Estudo Geralhttps://estudogeral.sib.uc.ptThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Mon, 17 Feb 2020 08:28:29 GMT2020-02-17T08:28:29Z50331Product of diagonal entries of the unitary orbit of a 3-by-3 normal matrixhttp://hdl.handle.net/10316/4578Title: Product of diagonal entries of the unitary orbit of a 3-by-3 normal matrix
Authors: Nakazato, Hiroshi; Bebiano, Natália; Providência, João da
Abstract: Let N be a 3×3 normal matrix. We investigate the sets where U(3) is the group of 3×3 unitary matrices and 1[less-than-or-equals, slant]k[less-than-or-equals, slant]3. Geometric properties of these sets are studied, namely, star-shapedness and simple connectedness are investigated. A method for the numerical estimation of is also provided for normal matrices of size 3.
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/10316/45782008-01-01T00:00:00ZThe J-numerical range of a J-Hermitian matrix and related inequalitieshttp://hdl.handle.net/10316/4585Title: The J-numerical range of a J-Hermitian matrix and related inequalities
Authors: Nakazato, Hiroshi; Bebiano, Natália; Providência, João da
Abstract: Recently, indefinite versions of classical inequalities of Schur, Ky Fan and Rayleigh-Ritz on Hermitian matrices have been obtained for J-Hermitian matrices that are J-unitarily diagonalizable, J=Ir[circle plus operator](-Is),r,s>0. The inequalities were obtained in the context of the theory of numerical ranges of operators on indefinite inner product spaces. In this paper, the subject is revisited, relaxing the constraint of the matrices being J-unitarily diagonalizable.
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/10316/45852008-01-01T00:00:00ZLong-wave fluctuations of the two-dimensional Heisenberg antiferromagnethttp://hdl.handle.net/10316/4532Title: Long-wave fluctuations of the two-dimensional Heisenberg antiferromagnet
Authors: Belinicher, Victor; Providência, João da
Abstract: We have reduced the Heisenberg model for the quantum antiferromagnet in the leading approximation over 1/s to the spin-rotator model which represents some sort of quantum lattice rotator model. We perform the long-wavelength reduction of this spin-rotator model. An intermediate scale [Lambda] was introduced and the short-wavelength antiferromagnetic spin fluctuations were integrated out. As a result, we obtained the continuum nonlinear sigma model with some additional contributions. We presented arguments showing that this model is renormalizable in the framework of 1/N perturbation theory. We separated out all ultraviolet divergencies and expressed all parameters of the theory in terms of the parameters of the primary Heisenberg model, in the one-loop approximation.
Sat, 01 Jan 2000 00:00:00 GMThttp://hdl.handle.net/10316/45322000-01-01T00:00:00ZExact solutions for the LMG model Hamiltonian based on the Bethe ansatzhttp://hdl.handle.net/10316/4412Title: Exact solutions for the LMG model Hamiltonian based on the Bethe ansatz
Authors: Morita, Hiroyuki; Ohnishi, Hiromasa; Providência, João da; Nishiyama, Seiya
Abstract: Exact solutions for the Lipkin-Meshkov-Glick (LMG) model Hamiltonian are obtained by solving the Bethe ansatz equation (BAE) which is derived from the variation equation based on the Bethe ansatz. Unlike Pan and Draayer, we do not use bosonization and infinite-dimensional algebra techniques. Consequently there are no restrictions on parameters specifying strengths of the interactions included in the LMG Hamiltonian. Thus, for all the regimes of the interaction parameters, we get the exact solutions for the LMG Hamiltonian by numerically solving the BAEs and give the numerical behaviour of an order parameter .
Sun, 01 Jan 2006 00:00:00 GMThttp://hdl.handle.net/10316/44122006-01-01T00:00:00ZThe numerical range of 2-dimensional Krein spaces operatorshttp://hdl.handle.net/10316/11374Title: The numerical range of 2-dimensional Krein spaces operators
Authors: Nakazato, Hiroshi; Bebiano, Natália; Providência, João da
Abstract: The tracial numerical range of operators on a 2-dimensional Krein
space is investigated. Results in the vein of those obtained in the context of Hilbert
spaces are stated
Sun, 01 Jan 2006 00:00:00 GMThttp://hdl.handle.net/10316/113742006-01-01T00:00:00ZConvexity of the Krein space tracial numerical range and Morse theoryhttp://hdl.handle.net/10316/13648Title: Convexity of the Krein space tracial numerical range and Morse theory
Authors: Nakazato, Hiroshi; Bebiano, Natália; Providência, João da
Abstract: In this paper we present a Krein space convexity theorem on the tracial-numerical range of a matrix. This theorem is the analogue of Westwick's theorem.
The proof is an application of Morse theory.
Thu, 01 Jan 2009 00:00:00 GMThttp://hdl.handle.net/10316/136482009-01-01T00:00:00ZProducts of Laurent operators and fields of valueshttp://hdl.handle.net/10316/44386Title: Products of Laurent operators and fields of values
Authors: Bebiano, Natália; da Providência, João
Tue, 01 Nov 2016 00:00:00 GMThttp://hdl.handle.net/10316/443862016-11-01T00:00:00ZOn the corners of certain determinantal rangeshttp://hdl.handle.net/10316/4598Title: On the corners of certain determinantal ranges
Authors: Kovačec, Alexander; Bebiano, Natália; Providência, João da
Abstract: Let A be a complex n×n matrix and let SO(n) be the group of real orthogonal matrices of determinant one. Define [Delta](A)={det(AoQ):Q[set membership, variant]SO(n)}, where o denotes the Hadamard product of matrices. For a permutation [sigma] on {1,...,n}, define It is shown that if the equation z[sigma]=det(AoQ) has in SO(n) only the obvious solutions (Q=([epsilon]i[delta][sigma]i,j), [epsilon]i=±1 such that [epsilon]1...[epsilon]n=sgn[sigma]), then the local shape of [Delta](A) in a vicinity of z[sigma] resembles a truncated cone whose opening angle equals , where [sigma]1, [sigma]2 differ from [sigma] by transpositions. This lends further credibility to the well known de Oliveira Marcus Conjecture (OMC) concerning the determinant of the sum of normal n×n matrices. We deduce the mentioned fact from a general result concerning multivariate power series and also use some elementary algebraic topology.
Mon, 01 Jan 2007 00:00:00 GMThttp://hdl.handle.net/10316/45982007-01-01T00:00:00ZVariational formulation of the Vlasov equationhttp://hdl.handle.net/10316/40854Title: Variational formulation of the Vlasov equation
Authors: Providência, João da; Fiolhais, Carlos
Abstract: A new variational framework for the Liouville equation is presented. The Vlasov equation is obtained from the complete factorisation of the trial distribution function, while the linearised Vlasov equation arises from the additional assumption of small oscillations around an equilibrium state. It is stressed that the definition of this state does not necessarily require minimisation of the free energy. The classical energy-weighted sum rule is derived.
Thu, 01 Jan 1987 00:00:00 GMThttp://hdl.handle.net/10316/408541987-01-01T00:00:00ZVlasov description of the collision between two slabshttp://hdl.handle.net/10316/40856Title: Vlasov description of the collision between two slabs
Authors: Providência, João da; Fiolhais, Carlos
Abstract: A simple analytical description of the adiabatic collision between two slabs of nuclear matter is presented, as an illustration of a recent variational formulation of the Vlasov equation.
Fri, 01 Jan 1988 00:00:00 GMThttp://hdl.handle.net/10316/408561988-01-01T00:00:00ZCollective and intrinsic degrees of freedom in the Heisenberg ferromagnet.http://hdl.handle.net/10316/40857Title: Collective and intrinsic degrees of freedom in the Heisenberg ferromagnet.
Authors: Providência, João da; Brajczewska, Marta; Fiolhais, Carlos
Abstract: A separation between collective and intrinsic degrees of freedom in the Heisenberg model of a ferromagnet at a finite temperature is achieved by reformulating the thermal boson expansion previously obtained by the present authors. In the new approach, all the collective modes (spin waves) have wavenumbers lower than a certain value. The intrinsic energy turns out to be the usual mean-field energy minus a correlation energy due to the excitations of spin waves. The influence of the cutoff in momentum space on the critical temperature is studied and the temperature dependence of the spin-wave renormalisation factor and the magnetisation are calculated.
Sun, 01 Jan 1989 00:00:00 GMThttp://hdl.handle.net/10316/408571989-01-01T00:00:00ZMean-filed theories with mixed states and associated boson expansionshttp://hdl.handle.net/10316/14707Title: Mean-filed theories with mixed states and associated boson expansions
Authors: Providência, João da; Fiolhais, C.
Abstract: A variational derivation of the Liouville-von Neumann equation of quantum-statistical mechanics is presented, in order to formulate mean-field approximations appropriate to mixed states. The Hartree-Fock and the RPA at finite temperatures are particular cases of the general formalism. A thermal boson expansion is defined, which allows us to describe anharmonic motion around a thermal excited state. In a numerical application on the basis of the Lipkin model, temperature-dependent phase transitions are observed.
Mon, 04 Mar 1985 00:00:00 GMThttp://hdl.handle.net/10316/147071985-03-04T00:00:00ZOn the random phase approximation based on the thermo field dynamics formalismhttp://hdl.handle.net/10316/40851Title: On the random phase approximation based on the thermo field dynamics formalism
Authors: Yamamura, Masatoshi; Providência, João da; Kuriyama, Atsushi; Fiolhais, Carlos
Abstract: A comparative stdy of the random phase approximation is reported in the case of the following three approches based on the thermo field dynamics formalism: (A) the Tanabe approach, (B) the Hatsuda approach and (C) the approach developed by the present authors. The basic viewpoint is to formulate the random phase approximation by picking up the quadratic terms with respect to bosons in the boson expansion theory for the thermo field dynamics formalism. It is concluded tha the approach (C) appears to be superior to the other two approaches.
Mon, 01 Jan 1990 00:00:00 GMThttp://hdl.handle.net/10316/408511990-01-01T00:00:00ZVariational principles in quantum statistical mechanicshttp://hdl.handle.net/10316/40853Title: Variational principles in quantum statistical mechanics
Authors: Providência, João da; Fiolhais, Carlos
Abstract: The principle of least information is used to derive the inequality between the arithmetic and the
geometric mean. Some important variational results in quantum statistical mechanics (Peierls and Bogolyubov inequalities) are pedagogically reviewed. Emphasis is put on the distinction between mixed states describing metastable and stable equilibrium.
Thu, 01 Jan 1987 00:00:00 GMThttp://hdl.handle.net/10316/408531987-01-01T00:00:00ZA note on thermal bosonshttp://hdl.handle.net/10316/41571Title: A note on thermal bosons
Authors: Providência, João da; Fiolhais, Carlos
Mon, 01 Jan 1990 00:00:00 GMThttp://hdl.handle.net/10316/415711990-01-01T00:00:00ZThermal boson expansion for the Heisenberg ferromagnethttp://hdl.handle.net/10316/41958Title: Thermal boson expansion for the Heisenberg ferromagnet
Authors: Brajczewska, Marta; Fiolhais, Carlos; Providência, João da
Wed, 01 Jan 1986 00:00:00 GMThttp://hdl.handle.net/10316/419581986-01-01T00:00:00ZMean field theories with mixed stateshttp://hdl.handle.net/10316/41959Title: Mean field theories with mixed states
Authors: Providência, João da; Fiolhais, Carlos
Tue, 01 Jan 1985 00:00:00 GMThttp://hdl.handle.net/10316/419591985-01-01T00:00:00ZThe EMM and the spectral analysis of a non self-adjoint Hamiltonian on an infinite dimensional Hilbert spacehttp://hdl.handle.net/10316/44378Title: The EMM and the spectral analysis of a non self-adjoint Hamiltonian on an infinite dimensional Hilbert space
Authors: Bebiano, Natália; da Providência, João
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/10316/443782016-01-01T00:00:00ZRevisiting the inverse field of values problemahttp://hdl.handle.net/10316/44404Title: Revisiting the inverse field of values problema
Authors: Bebiano, Natália; da Providência, João; Nata, Ana; da Providência, J. P.
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/10316/444042014-01-01T00:00:00ZComputing the numerical range of Krein space operatorshttp://hdl.handle.net/10316/44403Title: Computing the numerical range of Krein space operators
Authors: Bebiano, Natália; da Providência, João; Nata, Ana; da Providência, J. P.
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/10316/444032015-01-01T00:00:00ZAn inverse indefinite numerical range problemhttp://hdl.handle.net/10316/44379Title: An inverse indefinite numerical range problem
Authors: Bebiano, Natália; da Providência, João; Nata, Ana; da Providência, J. P.
Wed, 01 Apr 2015 00:00:00 GMThttp://hdl.handle.net/10316/443792015-04-01T00:00:00ZAn algorithm for constructing a pseudo-Jacobi matrix from given spectral datahttp://hdl.handle.net/10316/44377Title: An algorithm for constructing a pseudo-Jacobi matrix from given spectral data
Authors: Bebiano, Natália; Furtado, Susana; da Providência, João
Fri, 01 Mar 2013 00:00:00 GMThttp://hdl.handle.net/10316/443772013-03-01T00:00:00ZThe numerical range of banded biperiodic Toeplitz operatorshttp://hdl.handle.net/10316/44344Title: The numerical range of banded biperiodic Toeplitz operators
Authors: Bebiano, Natália; da Providência, João; Nata, Ana
Fri, 01 Feb 2013 00:00:00 GMThttp://hdl.handle.net/10316/443442013-02-01T00:00:00ZImplications of losing Hermiticity in quantum mechanicshttp://hdl.handle.net/10316/44380Title: Implications of losing Hermiticity in quantum mechanics
Authors: Bebiano, Natália; da Providência, João
Mon, 12 Sep 2016 00:00:00 GMThttp://hdl.handle.net/10316/443802016-09-12T00:00:00ZA fiedler-type theorem for the determinant of J-positive matriceshttp://hdl.handle.net/10316/44384Title: A fiedler-type theorem for the determinant of J-positive matrices
Authors: Bebiano, Natália; da Providência, João
Fri, 01 Apr 2016 00:00:00 GMThttp://hdl.handle.net/10316/443842016-04-01T00:00:00ZDeterminantal inequalities for J-accretive dissipative matriceshttp://hdl.handle.net/10316/44385Title: Determinantal inequalities for J-accretive dissipative matrices
Authors: Bebiano, Natália; da Providência, João
Abstract: In this note we determine bounds for the determinant of the sum of two J-accretive dissipative matrices with prescribed spectra.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/10316/443852017-01-01T00:00:00ZFields of values of linear pencils and spectral inclusion regionshttp://hdl.handle.net/10316/44402Title: Fields of values of linear pencils and spectral inclusion regions
Authors: Bebiano, Natália; da Providência, João; Nata, Ana; da Providência, J. P.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/10316/444022017-01-01T00:00:00ZThe characteristic polynomial of linear pencils of small size and the numerical rangehttp://hdl.handle.net/10316/44382Title: The characteristic polynomial of linear pencils of small size and the numerical range
Authors: Bebiano, Natália; da Providência, João; Esmaeili, F.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/10316/443822017-01-01T00:00:00ZEquation of collective submanifold with mixed stateshttp://hdl.handle.net/10316/41577Title: Equation of collective submanifold with mixed states
Authors: Yamamura, Masatoshi; Providência, João da; Kuriyama, Atsushi; Fiolhais, Carlos
Sun, 01 Jan 1989 00:00:00 GMThttp://hdl.handle.net/10316/415771989-01-01T00:00:00ZApplication of the thermal boson expansion to the Heisenberg antiferromagnet MnF2http://hdl.handle.net/10316/42217Title: Application of the thermal boson expansion to the Heisenberg antiferromagnet MnF2
Authors: Brajczewska, Marta; Fiolhais, Carlos; Providência, João da
Mon, 01 Jan 1990 00:00:00 GMThttp://hdl.handle.net/10316/422171990-01-01T00:00:00ZDiscontinuous phase transition obtained with the thermal boson expansionhttp://hdl.handle.net/10316/41575Title: Discontinuous phase transition obtained with the thermal boson expansion
Authors: Fiolhais, Carlos; Brajczewska, Marta; Providência, João da
Fri, 01 Jan 1993 00:00:00 GMThttp://hdl.handle.net/10316/415751993-01-01T00:00:00ZApplication of the thermal boson expansion to the Heisenberg ferromagnets EuO and EuShttp://hdl.handle.net/10316/41957Title: Application of the thermal boson expansion to the Heisenberg ferromagnets EuO and EuS
Authors: Fiolhais, Carlos; Brajczewska, Marta; Providência, João da
Thu, 01 Jan 1987 00:00:00 GMThttp://hdl.handle.net/10316/419571987-01-01T00:00:00ZVariational derivation of mean field theories with mixed stateshttp://hdl.handle.net/10316/40949Title: Variational derivation of mean field theories with mixed states
Authors: Providência, João da; Fiolhais, Carlos
Tue, 01 Jan 1985 00:00:00 GMThttp://hdl.handle.net/10316/409491985-01-01T00:00:00Z