33 research outputs found
Periodicity of bipartite walk on biregular graphs with conditional spectra
In this paper we study a class of discrete quantum walks, known as bipartite
walks. These include the well-known Grover's walks. Any discrete quantum walk
is given by the powers of a unitary matrix indexed by arcs or edges of the
underlying graph. The walk is periodic if for some positive integer
. Kubota has given a characterization of periodicity of Grover's walk when
the walk is defined on a regular bipartite graph with at most five eigenvalues.
We extend Kubota's results--if a biregular graph has eigenvalues whose
squares are algebraic integers with degree at most two, we characterize
periodicity of the bipartite walk over in terms of its spectrum. We apply
periodicity results of bipartite walks to get a characterization of periodicity
of Grover's walk on regular graphs
Rational Cherednik algebras and Schubert cells
The representation theory of rational Cherednik algebras of type A at t =β0 gives rise, by considering supports, to a natural family of smooth Lagrangian subvarieties of the Calogero-Moser space. The goal of this article is to make precise the relationship between these Lagrangian families and Schubert cells in the adelic Grassmannian. In order to do this we show that the isomorphism, as constructed by Etingof and Ginzburg, from the spectrum of the centre of the rational Cherednik algebra to the Calogero-Moser space is compatible with the factorization property of both of these spaces. As a consequence, the space of homomorphisms between certain representations of the rational Cherednik algebra can be identified with functions on the intersection of some Schubert cells
Fusion Frame Homotopy and Tightening Fusion Frames by Gradient Descent
Finite frames, or spanning sets for finite-dimensional Hilbert spaces, are a
ubiquitous tool in signal processing. There has been much recent work on
understanding the global structure of collections of finite frames with
prescribed properties, such as spaces of unit norm tight frames. We extend some
of these results to the more general setting of fusion frames -- a fusion frame
is a collection of subspaces of a finite-dimensional Hilbert space with the
property that any vector can be recovered from its list of projections. The
notion of tightness extends to fusion frames, and we consider the following
basic question: is the collection of tight fusion frames with prescribed
subspace dimensions path connected? We answer (a generalization of) this
question in the affirmative, extending the analogous result for unit norm tight
frames proved by Cahill, Mixon and Strawn. We also extend a result of Benedetto
and Fickus, who defined a natural functional on the space of unit norm frames
(the frame potential), showed that its global minimizers are tight, and showed
that it has no spurious local minimizers, meaning that gradient descent can be
used to construct unit-norm tight frames. We prove the analogous result for the
fusion frame potential of Casazza and Fickus, implying that, when tight fusion
frames exist for a given choice of dimensions, they can be constructed via
gradient descent. Our proofs use techniques from symplectic geometry and
Mumford's geometric invariant theory
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Spherical Tropicalization
In this thesis, I extend tropicalization of subvarieties of algebraic tori over a trivially valued algebraically closed field to subvarieties of spherical homogeneous spaces. I show the existence of tropical compactifications in a general setting. Given a tropical compactification of a closed subvariety of a spherical homogeneous space, I show that the support of the colored fan of the ambient spherical variety agrees with the tropicalization of the closed subvariety. I provide examples of tropicalization of subvarieties of GL(n), SL(n), and PGL(n)
Quantization of nilpotent coadjoint orbits
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mathematics, 1996.Includes bibliographical references (p. 59-60).by Diko Mihov.Ph.D
IST Austria Thesis
In the first part of the thesis we consider Hermitian random matrices. Firstly, we consider sample covariance matrices XXβ with X having independent identically distributed (i.i.d.) centred entries. We prove a Central Limit Theorem for differences of linear statistics of XXβ and its minor after removing the first column of X. Secondly, we consider Wigner-type matrices and prove that the eigenvalue statistics near cusp singularities of the limiting density of states are universal and that they form a Pearcey process. Since the limiting eigenvalue distribution admits only square root (edge) and cubic root (cusp) singularities, this concludes the third and last remaining case of the Wigner-Dyson-Mehta universality conjecture. The main technical ingredients are an optimal local law at the cusp, and the proof of the fast relaxation to equilibrium of the Dyson Brownian motion in the cusp regime.
In the second part we consider non-Hermitian matrices X with centred i.i.d. entries. We normalise the entries of X to have variance N β1. It is well known that the empirical eigenvalue density converges to the uniform distribution on the unit disk (circular law). In the first project, we prove universality of the local eigenvalue statistics close to the edge of the spectrum. This is the non-Hermitian analogue of the TracyWidom universality at the Hermitian edge. Technically we analyse the evolution of the spectral distribution of X along the Ornstein-Uhlenbeck flow for very long time
(up to t = +β). In the second project, we consider linear statistics of eigenvalues for macroscopic test functions f in the Sobolev space H2+Ο΅ and prove their convergence to the projection of the Gaussian Free Field on the unit disk. We prove this result for non-Hermitian matrices with real or complex entries. The main technical ingredients are: (i) local law for products of two resolvents at different spectral parameters, (ii) analysis of correlated Dyson Brownian motions.
In the third and final part we discuss the mathematically rigorous application of supersymmetric techniques (SUSY ) to give a lower tail estimate of the lowest singular value of X β z, with z β C. More precisely, we use superbosonisation formula to give an integral representation of the resolvent of (X β z)(X β z)β which reduces to two and three contour integrals in the complex and real case, respectively. The rigorous analysis of these integrals is quite challenging since simple saddle point analysis cannot be applied (the main contribution comes from a non-trivial manifold). Our result
improves classical smoothing inequalities in the regime |z| β 1; this result is essential to prove edge universality for i.i.d. non-Hermitian matrices