Questions tagged [total-positivity]

For questions related to totally positive (or totally nonnegative) matrices, and related topics such as total positivity in a more general Lie-theoretic setting. (Not related to "totally positive integers" in the number-theoretic sense.)

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3 votes
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132 views

The Grassmann twist-map, an associated semi-group action, and RSK

Let me begin by setting some notation: Let $\mathrm{Mat}_{k,n}(\Bbb{R})$ denote the vector space of all $k \times n$ real-valued matrices. Given $g \in \mathrm{Mat}_{k,n}(\Bbb{R})$ and two (ordered) ...
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4 votes
1 answer
75 views

Total positivity tests: optimal in the number of minors vs. the computational cost

A real matrix is called totally positive if all of its minors are positive. Of course, to check that a given matrix is totally positive it is not necessary to check all the minors: for example, it ...
1 vote
0 answers
73 views

Embedding of co-oriented subspaces into positive Grassmannian

$\def\R{\mathbb{R}}$Let $P_1$, $P_2$, $P_3$ be three $m$-dimensional subspaces in $\R^n$. With a slight abuse of notation they will also denote the ortho-projectors on the respective subspaces. We ...
3 votes
0 answers
96 views

Positivity of sequences

Totally positive sequences $\lbrace a_n\rbrace_{n\in\mathbb{Z}}$ are defined as those such that the Töplitz matrix $A_{ij}=a_{i-j}$ is totally positive (all its minors are non-negative). An ...
2 votes
0 answers
51 views

Total positivity of order 2 of generalized absolute value density or likelihood ratio order of "shifted" generalized absolute value

If $f$ is the Lebesgue density of a real valued symmetric random variable $X$ (symmetric means $X \overset{d}{=} -X$) then for fixed $u > 0$ $$f^*(v,u) := f(-u -v) + f(-u+v)$$ is the density of $\...
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4 votes
0 answers
251 views

Infinite tridiagonal matrices and a special class of totally positive sequences

Let $\Bbb{y} = \big(y_1, y_2, y_3, \dots \big)$ be an infinite sequence of positive real numbers such that following $\Bbb{N} \times \Bbb{N}$ tridiagonal matrix \begin{equation} T(\Bbb{y}) := \, \...
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10 votes
1 answer
250 views

Map from Bruhat stratification to Catalan stratification for the space of totally nonnegative upper-triangular matrices

$\DeclareMathOperator\SL{SL}$This question came up in a class "Total Positivity and Cluster Algebras" being taught by Chris Fraser. Let $N^+$ denote the space of uni-upper-triangular ...
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2 votes
0 answers
186 views

Finding the decorated permutation of a non-reduced plabic graph

This is a question about Postnikov's theory of positroids and plabic graphs. The short version is If we have an non-reduced plabic graph $G$, how do we look at the alternating strands and read off ...
11 votes
2 answers
520 views

$q$-analogs of total positivity

A real matrix $M$ is called totally positive if all of its minors are positive; these matrices have been extensively studied, and there are generalizations to other Lie types, for example by Lusztig. ...
7 votes
0 answers
243 views

Cyclic shift acting on finite Grassmannian

The (twisted) cyclic shift $(v_1,v_2,\ldots,v_n) \mapsto (v_2,v_3,\ldots,v_n,(-1)^{k-1}v_1)$ acting on the Grassmannian $\mathrm{Gr}(\mathbb{C};k,n)$ of $k$-planes in $\mathbb{C}^n$ is an important ...
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18 votes
1 answer
695 views

Biographical information on Anne Marie Whitney

I am looking for information on the mathematician Anne Marie Whitney. She wrote a number of significant papers related to total positivity with her thesis adviser Isaac Schoenberg. All I could find on ...
2 votes
0 answers
114 views

What is the symmetry group of the totally nonnegative Grassmannian $Gr_{tnn}(k,n)$?

What is the symmetry group of the totally nonnegative Grassmannian $Gr_{tnn}(k,n)$? [The latter consists of those elements of the Grassmannian that can be represented by $k \times n$-matrices all of ...
14 votes
4 answers
2k views

Vandermonde matrix is totally positive

A totally positive matrix $M\in \mathcal{M}_{n\times m}(\mathbb R)$ is such that all of its minors of all sizes are positive. It is true that any Vandermonde matrix (with well-ordered positive entries)...
4 votes
1 answer
258 views

Big cells in a Grassmann and permutations

In the lecture notes, it is said that (Theorem 3.1.3) the set of positroid cells in $Gr(k,n)$ are in one to one correspondence with the set of bounded affine permutations of type $(k,n)$. In Example 4....
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12 votes
1 answer
231 views

Total positivity of $q$-Pascal matrix?

A matrix of real numbers is called totally positive if all its minors are non-negative. A well-known example is the Pascal matrix $(\binom{i}{j})$. Is it true that the minors of the $q$-Pascal matrix ...
3 votes
0 answers
205 views

Definition of loop amplituhedrons

In the paper The Amplituhedron , Nima Arkani-Hamed and Jaroslav Trnka introduced the geometric object amplituhedron. It is defined as follows (see also the lecture notes). Let $Z$ be a $(k+m)\times ...
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7 votes
2 answers
1k views

Decorated permutations and subset permutations

Decorated permutations are defined as permutations where the fix-points come in two colors (say $\overline{\cdot}$ and $\underline{\cdot}$). For example, the 16 decorated permutations of length 3 are $...
3 votes
0 answers
171 views

Spectra of certain totally positive matrices

Let $S$ be the set of $3 \times 3$ matrices $A$ satisfying the following conditions: All minors are $>0$ (i.e., $A$ is a strictly totally positive matrix); all principal minors are $>1$, except ...
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6 votes
3 answers
2k views

An Intriguing Tapestry: Number triangles, polytopes, Grassmannians, and scattering amplitudes

What are the roles that the classic number arrays-- Eulerian, Narayana--play in the application of totally non-negative Grassmannians, or amplituhedrons, to string / twistor scattering theory? (This ...
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3 votes
1 answer
576 views

Positroids and Totally Nonnegative Complex Grassmanian

Recently I begin working on matroids, in particular to a generalization of oriented matroids to the complex case. I found on arxiv the following interesting articles: 1)Alexander Postnikov: Total ...
74 votes
4 answers
6k views

What is the amplituhedron?

The paper ”Scattering Amplitudes and the Positive Grassmannian” by Nima Arkani-Hamed, Jacob L. Bourjaily, Freddy Cachazo, Alexander B. Goncharov, Alexander Postnikov, and Jaroslav Trnka, introduces ...
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72 votes
2 answers
9k views

The amplituhedron minus the physics

Is it possible to appreciate the geometric/polytopal properties of the amplituhedron without delving into the physics that gave rise to it? All the descriptions I've so far encountered assume ...
5 votes
1 answer
514 views

On totally nonnegative Grassmannian

I was reading Postnikov's paper [TOTAL POSITIVITY, GRASSMANNiANS, AND NETWORKS][1] when I came across the definition of the totally nonnegative Grassmannian $Gr_{kn}^{tnn} \subset Gr_{kn}$ as the ...
9 votes
3 answers
4k views

Eigenvectors of a certain big upper triangular matrix

I'm looking at this matrix: $$ \begin{pmatrix} 1 & 1/2 & 1/8 & 1/48 & 1/384 & \dots \\ 0 & 1/2 & 1/4 & 1/16 & 1/96 & \dots \\ 0 & 0 & 1/8 & 1/16 &...