Search

In this article, we define Dirichlet-type space $\mathcal{D}^{2}(\boldsymbol{\mu})$ over the bidisc $\mathbb D^2$ for any measure $\boldsymbol{\mu}\in\mathcal{P}\mathcal{M}_{+}(\mathbb T^2).$ We show that the set of polynomials is dense in $\mathcal{D}^{2}(\boldsymbol{\mu})$ and the pair $(M_{z_1}, M_{z_2})$ of multiplication operator by co-ordinate functions on $\mathcal{D}^{2}(\boldsymbol{\mu})$ is a pair of commuting $2$-isometries. Moreover, the pair $(M_{z_1}, M_{z_2})$ is a left-inverse commuting pair in the following sense: $L_{M_{z_i}} M_{z_j}=M_{z_j}L_{M_{z_i}}$ for $1\leqslant i\neq j\leqslant n,$ where $L_{M_{z_i}}$ is the left inverse of $M_{z_i}$ with $\ker L_{M_{z_i}} =\ker M_{z_i}^*$, $1\leqslant i \leqslant n$. Furthermore, it turns out that, for the class of left-inverse commuting tuple $\boldsymbol T=(T_1, \ldots, T_n)$ acting on a Hilbert space $\mathcal{H}$, the joint wandering subspace property is equivalent to the individual wandering subspace property. As an application of this, the article shows that the class of left-inverse commuting pair with certain splitting property is modelled by the pair of multiplication by co-ordinate functions $(M_{z_1}, M_{z_2})$ on $\mathcal{D}^{2}(\boldsymbol{\mu})$ for some $\boldsymbol{\mu}\in\mathcal{P}\mathcal{M}_{+}(\mathbb T^2).$, Comment: The article has been significantly revised

For a positive integer $m$ and a finite non-negative Borel measure $\mu$ on the unit circle, we study the Hadamard multipliers of higher order weighted Dirichlet-type spaces $\mathcal H_{\mu, m}$. We show that if $\alpha>\frac{1}{2},$ then for any $f$ in $\mathcal H_{\mu, m},$ the sequence of generalized Ces{\`a}ro sums $\{\sigma_n^{\alpha}[f]\}$ converges to $f$. We further show that if $\alpha=\frac{1}{2}$ then for the Dirac delta measure supported at any point on the unit circle, the previous statement breaks down for every positive integer $m$., Comment: 14 pages, comments and suggestions are welcome

In this paper we propose a generalization of the Grothendieck inequality for pairs of Banach spaces $E$ and $F$ with $E$ being finite dimensional and investigate the behaviour of the Grothendieck constant $K_G(E,F)$ implicit in such an inequality. We show that if $\sup\{K_G(E_n,F): n\geqslant 1\}$ is finite for some sequence of finite dimensional Banach spaces $(E_n)_{n\geqslant 1}$ with $\dim E_n=n$, and an infinite dimensional Banach space $F$, then both $F$ and $F^*$ must have finite cotype. In addition to that if $F$ has the bounded approximation property, we conclude that $(E_n^*)_{n\geqslant 1}$ satisfies G.T. uniformly by assuming the validity of a conjecture due to Pisier. We also show that $K_G(E,F)$ is closely related to the constant $\rho(E,F)$, introduced recently, comparing the projective and injective norms on the tensor product of two finite dimensional Banach spaces $E$ and $F$. We also study, analogously, these constants by computing the supremum only on non-negative tensors., Comment: Preliminary version, 21 pages

We prove a local Douglas formula for higher order weighted Dirichlet-type integrals. With the help of this formula, we study the multiplier algebra of the associated higher order weighted Dirichlet-type spaces $\mathcal H_{\pmb\mu},$ induced by an $m$-tuple $\pmb \mu =(\mu_1,\ldots,\mu_{m})$ of finite non-negative Borel measures on the unit circle. In particular, it is shown that any weighted Dirichlet-type space of order $m,$ for $m\geqslant 3,$ forms an algebra under pointwise product. We also prove that every non-zero closed $M_z$-invariant subspace of $\mathcal H_{\pmb\mu},$ has codimension $1$ property if $m\geqslant 3$ or $\mu_2$ is finitely supported. As another application of local Douglas formula obtained in this article, it is shown that for any $m\geqslant 2,$ weighted Dirichlet-type space of order $m$ does not coincide with any de Branges-Rovnyak space $\mathcal H(b)$ with equivalence of norms., Comment: 21 pages

In this manuscript, we report room temperature structural, microstructural, optical, dielectric, and magnetic properties of CuO and Cu0.995La0.005 ceramics, synthesized by solid-state reaction method. La doping in CuO leads to the evolution of compact and dense microstructure with reduced porosity. Due to noticeable differences in the ionic radii of, La doping creates vacancy defects which induce considerable strain in the CuO lattice resulting in a reduction in the lattice parameters and cell volume. However, both ceramics processes a similar monoclinic structure with the C2/c space group. Detailed characterization using XPS, Raman, and FTIR spectroscopy confirmed the incorporation of the La3+ in CuO lattice. Interestingly, La doping enhances the dielectric constant by more than three times and results in a reduced leakage current. The onset of a large dielectric constant is attributed to dense microstructure and strain/distortion in CuO lattice after La doping. Additionally, the band-gap of Cu0.995La0.005 ceramics decreases which is attributed to increased vacancy defect concentration that creates intermediate dopant energy level within bandgap of CuO matrix. Furthermore, improvement in magnetic and dielectric properties is also discussed and correlated with the grain size in La-doped CuO.

The bulk samples of CuO and Fe-doped CuO were synthesized by ceramics methods. Structural and compositional analyses were performed by using X-ray diffraction, SEM, and EDAX. Through this manuscript, we are going to report the effect of trivalent iron doping (Fe$^{3+}$) in copper (II) oxide (Cu$_{0.95}$Fe$_{0.05}$O) bulk samples on magnetic and dielectric behavior. The paramagnetic phase has been established in CuO as a result of Fe$^{3+}$ doping. The strong correlation between magnetic and dielectric properties indicated spin-polaron interaction at the transition temperature. Bulk CuO and also Cu$_{0.95}$Fe$_{0.05}$O exhibit the multiferroic phase in a narrow temperature range (190 K to 230 K). Two transitions happened from a paramagnetic-paraelectric phase to incommensurate or asymmetrical antiferromagnetic (AF) and ferroelectric state near highest Neel temperature (TN1) ~230 K and another second phase transition, the order of AF phase transformed to commensurate AF phase and ferroelectricity disappeared at around the Neel temperature (TN2) ~210 K in all samples. This Cu$_{0.95}$Fe$_{0.05}$O would show its potential in the spintronic application for a high dielectric constant with low loss and high magnetic susceptibility., Comment: We have submitted 12 figure with 22 pages. arXiv admin note: text overlap with arXiv:1807.09697 by other authors

We obtain a formula that relates the spherical moments of the multiplication tuple on a Dirichlet-type space to a complex moment problem in several variables. This can be seen as the ball-analogue of a formula originally invented by Richter. We capitalize on this formula to study Dirichlet-type spaces on the unit ball and joint $2$-isometries., Comment: 24 pages, major revision, appendix added

We systematically study various aspects of operator-valued multishifts. Beginning with basic properties, we show that the class of multishifts on the directed Cartesian product of rooted directed trees is contained in that of operator-valued multishifts. Further, we establish circularity, analyticity and wandering subspace property of these multishifts. In the rest part of the paper, we study the function theoretic behaviour of operator-valued multishifts. We determine the bounded point evaluation, reproducing kernel structure and the unitary equivalence of operator-valued multishifts with invertible operator weights. In contrast with a result of Lubin, it appears that the set of all bounded point evaluations of an operator-valued multishift may be properly contained in the joint point spectrum of the adjoint of underlying multishift., Comment: 23 pages