Weakly Terminal Objects in Quasicategories of $\mathbb{SET}$ Endofunctors

The quasicategory ℚ of all set functors (i.e. endofunctors of the category of all sets and mappings) and all natural transformations has a terminal object – the constant functor C₁. We construct here the terminal (or at least the smallest weakly terminal object, which is rigid) in some important subquasicategories of ℚ – in the quasicategory of faithful connected set functors and all natural transformations, and in the quasicategories of all set functors and natural transformations which preserve filters of points (up to cardinality κ).Mathematics Subject Classifications (2000) 18A22, 18A25.Libor Barto: This work was completed with the support of the Grant Agency of the Czech Republic under the grant 201/02/0148; supported also by MSM 113200007.     

Tameness of Pseudovariety Joins Involving ${\sf R}$

In this paper, we establish several decidability results for pseudovariety joins of the form , where is a subpseudovariety of or the pseudovariety . Here, (resp. ) denotes the pseudovariety of all -trivial (resp. -trivial) semigroups. In particular, we show that the pseudovariety is (completely) κ-tame when is a subpseudovariety of with decidable κ-word problem and is (completely) κ-tame. Moreover, if is a κ-tame pseudovariety which satisfies the pseudoidentity x₁ ⋯ x_ry^ω+1zt^ω = x₁ ⋯ x_ryzt^ω, then we prove that is also κ-tame. In particular the joins , , , and are decidable. Partial support by FCT, through the Centro de Matemática da Universidade do Porto, is also gratefully acknowledged. Partial support by FCT, through the Centro de Matemática da Universidade do Minho, is also gratefully acknowledged.     

Characterizations of the Vertex Operator Algebras ${V_{L}^{T}}$ and VLO${V_{L}^{O}}$

In this paper, we give characterizations of the rational vertex operator algebras \({V_{L}^{T}}\) and \({V_{L}^{O}}\), where L is the root lattice of type A ₁, T is the tetrahedral group, and O is the octahedral group. By these two characterizations, the classification of rational VOAs of central charge 1 is reduced to the characterization of \({V_{L}^{I}}\) where I is the icosahedral group.     

Cartesian Closed Subcategories of $CONT_{\ll }^{{\kern 8pt}\ast }$

Let CONT _? be the category of continuous domains and Scott continuous mappings that preserve the way-below relation on domains. Let ω-ALG _? be the full subcategory of CONT _? consisting of all countably based algebraic domains, and F I N be the category of finite posets and monotone mappings. The main result proved in this paper is that F I N is the largest Cartesian closed full subcategory of ω-ALG _?. On the other hand, it is shown that the algebraic L-domains form a Cartesian closed full subcategory of ALG _?.     

Dominated polynomials on $\mathcal{L}_P $ -spaces

It is well known that continuous bilinear forms on C(K) × C(K) are 2-dominated. This paper shows that generalizations of this result are not to be expected. The main result asserts that for every -space E(1 p ), every n 2, every r > 0 and every Banach space F , there exists an n-homogeneous polynomial P : E F such that P is not of type [_r], hence P is neither r-dominated nor r-semi-integral (if n = 2 and p = , F is supposed to contain an isomorphic copy of some , 1_q < ).Received: 24 November 2003     

On $ {p} $‐groups of maximal class

Let R be the ring $ {\mathbb Z}[x]/\left({{x^p-1}\over{x-1}}\right) = {\mathbb Z}[\bar{x}] $ and let $ \mathfrak {a} $ be the ideal of R generated by $ (\bar{x}-1) $ . In this paper, we discuss the structure of the $ {\mathbb Z}[C_p] $‐module $ (R/\mathfrak {a}^{n-1}) \wedge (R/\mathfrak {a}^{n-1}) $, which plays an important role in the theory of p‐groups of maximal class (see 2 - 5 ). The generators of this module allow us to obtain the defining relations of some important examples of p‐groups of maximal class with Y₁ of class two. In particular we obtain the best possible estimates for the degree of commutativity of p‐groups of maximal class with Y₁ of class two. © 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim     

Towards factoring in {SL(2,\,\mathbb{F}_{2^n})}

The security of many cryptographic protocols relies on the hardness of some computational problems. Besides discrete logarithm or integer factorization, other problems are regularly proposed as potential hard problems. The factorization problem in finite groups is one of them. Given a finite group G, a set of generators generators for this group and an element ${g\in G}$ , the factorization problem asks for a “short” representation of g as a product of the generators. The problem is related to a famous conjecture of Babai on the diameter of Cayley graphs. It is also motivated by the preimage security of Cayley hash functions, a particular kind of cryptographic hash functions. The problem has been solved for a few particular generator sets, but essentially nothing is known for generic generator sets. In this paper, we make significant steps towards a solution of the factorization problem in the group ${G:=\,SL(2,\,\mathbb{F}_{2^n})}$ , a particularly interesting group for cryptographic applications. To avoid considering all generator sets separately, we first give a new reduction tool that allows focusing on some generator sets with a “nice” special structure. We then identify classes of trapdoor matrices for these special generator sets, such that the factorization of a single one of these matrices would allow efficiently factoring any element in the group. Finally, we provide a heuristic subexponential time algorithm that can compute subexponential length factorizations of any element for any pair of generators of ${SL(2,\,\mathbb{F}_{2^n})}$ . Our results do not yet completely remove the factorization problem in ${SL(2,\,\mathbb{F}_{2^n})}$ from the list of potential hard problems useful for cryptography. However, we believe that each one of our individual results is a significant step towards a polynomial time algorithm for factoring in ${SL(2,\,\mathbb{F}_{2^n})}$ .     

Covering and packing in ${\Bbb Z}^n$ and ${\Bbb R}^n$, (I)

Given a finite subset of an additive group such as or , we are interested in efficient covering of by translates of , and efficient packing of translates of in . A set provides a covering if the translates with cover (i.e., their union is ), and the covering will be efficient if has small density in . On the other hand, a set will provide a packing if the translated sets with are mutually disjoint, and the packing is efficient if has large density. In the present part (I) we will derive some facts on these concepts when , and give estimates for the minimal covering densities and maximal packing densities of finite sets . In part (II) we will again deal with , and study the behaviour of such densities under linear transformations. In part (III) we will turn to . Authors’ address: Department of Mathematics, University of Colorado at Boulder, Campus Box 395, Boulder, Colorado 80309-0395, USA The first author was partially supported by NSF DMS 0074531.     

Sharp Subcritical and Critical Trudinger-Moser Inequalities on $\mathbb {R}^{2}$ and their Extremal Functions

In this paper, we study on \(\mathbb {R}^{2}\) some new types of the sharp subcritical and critical Trudinger-Moser inequality that have close connections to the study of the optimizers for the classical Trudinger-Moser inequalities. For instance, one of our results can be read as follows: Let 0 ≤ β < 2, p ≥ 0, α ≥ 0. Then

$$\sup_{\left\Vert \nabla u\right\Vert_{2}^{2}+\left\Vert u\right\Vert_{2} ^{2}\leq1}\left\Vert u\right\Vert_{2}^{p}{\int}_{\mathbb{R}^{2}}\exp\left( \alpha\left( 1-\frac{\beta}{2}\right) \left\vert u\right\vert^{2}\right) \left\vert u\right\vert^{2}\frac{dx}{\left\vert x\right\vert^{\beta}}<\infty $$

if and only if α < 4π or α = 4π, p ≥ 2. The attainability and inattainability of these sharp inequalties will be also investigated using a new approach, namely the relations between the supremums of the sharp subcritical and critical ones. This new method will enable us to compute explicitly the supremums of the subcritical Trudinger-Moser inequalities in some special cases. Also, a version of Concentration-compactness principle in the spirit of Lions ( Lions, I. Rev. Mat. Iberoam. 1(1) 145–01 1985) will also be studied.

     

Programmable criteria for strong $\mathcal {H}$-tensors

Strong \(\mathcal {H}\)-tensors play an important role in identifying positive semidefiniteness of even-order real symmetric tensors. We provide several simple practical criteria for identifying strong \(\mathcal {H}\)-tensors. These criteria only depend on the elements of the tensors; therefore, they are easy to be verified. Meanwhile, a sufficient and necessary condition of strong \(\mathcal {H}\)-tensors is obtained. We also propose an algorithm for identifying the strong \(\mathcal {H}\)-tensors based on these criterions. Some numerical results show the feasibility and effectiveness of the algorithm.     

Constant angle surfaces in $ {\Bbb S}^2\times {\Bbb R} $

In this article we study surfaces in for which the unit normal makes a constant angle with the -direction. We give a complete classification for surfaces satisfying this simple geometric condition.     

Orthogonal Polynomials Associated with Equilibrium Measures on $\mathbb {R}$

Let K be a non-polar compact subset of \(\mathbb {R}\) and μ _K denote the equilibrium measure of K. Furthermore, let P _n (?;μ _K ) be the n-th monic orthogonal polynomial for μ _K . It is shown that \(\|P_{n}\left (\cdot ; \mu _{K}\right )\|_{L^{2}(\mu _{K})}\), the Hilbert norm of P _n (?;μ _K ) in L ²(μ _K ), is bounded below by Cap(K) ⁿ for each \(n\in \mathbb {N}\). A sufficient condition is given for\(\left (\|P_{n}\left (\cdot ;\mu _{K}\right )\|_{L^{2}(\mu _{K})}/\text {Cap}(K)^{n}\right )_{n=1}^{\infty }\) to be unbounded. More detailed results are presented for sets which are union of finitely many intervals.     

The Idempotents of the TL n -Module {\otimes^{n}\mathbb{C}2} in Terms of Elements of {{\rm U}_{q} \mathfrak{sl}_2}

The vector space \({\otimes^{n}\mathbb{C}^2}\) upon which the XXZ Hamiltonian with n spins acts bears the structure of a module over both the Temperley–Lieb algebra \({{\rm TL}_{n}(\beta = q + q^{-1})}\) and the quantum algebra \({{\rm U}_{q} \mathfrak{sl}_2}\) . The decomposition of \({\otimes^{n}\mathbb{C}^2}\) as a \({{\rm U}_{q} \mathfrak{sl}_2}\) -module was first described by Rosso (Commun Math Phys 117:581–593, 1988), Lusztig (Cont Math 82:58–77, 1989) and Pasquier and Saleur (Nucl Phys B 330:523–556, 1990) and that as a TL _n -module by Martin (Int J Mod Phys A 7:645–673, 1992) (see also Read and Saleur Nucl Phys B 777(3):316–351, 2007; Gainutdinov and Vasseur Nucl Phys B 868:223–270, 2013). For q generic, i.e. not a root of unity, the TL _n -module \({\otimes^{n}\mathbb{C}^2}\) is known to be a sum of irreducible modules. We construct the projectors (idempotents of the algebra of endomorphisms of \({\otimes^{n}\mathbb{C}^2}\) ) onto each of these irreducible modules as linear combinations of elements of \({{\rm U}_{q} \mathfrak{sl}_2}\) . When q = q _c is a root of unity, the TL _n -module \({\otimes^{n}\mathbb{C}^2}\) (with n large enough) can be written as a direct sum of indecomposable modules that are not all irreducible. We also give the idempotents projecting onto these indecomposable modules. Their expression now involves some new generators, whose action on \({\otimes^{n}\mathbb{C}^2}\) is that of the divided powers \({(S^{\pm})^{(r)} = \lim_{q \rightarrow q_{c}} (S^{\pm})^r/[r]!}\) .     

Representation theory of $\mathcal{W}$-algebras

We study the representation theory of the -algebra associated with a simple Lie algebra at level k. We show that the “-” reduction functor is exact and sends an irreducible module to zero or an irreducible module at any level k∈ℂ. Moreover, we show that the character of each irreducible highest weight representation of is completely determined by that of the corresponding irreducible highest weight representation of affine Lie algebra of . As a consequence we complete (for the “-” reduction) the proof of the conjecture of E. Frenkel, V. Kac and M. Wakimoto on the existence and the construction of the modular invariant representations of -algebras. Mathematics Subject Classification (1991)  17B68, 81R10     

Second Order Elliptic Equations in $\mathbb{R}^{d} $ with Piecewise Continuous Coefficients

The existence and uniqueness of solutions of second order elliptic differential equations in are proved. The coefficients of second order terms are allowed to have discontinuity at finitely many parallel hyper-planes in and the first derivatives of solutions can have jumps at the hyper-planes.      

Improving algebraic attacks on stream ciphers based on linear feedback shift register over $$\mathbb {F}_{2^k}$$

In this paper we investigate univariate algebraic attacks on filter generators over extension fields \(\mathbb {F}_q=\mathbb {F}_{2^n}\) with focus on the Welch–Gong (WG) family of stream ciphers. Our main contribution is to reduce the general algebraic attack complexity on such cipher by proving new and lower bounds for the spectral immunity of such ciphers. The spectral immunity is the univariate analog of algebraic immunity and instead of measuring degree of multiples of a multivariate polynomial, it measures the minimum number of nonzero coefficients of a multiple of a univariate polynomial. In particular, there is an algebraic degeneracy in these constructions, which, when combined with attacks based on low-weight multiples over \(\mathbb {F}_q\), provides much more efficient attacks than over \(\mathbb {F}_2\). With negligible computational complexity, our best attack breaks the primitive WG-5 if given access to 4 kilobytes of keystream, break WG-7 if given access to 16 kilobytes of keystream and break WG-8 if given access to half a megabyte of keystream. Our best attack on WG-16 targeted at 4G-LTE is less practical, and requires \(2^{103}\) computational complexity and \(2^{61}\) bits of keystream. In all instances, we significantly lower both keystream and computational complexity in comparison to previous estimates. On a side note, we resolve an open problem regarding the rank of a type of equation systems used in algebraic attacks.     

A variational characterization for $\sigma_{n/2}$

We present here a conformal variational characterization in dimension n = 2k of the equation , where A is the Schouten tensor. Using the fully nonlinear parabolic flow introduced in [3], we apply this characterization to the global minimization of the functional.Received: 31 March 2003, Accepted: 10 July 2003, Published online: 25 February 2004Research supported in part by an NSF Postdoctoral Fellowship.     

A Single Cell in an Arrangement of Convex Polyhedra in ${\Bbb R}^3$

We show that the combinatorial complexity of a single cell in an arrangement of k convex polyhedra in 3-space having n facets in total is , for any , thus settling a conjecture of Aronov et al. We then extend our analysis and show that the overall complexity of the zone of a low-degree algebraic surface, or of the boundary of an arbitrary convex set, in an arrangement of k convex polyhedra in 3-space with n facets in total, is also , for any . Finally, we present a deterministic algorithm that constructs a single cell in an arrangement of this kind, in time , for any .