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In this paper we consider functions in the Hardy space $mathbf{H}_2^{ptimes q}$ defined in the unit disc of matrix-valued. We show that it is possible, as in the scalar case, to decompose those functions as linear combinations of suitably modified matrix-valued Blaschke product, in an adaptive way. The procedure is based on a generalization to the matrix-valued case of the maximum selection principle which involves not only selections of suitable points in the unit disc but also suitable orthogonal projections. We show that the maximum selection principle gives rise to a convergent algorithm. Finally, we discuss the case of real-valued signals.
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Finite-temperature quantum field theories are formulated in terms of Greens functions and self-energies on the Matsubara axis. In multi-orbital systems, these quantities are related to positive semidefinite matrix-valued functions of the Caratheodory and Schur class. Analysis, interpretation and evaluation of derived quantities such as real-frequency response functions requires analytic continuation of the off-diagonal elements to the real axis. We derive the criteria under which such functions exist for given Matsubara data and present an interpolation algorithm that intrinsically respects their mathematical properties. For small systems with precise Matsubara data, we find that the continuation exactly recovers all off-diagonal and diagonal elements. In real-materials systems, we show that the precision of the continuation is sufficient for the analytic continuation to commute with the Dyson equation, and we show that the commonly used truncation of off-diagonal self-energy elements leads to considerable approximation artifacts. Our method paves the way for the systematic evaluation of Matsubara data with equations of many-body theory on the real-frequency axis.
In this article, we prove a Strichartz type inequality %associated with Schrodinger equation for a system of orthonormal functions associated with the special Hermite operator $mathcal{L}=-Delta+frac{1}{4}|z|^{2}-i sum_{1}^{n}left(x_{j} frac{partial}{partial y_{j}}-y_{j} frac{partial}{partial x_{j}}right), $ where $Delta$ denotes the Laplacian on $mathbb{C}^{n}$.
The Strichartz inequality for the system of orthonormal functions for the Hermite operator $H=-Delta+|x|^2$ on $mathbb{R}^n$ has been proved in cite{lee}, using the classical Strichartz estimates for the free Schrodinger propagator for orthonormal systems cite{frank, frank1} and the link between the Schrodinger kernel and the Mehler kernel associated with the Hermite semigroup cite{SjT}. In this article, we give an alternative proof of the above result in connection with the restriction theorem with respect to the Hermite transform with an optimal behavior of the constant in the limit of a large number of functions. As an application, we show the well-posedness results in Schatten spaces for the nonlinear Hermite-Hartree equation.
In this paper, we study the (possible) solutions of the equation $exp_{*}(f)=g$, where $g$ is a slice regular never vanishing function on a circular domain of the quaternions $mathbb{H}$ and $exp_{*}$ is the natural generalization of the usual exponential to the algebra of slice regular functions. Any function $f$ which satisfies $exp_{*}(f)=g$ is called a $*$-logarithm of $g$. We provide necessary and sufficient conditions, expressed in terms of the zero set of the ``vector part $g_{v}$ of $g$, for the existence of a $*$-logarithm of $g$, under a natural topological condition on the domain $Omega$. By the way, we prove an existence result if $g_{v}$ has no non-real isolated zeroes; we are also able to give a comprehensive approach to deal with more general cases. We are thus able to obtain an existence result when the non-real isolated zeroes of $g_{v}$ are finite, the domain is either the unit ball, or $mathbb{H}$, or $mathbb{D}$ and a further condition on the ``real part $g_{0}$ of $g$ is satisfied (see Theorem 6.19 for a precise statement). We also find some unexpected uniqueness results, again related to the zero set of $g_{v}$, in sharp contrast with the complex case. A number of examples are given throughout the paper in order to show the sharpness of the required conditions.
Slice regular functions have been extensively studied over the past decade, but much less is known about their boundary behavior. In this paper, we initiate the study of Julia theory for slice regular functions. More specifically, we establish the quaternion
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