145,726 research outputs found
Vertex critical 4-dichromatic circulant tournaments
No full textAbstractAn infinite family of vertex critical 4-dichromatic circulant tournaments is presented, answering a problem posed by Neumann-Lara and Urrutia (1984)
A conjecture of Neumann-Lara on infinite families of r-dichromatic circulant tournaments
No full textAbstractIn this paper, we exhibit infinite families of vertex critical r-dichromatic circulant tournaments for all r≥3. The existence of these infinite families was conjectured by Neumann-Lara [V. Neumann-Lara, Note on vertex critical 4-dichromatic circulant tournaments, Discrete Math. 170 (1997) 289–291], who later proved it for all r≥3 and r≠7. Using different methods, we provide new constructions of such infinite families for all r≥3, which covers the case r=7 and thus settles the conjecture
Clique divergent clockwork graphs and partial orders
No full textAbstractS. Hazan and V. Neumann-Lara proved in 1996 that every finite partially ordered set whose comparability graph is clique null has the fixed point property and they asked whether there is a finite poset with the fixed point property whose comparability graph is clique divergent. In this work we answer that question by exhibiting such a finite poset. This is achieved by developing further the theory of clockwork graphs. We also show that there are polynomial time algorithms that recognize clockwork graphs and decide whether they are clique divergent
Three nontrivial solutions for the p-Laplacian Neumann problems with a concave nonlinearity near the origin
No full textWe consider a nonlinear Neumann problem driven by the p-
Laplacian, with a right-hand side nonlinearity which is concave near the
origin. Using variational techniques, combined with the method of upper-lower
solutions and with Morse theory, we show that the problem has at least three
nontrivial smooth solutions, two of which have a constant sign (one positive
and one negative).FCTPOCI/MAT/55524/200
Kochen-Specker theorem for von Neumann algebras
No full textThe Kochen-Specker theorem has been discussed intensely ever since its original proof in 1967. It is one of the central no-go theorems of quantum theory, showing the non-existence of a certain kind of hidden states models. In this paper, we first offer a new, non-combinatorial proof for quantum systems with a type I_n factor as algebra of observables, including I_infinity. Afterwards, we give a proof of the Kochen-Specker theorem for an arbitrary von Neumann algebra R without summands of types I_1 and I_2, using a known result on two-valued measures on the projection lattice P(R). Some connections with presheaf formulations as proposed by Isham and Butterfield are made
On clique-divergent graphs
No full textThis article presents results on expansive graphs that Víctor Neumann-Lara obtained in the late 1970’s and reported mostly without proofs in [8] and [9]. The untimely death of Víctor left us to complete this project, which had started in [7]. We had at hand [8, 9] and the manuscript [10] that, using his old notebooks
Álgebras de von Neumann - fatores
No full textDissertação (mestrado) - Universidade Federal de Santa Catarina, Centro de Ciências Físicas e Matemáticas, Programa de Pós-Graduação em Matemática e Computação Científica, Florianópolis, 2011Dada uma álgebra de von Neumann M em L(H), onde L(H) é o espaço dos operadores lineares e limitados sobre um espaço de Hilbert H, dizemos que M é um fator se seu centro consiste somente por múltiplos escalares do operador identidade de L(H). Quando M é um fator, podemos classificá-lo em tipo I, II e III. Além disso, o tipo II pode ser dividido em dois sub-tipos. O objetivo dessa dissertação é exibir exemplos de fatores, bem como exemplos dos tipos I, II e seus sub-tipos
Groupoid normalizers of tensor products
No full textWe consider an inclusion B [subset of or equal to] M of finite von Neumann algebras satisfying B′∩M [subset of or equal to] B. A partial isometry vset membership, variantM is called a groupoid normalizer if vBv*,v*Bv[subset of or equal to] B. Given two such inclusions B<sub>i</sub> [subset of or equal to] M<sub>i</sub>, i=1,2, we find approximations to the groupoid normalizers of [formula] in [formula], from which we deduce that the von Neumann algebra generated by the groupoid normalizers of the tensor product is equal to the tensor product of the von Neumann algebras generated by the groupoid normalizers. Examples are given to show that this can fail without the hypothesis [formula], i=1,2. We also prove a parallel result where the groupoid normalizers are replaced by the intertwiners, those partial isometries vset membership, variantM satisfying vBv*[subset of or equal to] B and v*v,vv*[set membership, variant] B
Mesh-based numerical implementation of the localized boundary-domain integral equation method to a variable-coefficient Neumann problem
No full textAn implementation of the localized boundary-domain integral-equation (LBDIE) method for the numerical solution of the Neumann boundary-value problem for a second-order linear elliptic PDE with variable coefficient is discussed. The LBDIE method uses a specially constructed localized parametrix (Levi function) to reduce the BVP to a LBDIE. After employing a mesh-based discretization, the integral equation is reduced to a sparse system of linear algebraic equations that is solved numerically. Since the Neumann BVP is not unconditionally and uniquely solvable, neither is the LBDIE. Numerical implementation of the finite-dimensional perturbation approach that reduces the integral equation to an unconditionally and uniquely solvable equation, is also discussed
- …
