The smallest one-realization of a given set

For any set $S$ of positive integers, a mixed hypergraph ${\cal H}$ is a realization of $S$ if its feasible set is $S$, furthermore, ${\cal H}$ is a one-realization of $S$ if it is a realization of $S$ and each entry of its chromatic spectrum is either 0 or 1. Jiang et al. \cite{Jiang} showed that the minimum number of vertices of realization of $\{s,t\}$ with $2\leq s\leq t-2$ is $2t-s$. Kr$\acute{\rm a}$l \cite{Kral} proved that there exists a one-realization of $S$ with at most $|S|+2\max{S}-\min{S}$ vertices. In this paper, we improve Kr$\acute{\rm a}$l's result, and determine the size of the smallest one-realization of a given set. As a result, we partially solve an open problem proposed by Jiang et al. in 2002 and by Kr$\acute{\rm a}$l in 2004

Minimal Dynamics and K-theoretic Rigidity: Elliott's Conjecture

Let X be an infinite, compact, metrizable space of finite covering dimension and h a minimal homeomorphism of X. We prove that the crossed product of C(X) by h absorbs the Jiang-Su algebra tensorially and has finite nuclear dimension. As a consequence, these algebras are determined up to isomorphism by their graded ordered K-theory under the necessary condition that their projections separate traces. This result applies, in particular, to those crossed products arising from uniquely ergodic homeomorphisms.Comment: 19 page

A Note On Subhomogeneous C*-Algebras

We show that finitely generated subhomogeneous C*-algebras have finite decomposition rank. As a consequence, any separable ASH C*-algebra can be written as an inductive limit of subhomogeneous C*-algebras each of which has finite decomposition rank. It then follows from work of H. Lin and of the second named author that the class of simple unital ASH algebras which have real rank zero and absorb the Jiang-Su algebra tensorially satisfies the Elliott conjecture.Comment: 5 page

Exponential rank and exponential length for Z-stable simple C*-algebras

Let $A$ be a unital separable simple ${\cal Z}$-stable C*-algebra which has rational tracial rank at most one and let $u\in U_0(A),$ the connected component of the unitary group of $A.$ We show that, for any $\epsilon>0,$ there exists a self-adjoint element $h\in A$ such that $$|u-\exp(ih)|<\epsilon.$$ The lower bound of $|h|$ could be as large as one wants. If $u\in CU(A),$ the closure of the commutator subgroup of the unitary group, we prove that there exists a self-adjoint element $h\in A$ such that $$|u-\exp(ih)| <\epsilon and |h|\le 2\pi.$$ Examples are given that the bound $2\pi$ for $|h|$ is the optimal in general. For the Jiang-Su algebra ${\cal Z},$ we show that, if $u\in U_0({\cal Z})$ and $\epsilon>0,$ there exists a real number $-\pi<t\le \pi$ and a self-adjoint element $h\in {\cal Z}$ with $|h|\le 2\pi$ such that $$ |e^{it}u-\exp(ih)|<\epsilon. $

Doubles of (quasi) Hopf algebras and some examples of quantum groupoids and vertex groups related to them

Let A be a finite dimensional Hopf algebra and (H, R) a quasitriangular bialgebra. Denote by H^*_R a certain deformation of the multiplication of H^* via R. We prove that H^*_R is a quantum commutative left H\otimes H^{op cop}-module algebra. If H is the Drinfel'd double of A then H^*_R is the Heisenberg double of A. We study the relation between H^*_R and Majid's "covariantised product". We give a formula for the canonical element of the Heisenberg double of A, solution to the pentagon equation, in terms of the R-matrix of the Drinfel'd double of A. We generalize a theorem of Jiang-Hua Lu on quantum groupoids and using this and the above we obtain an example of a quantum groupoid having the Heisenberg double of A as base. If, in Richard Borcherds' concept of a "vertex group" we allow the "ring of singular functions" to be noncommutative, we prove that if A is a finite dimensional cocommutative Hopf algebra then the Heisenberg double of A is a vertex group over A. The construction and properties of H^*_R are given also for quasi-bialgebras and a definition for the Heisenberg double of a finite dimensional quasi-Hopf algebra is proposed.Comment: 30 pages, Latex, no figure

Dirac actions and Lu's Lie algebroid

Poisson actions of Poisson Lie groups have an interesting and rich geometric structure. We will generalize some of this structure to Dirac actions of Dirac Lie groups. Among other things, we extend a result of Jiang-Hua-Lu, which states that the cotangent Lie algebroid and the action algebroid for a Poisson action form a matched pair. We also give a full classification of Dirac actions for which the base manifold is a homogeneous space $H/K$, obtaining a generalization of Drinfeld's classification for the Poisson Lie group case.Comment: 41 pages. To appear in "Transformation Groups

Local well-posedness for the quadratic Schrodinger equation in two-dimensional compact manifolds with boundary

We consider the quadractic NLS posed on a bidimensional compact Riemannian manifold $(M, g)$ with $\partial M \neq \emptyset$. Using bilinear and gradient bilinear Strichartz estimates for Schr\"odinger operators in two-dimensional compact manifolds proved by J. Jiang in \cite{JIANG} we deduce a new evolution bilinear estimates. Consequently, using Bourgain's spaces, we obtain a local well-posedness result for given data $u_0\in H^s(M)$ whenever $s> \frac{2}{3}$ in such manifolds.Comment: 25 page

Turan Problems and Shadows I: Paths and Cycles

A $k$-path is a hypergraph P_k = e_1,e_2,...,e_k such that |e_i \cap e_j| = 1 if |j - i| = 1 and e_i \cap e_j is empty otherwise. A k-cycle is a hypergraph C_k = e_1,e_2,.. ,e_k obtained from a (k-1)-path e_1,e_2,...,e_{k-1} by adding an edge e_k that shares one vertex with e_1, another vertex with e_{k-1} and is disjoint from the other edges. Let ex_r(n,G) be the maximum number of edges in an r-graph with n vertices not containing a given r-graph G. We determine ex_r(n, P_k) and ex_r(n, C_k) exactly for all k \ge 4 and r \ge 3 and $n$ sufficiently large and also characterize the extremal examples. The case k = 3 was settled by Frankl and F\"{u}redi. This work is the next step in a long line of research beginning with conjectures of Erd\H os and S\'os from the early 1970's. In particular, we extend the work (and settle a recent conjecture) of F\"uredi, Jiang and Seiver who solved this problem for P_k when r \ge 4 and of F\"uredi and Jiang who solved it for C_k when r \ge 5. They used the delta system method, while we use a novel approach which involves random sampling from the shadow of an r-graph

Hypergraph Tur\'an numbers of vertex disjoint cycles

The Tur\'an number of a $k$-uniform hypergraph $H$, denoted by $e{x_k}\left({n;H} \right)$, is the maximum number of edges in any $k$-uniform hypergraph $F$ on $n$ vertices which does not contain $H$ as a subgraph. Let $\mathcal{C}_{\ell}^{\left(k \right)}$ denote the family of all $k$-uniform minimal cycles of length $\ell$, $\mathcal{S}(\ell_1,\ldots,\ell_r)$ denote the family of hypergraphs consisting of unions of $r$ vertex disjoint minimal cycles of length $\ell_1,\ldots,\ell_r$, respectively, and $\mathbb{C}_{\ell}^{\left(k \right)}$ denote a $k$-uniform linear cycle of length $\ell$. We determine precisely $e{x_k}\left({n;\mathcal{S}(\ell_1,\ldots,\ell_r)} \right)$ and $e{x_k}\left({n;\mathbb{C}_{{\ell_1}}^{\left(k \right)}, \ldots, \mathbb{C}_{{\ell_r}}^{\left(k \right)}} \right)$ for sufficiently large $n$. The results extend recent results of F\"{u}redi and Jiang who determined the Tur\'an numbers for single $k$-uniform minimal cycles and linear cycles.Comment: 9 pages. arXiv admin note: text overlap with arXiv:1302.2387 by other author

Permanence of stable rank one for centrally large subalgebras and crossed products by minimal homeomorphisms

We define centrally large subalgebras of simple unital C*-algebras, strengthening the definition of large subalgebras in previous work. We prove that if A is any infinite dimensional simple separable unital C*-algebra which contains a centrally large subalgebra with stable rank one, then A has stable rank one. We also prove that large subalgebras of crossed product type are automatically centrally large. We use these results to prove that if X is a compact metric space which has a surjective continuous map to the Cantor set, and h is a minimal homeomorphism of X, then C* (Z, X, h) has stable rank one, regardless of the dimension of X or the mean dimension of h. In particular, the Giol-Kerr examples give crossed products with stable rank one but which are not stable under tensoring with the Jiang-Su algebra and are therefore not classifiable in terms of the Elliott invariant.Comment: 32 pages; 2 figures; AMSLaTeX. Changes from first version: Improved exposition. Corrected misprints. Added mention of work using centrally large subalgebras constructed using methods which are not generalizations of Putnam's construction. Added reference