Dans cet article nous donnons une formule pour les coefficients de l’inverse des matrices de Toeplitz respectivement de symboles $f\left(e^{i\theta }\right)=(1-cos\theta ){\left|f_1\left(e^{i\theta }\right)\right|}^2$ (cas singulier) et $|f_1\left(e^{i\theta }\right){|}^2$ (cas régulier) où $f_1$ est une fonction appartenant à  une classe de fonctions holomorphes sur un disque ouvert contenant le tore $𝕋$ et sans zéro sur $𝕋$. Un cas particulier défini par $f_1=\frac{Q}{P}$ où $P$ et $Q$ sont des polynômes sans zéro sur $𝕋$ est traité. Dans le cas où le symbole est singulier, cette formule présente l’intérêt d’avoir un second ordre. Dans tous les...

In this paper a class of injective unilateral weighted shift operators is introduced which contains strictly the class of the strictly cyclic operators and which can only be unicellular if they are quasinilpotent.

We study unbounded Hermitian operators with dense domain in Hilbert space. As is known, the obstruction for a Hermitian operator to be selfadjoint or to have selfadjoint extensions is measured by a pair of deficiency indices, and associated deficiency spaces; but in practical problems, the direct computation of these indices can be difficult. Instead, in this paper we identify additional structures that throw light on the problem. We will attack the problem of computing deficiency spaces for a single...

Sufficient conditions for the absence of absolutely continuous spectrum for unbounded Jacobi operators are given. A class of unbounded Jacobi operators with purely singular continuous spectrum is constructed as well.

We discuss Laplacians on graphs in a framework of regular Dirichlet forms. We focus on phenomena related to unboundedness of the Laplacians. This includes (failure of) essential selfadjointness, absence of essential spectrum and stochastic incompleteness.

Suppose A is a (possibly unbounded) linear operator on a Banach space. We show that the following are equivalent. (1) A is well-bounded on [0,∞). (2) -A generates a strongly continuous semigroup ${e^{-sA}}_{s\le 0}$ such that ${(1/s^2)e^{-sA}}_{s>0}$ is the Laplace transform of a Lipschitz continuous family of operators that vanishes at 0. (3) -A generates a strongly continuous differentiable semigroup ${e^{-sA}}_{s\ge 0}$ and ∃ M < ∞ such that $\parallel H_n\left(s\right)\parallel \equiv \parallel ({\sum }_{k=0}^n\left(s^k{A}^k\right)/k!)e^{-sA}\parallel \le M$, ∀s > 0, n ∈ ℕ ∪ 0. (4) -A generates a strongly continuous holomorphic semigroup ${e^{-zA}}_{Re\left(z\right)>0}$ that is O(|z|) in all...