Counterexamples undeniably play an outstanding role in mathematics. As is known,
counterexamples are patently examples but with a different purpose. The commonly agreed …

We prove, under some conditions on the domains, that the adjoint of the sum of two
unbounded operators is the sum of their adjoints in both Hilbert and Banach space settings …

STABILITY OF ALMOST CLOSED OPERATORS ON A HILBERT SPACE 1. Introduction Let us
denote by B(H) the algebra of bounded linear oper Page 1 SARAJEVO JOURNAL OF …

COLLOQUIUM MATHEMATICUM Page 1 COLLOQUIUM MATHEMATICUM VOL. 112 2008 NO.
2 A TOPOLOGICAL CHARACTERIZATION OF THE PRODUCT OF TWO CLOSED OPERATORS …

The purpose of this paper is to introduce some new metrics on the set C (H) of closed
densely defined operators on a Hilbert space H and we characterize the closure of the …

In this paper we give some topological conditions which ensure the closedness of the sum
and product of two closed linear operators acting in a Hilbert space. We get also a general …

Abstract Let A: D (A)⊂ H→ K be a densely defined linear operator, where H and K are two
Hilbert spaces. Then D (A∗) is the set constituted of all y∈ K for which there exists az∈ H …

The paper present a survey of results concerning the fundamental properties of the Drazin
inverse for bounded operators and an interesting study of the Drazin inverse for a closed …

Il y'a quatres ans de cela, je me suis rendu à l'université d'oran pour prendre contact avec
mon directeur de thèse, on s' est fixé l'objectif de résoudre les conditions pour lesquelles les …