2\times l}$ and $\dim S_i=l/2$ for any $i\in\{1,\ldots,k\}$;
(ii)...

**4**

votes

**1**answer

323 views

### Homogeneous polynomials and symmetric binary forms

Let $f\in k[x_0,...,x_n]_d$ be a degree $d$ homogeneous polynomial in $n+1$ variables.
Is there a way to associate to $f$ a form $g(y_1,...,y_m)$ which is symmetric in the sets of binary variables $...

**5**

votes

**2**answers

401 views

### (Efficient) computation of symmetric powers of square matrices

I'm looking for software that can compute symmetric powers of medium-size square (say rational, 100 by 100) matrices, and ideally can do so efficiently if the matrix is sparse enough. I haven't found ...

**3**

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**0**answers

60 views

### Multilinear maps that preserve unitarity

Let $M_1, M_2, M_3$ be spaces of square complex matrices, respectively acting on finite-dimensional Hilbert spaces $V_1, V_2$, and $V_3 = V_1 \otimes V_2$. Consider bilinear maps
$$\phi: M_1 \times ...

**2**

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94 views

### Spectrum of a special sum of matrices

Suppose I have a vector space with a tensor product structure $H = V^{\otimes m}$ and suppose I have some traceless Hermitian operator $A: V \otimes V \rightarrow V \otimes V$. Denote by $A_{i,j}$ ...

**1**

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**1**answer

348 views

### What is a multivariable Boolean polynomial? [closed]

When I was reading a paper about optimization, I encountered to multivariable Boolean polynomials which was undefined. What is the exact definition of a multivariable Boolean polynomial and can you ...

**6**

votes

**1**answer

168 views

### Some intuition on the $SL_n$-module $V_{[1,1,…,1]}$

(This question highly overlaps with this and also this.)
The irreducible ${\sf SL}_{n-1}$-module $V_{[1,1,\ldots,1]}$ is the one providing the minimal projective embedding $\mathbb{P}(V_{[1,1,\ldots,...

**5**

votes

**1**answer

277 views

### Subgroups of the tensor product $A\otimes A$

I have this problem about subgroups of the tensor product of an abelian group $A$ with itself which arises from a complete different setting.
I fell into this question studying quandles and quandle ...

**3**

votes

**1**answer

300 views

### Properties of one dimensional null space

Let $\mathcal{G}$ be denote the set of all $3 \times 3$ real symmetric matrices and let $\mathcal{G}^+$ denote the set of all $3 \times 3$ positive semidefinite matrices (see definition).
Let $S: \...

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**1**answer

159 views

### Conditions on $\beta$ under which the trace pairing restricted to $\mathfrak{so}(V,\beta)$ is positive (negative) definite

Let $V$ be a finite dimensional vector space over $ \mathbb{R}$. Let
\begin{equation} \left\langle\:,\:\right\rangle:\mbox{End}(V)\otimes\mbox{End}(V)\rightarrow \mathbb{R}\end{equation}
denote the ...

**1**

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75 views

### About a particular definition of a Hessian of a function of tuples of matrices

Say I have a function $L : (W_1,..,W_{H+1}) \rightarrow \mathbb{R}$ i.e it takes a tuple of $n$ matrices of different dimensions and computes a number from them.
Then I see being defined a ...

**3**

votes

**1**answer

144 views

### A question on surjectivity of a bilinear quadratic map

Let $a=(a_0, a_1, ..., a_n )$, $b=(b_0, b_1, ..., b_n )$ that belong to ${\mathbb R}^{n+1}$. Define polynomials $f_a (t)=a_0 +a_1 t+ ... + a_n t^n$ and $f_b (t)=b_0 +b_1 t+ ... + b_n t^n$ and let $f_{...

**4**

votes

**1**answer

130 views

### Exterior Powers of finite abelian group

Let $A$ be a finite $\mathbb{Z}$-module (i.e., a finite abelian group). My question is: for what $n\in \mathbb{Z}^{n\geq 2}$ the map
\begin{align}
\alpha_{n}:\bigwedge^nA&\to A^{\otimes n}\\
a_1\...