Complex Grassmannians Research Articles

Symplectic capacities of Hermitian symmetric spaces of compact and noncompact type

Inspired by the work of G. Lu on pseudo symplectic capacities we obtain several results on the Gromov width and the Hofer--Zehnder capacity of Hermitian symmetric spaces of compact type. Our results and proofs extend those obtained by Lu for complex Grassmannians to Hermitian symmetric spaces of compact type. We also compute the Gromov width and the Hofer--Zehnder capacity for Cartan domains and their products.

Cohomology classes of rank varieties and a counterexample to a conjecture of Liu

To each finite subset of a discrete grid $\mathbb{N}×\mathbb{N}$ (a diagram), one can associate a subvariety of a complex Grassmannian (a diagram variety), and a representation of a symmetric group (a Specht module). Liu has conjectured that the cohomology class of a diagram variety is represented by the Frobenius characteristic of the corresponding Specht module. We give a counterexample to this conjecture.However, we show that for the diagram variety of a permutation diagram, Liu's conjectured cohomology class $\sigma$ is at least an upper bound on the actual class $\tau$, in the sense that $\sigma - \tau$ is a nonnegative linear combination of Schubert classes. To do this, we consider a degeneration of Coskun's rank varieties which contains the appropriate diagram variety as a component. Rank varieties are instances of Knutson-Lam-Speyer's positroid varieties, whose cohomology classes are represented by affine Stanley symmetric functions. We show that the cohomology class of a rank variety is in fact represented by an ordinary Stanley symmetric function. A chaque sous-ensemble fini de $\mathbb{N}×\mathbb{N}$ (un diagramme), on peut associer une sous-variété d’une grassmannienne complexe et une représentation d’un groupe symétrique (un module de Specht). Liu a conjecturé que la classe de cohomologie de la variété d’un diagramme est représentée par la caractéristique de Frobenius du module de Specht correspondant. Nous donnons un contre-exemple à cette conjecture.Cependant, nous montrons que dans le cas de la variété du diagramme de permutation, la classe de cohomologie conjecturée par Liu est au moins un majorant de la classe juste $\tau$ , c’est-à-dire que $\sigma - \tau$ est une combinaison linéaire non-négative des classes de Schubert. Pour ce faire, nous considérons une dégénérescence des variétés de rang de Coskun qui contient la variété appropriée d’un diagramme comme une composante irréductible. Les variétés de rang sont des exemples de variétés de positroïde, dont les classes de cohomologie sont représentées par des fonctions symétriques de Stanley affines. En effet, nous montrons que la classe de cohomologie d’une variété de rang est représentée par une fonction symétrique de Stanley ordinaire.

Braid groups in complex Grassmannians

We describe the fundamental group and second homotopy group of ordered k-point sets in Gr(k,n) generating a subspace of fixed dimension.

Minimal two-spheres with constant curvature in the complex Grassmannians

In this paper we use the Cartan embedding and the method of moving frames to study the geometry of 2-dimensional SU(2)-orbits in the complex Grassmann manifold G(k, n), and we express their geometric quantities explicitly. We obtain various minimal two-spheres in G(k, n) with constant curvature by studying the 2-dimensional orbits.

Coherent states on the Grassmannian U(4)/U(2)2: oscillator realization and bilayer fractional quantum Hall systems

Bilayer quantum Hall (BLQH) systems, which underlie a U(4) symmetry, display unique quantum coherence effects. We study coherent states (CS) on the complex Grassmannian , orthonormal basis, U(4) generators and their matrix elements in the reproducing kernel Hilbert space of analytic square-integrable holomorphic functions on , which carries a unitary irreducible representation of U(4) with index . A many-body representation of the previous construction is introduced through an oscillator realization of the U(4) Lie algebra generators in terms of eight boson operators. This particle picture allows us to make a physical interpretation of our abstract mathematical construction in the BLQH jargon. In particular, the index λ is related to the number of flux quanta bound to a bi-fermion in the composite fermion picture of Jain for fractions of the filling factor ν = 2. The simpler, and better known, case of spin-s CS on the Riemann–Bloch sphere is also treated in parallel, of which Grassmannian -CS can be regarded as a generalized (matrix) version.

Generalized normal homogeneous Riemannian metrics on spheres and projective spaces

In this paper, we develop new methods to study generalized normal homogeneous Riemannian manifolds. In particular, we obtain a complete classification of generalized normal homogeneous Riemannian metrics on spheres \({S^n}\). We prove that for any connected (almost effective) transitive on \(S^n\) compact Lie group \(G\), the family of \(G\)-invariant Riemannian metrics on \(S^n\) contains generalized normal homogeneous but not normal homogeneous metrics if and only if this family depends on more than one parameters and \(n\ge 5\). Any such family (that exists only for \(n=2k+1\)) contains a metric \(g_\mathrm{can}\) of constant sectional curvature \(1\) on \(S^n\). We also prove that \((S^{2k+1}, g_\mathrm{can})\) is Clifford–Wolf homogeneous, and therefore generalized normal homogeneous, with respect to \(G\) (except the groups \(G={ SU}(k+1)\) with odd \(k+1\)). The space of unit Killing vector fields on \((S^{2k+1}, g_\mathrm{can})\) from Lie algebra \(\mathfrak g \) of Lie group \(G\) is described as some symmetric space (except the case \(G=U(k+1)\) when one obtains the union of all complex Grassmannians in \(\mathbb{C }^{k+1}\)).

The Cut-off Phenomenon for Brownian Motions on Compact Symmetric Spaces

In this paper, we prove the cut-off phenomenon in total variation distance for the Brownian motions traced on the classical symmetric spaces of compact type, that is to say: 1. the classical simple compact Lie groups: special orthogonal groups SO(n), special unitary groups SU(n) and compact symplectic groups USp(n); 2. the real, complex and quaternionic Grassmannian varieties (including the real spheres, and the complex or quaternionic projective spaces when q = 1): SO(p + q)/(SO(p)×SO(q)), SU(p + q)/S(U(p)×U(q)) and USp(p + q)/(USp(p)×USp(q)); 3. the spaces of real, complex and quaternionic structures: SU(n)/SO(n), SO(2n)/ U(n), SU(2n)/USp(n) and USp(n)/UU(n). Denoting μt the law of the Brownian motion at time t, we give explicit lower bounds for dTV(μt,Haar) if \(t t_{\text{cut-of\/f}}\). This provides in particular an answer to some questions raised in recent papers by Chen and Saloff-Coste. Our proofs are inspired by those given by Rosenthal and Porod for products of random rotations in SO(n), and by Diaconis and Shahshahani for products of random transpositions in \(\mathfrak{S}_{n}\).

On the classification of homogeneous $2$-spheres in complex Grassmannians

In this paper we discuss a classification problem of homogeneous 2-spheres in the complex Grassmann manifold $G(k + 1, n + 1)$ by theory of unitary representations of the 3-dimensional special unitary group $\mathit{SU}(2)$. First we observe that if an immersion $x\colon S^{2} \to G(k + 1, n + 1)$ is homogeneous, then its image $x(S^{2})$ is a 2-dimensional $\rho(\mathit{SU}(2))$-orbit in $G(k + 1, n + 1)$, where $\rho\colon \mathit{SU}(2) \to U(n + 1)$ is a unitary representation of $\mathit{SU}(2)$. Then we give a classification theorem of homogeneous 2-spheres in $G(k + 1, n + 1)$. As an application we describe explicitly all homogeneous 2-spheres in $G(2, 4)$. Also we mention about an example of non-homogeneous holomorphic 2-sphere with constant curvature in $G(2, 4)$.

Explicit construction of harmonic two-spheres into the complex Grassmannian

We present an explicit description of all harmonic maps of finite uniton number from a Riemann surface into a complex Grassmannian. Namely, starting from a constant map Q and a collection of meromorphic functions and their derivatives, we show how to algebraically construct all harmonic maps from the two-sphere into a given Grassmannian \({G_p(\mathbb C^n)}\) . In this setting the uniton number depends on Q and p and we obtain a sharp estimate for it.

Construction of homogeneous minimal 2-spheres in complex Grassmannians

In this paper, we construct a class of homogeneous minimal 2-spheres in complex Grassmann manifolds by applying the irreducible unitary representations of SU(2). Furthermore, we compute induced metrics, Gaussian curvatures, Kähler angles and the square lengths of the second fundamental forms of these homogeneous minimal 2-spheres in G(2,n+1) by making use of Veronese sequence.

On conformal minimal 2-spheres in complex Grassmann manifold G(2,n)

For a harmonic map f from a Riemann surface into a complex Grassmann manifold, Chern and Wolfson [4] constructed new harmonic maps \(\partial\!f\) and \(\bar{\partial}\!f\) through the fundamental collineations \(\partial\) and \(\bar{\partial}\) respectively. In this paper, we study the linearly full conformal minimal immersions from S2 into complex Grassmannians G(2,n), according to the relationships between the images of \(\partial\!f\) and \(\bar{\partial}\!f\). We obtain various pinching theorems and existence theorems about the Gaussian curvature, Kahler angle associated to the given minimal immersions, and characterize some immersions under special conditions. Some examples are given to show that the hypotheses in our theorems are reasonable.

A note on the Witt group and theKO-theory of complex Grassmannians

AbstractFor a complex GrassmannianX, there is an isomorphismbetween Balmer's Witt group and the quotient of topologicalK-theories.

Dvoretzky Type Theorems for Multivariate Polynomials and Sections of Convex Bodies

In this paper we prove the Gromov--Milman conjecture (the Dvoretzky type theorem) for homogeneous polynomials on $\mathbb R^n$, and improve bounds on the number $n(d,k)$ in the analogous conjecture for odd degrees $d$ (this case is known as the Birch theorem) and complex polynomials. We also consider a stronger conjecture on the homogeneous polynomial fields in the canonical bundle over real and complex Grassmannians. This conjecture is much stronger and false in general, but it is proved in the cases of $d=2$ (for $k$'s of certain type), odd $d$, and the complex Grassmannian (for odd and even $d$ and any $k$). Corollaries for the John ellipsoid of projections or sections of a convex body are deduced from the case $d=2$ of the polynomial field conjecture.

Almost complex structures on quaternion-Kähler manifolds and inner symmetric spaces

We prove that compact quaternionic-K\"ahler manifolds of positive scalar curvature admit no almost complex structure, even in the weak sense, except for the complex Grassmannians $Gr_2(C^{n+2})$. We also prove that irreducible inner symmetric spaces $M^{4n}$ of compact type are not weakly complex, except for spheres and Hermitian symmetric spaces.

Quantum dimensions and their non-Archimedean degenerations

We derive explicit dimension formulas for irreducible $M_F$-spherical $K_F$-representations where $K_F$ is the maximal compact subgroup of the general linear group $GL(d,F)$ over a local field $F$ and $M_F$ is a closed subgroup of $K_F$ such that $K_F/M_F$ realizes the Grassmannian of $n$-dimensional $F$-subspaces of $F^d$. We explore the fact that $(K_F,M_F)$ is a Gelfand pair whose associated zonal spherical functions identify with various degenerations of the multivariable little $q$-Jacobi polynomials. As a result, we are led to consider generalized dimensions defined in terms of evaluations and quadratic norms of multivariable little $q$-Jacobi polynomials, which interpolate between the various classical dimensions. The generalized dimensions themselves are shown to have representation theoretic interpretations as the quantum dimensions of irreducible spherical quantum representations associated to quantum complex Grassmannians.

ON THE ORIGIN OF TIME AND THE UNIVERSE

We present a novel solution to the low entropy and arrow of time puzzles of the initial state of the universe. Our approach derives from the physics of a specific generalization of Matrix theory put forth in earlier work as the basis for a quantum theory of gravity. The particular dynamical state space of this theory, the infinite-dimensional analogue of the Fubini–Study metric over a complex nonlinear Grassmannian, has recently been studied by Michor and Mumford. The geodesic distance between any two points on this space is zero. Here we show that this mathematical result translates to a description of a hot, zero entropy state and an arrow of time after the Big Bang. This is modeled as a far from equilibrium, large fluctuation driven, "freezing by heating" metastable ordered phase transition of a nonlinear dissipative dynamical system.

On pseudo-holomorphic curves from two-spheres into a complex Grassmannian G(2, 5)

Let s: S 2 → G(2, 5) be a linearly full totally unramified pseudo-holomorphic curve with constant Gaussian curvature K in a complex Grassmann manifold G(2, 5). It is prove that K is either 1/2 or 4/5 if s is non-±holomorphic. Furthermore, K = 1/2 if and only if s is totally real. We also prove that the Gaussian curvature K is either 1 or 4/3 if s is a non-degenerate holomorphic curve under some conditions.

Homogeneous Lagrangian submanifolds of positive Euler characteristic

We fully classify all Lagrangian submanifolds of a complex Grassmannian which are an orbit of a compact group of isometries and have positive Euler characteristic.

Fuzzy complex Grassmannians and quantization of line bundles

We construct by purely representation-theoretic methods fuzzy versions of an arbitrary complex Grassmannian M=Grn(ℂn+m), i.e., a sequence of matrix algebras tending SU(n+m)-equivariantly to the algebra of smooth functions on M. We also show that this approximation can be interpreted in terms of the Berezin-Toeplitz quantization of M. Furthermore, we use branching rules to prove that the quantization of every complex line bundle over M is given by a SU(n+m)-equivariant truncation of the space of its L2-sections.

Bounds for codes and designs in complex subspaces

We introduce the concepts of complex Grassmannian codes and designs. Let $\mathcal{G}_{m,n}$ denote the set of m-dimensional subspaces of ℂ n : then a code is a finite subset of $\mathcal{G}_{m,n}$ in which few distances occur, while a design is a finite subset of $\mathcal{G}_{m,n}$ that polynomially approximates the entire set. Using Delsarte’s linear programming techniques, we find upper bounds for the size of a code and lower bounds for the size of a design, and we show that association schemes can occur when the bounds are tight. These results are motivated by the bounds for real subspaces recently found by Bachoc, Bannai, Coulangeon and Nebe, and the bounds generalize those of Delsarte, Goethals and Seidel for codes and designs on the complex unit sphere.