Characterization of Compact Operators in Pre-Hilbert and Hilbert Spaces
Achiles Nyongesa Simiyu *
Department of Mathematics, Masinde Muliro University of Science and Technology, P.O.Box 190-50100, Kakamega, Kenya.
Hillary Amonyela Isabu
Department of Mathematics, Masinde Muliro University of Science and Technology, P.O.Box 190-50100, Kakamega, Kenya.
Aldril Wekesa Wanambisi
Department of Mathematics, Masinde Muliro University of Science and Technology, P.O.Box 190-50100, Kakamega, Kenya.
*Author to whom correspondence should be addressed.
Abstract
The concept of a compact operator on a Hilbert space, H is an extension of the concept of a matrix acting on a finite-dimensional vector space. In Hilbert space, compact operators are precisely the closure of finite rank operators in the topology induced by the operator norm. In this paper, we provide an elementary exposition of compact linear operators in pre-Hilbert and Hilbert spaces. However, whenever advantageous, we may prove a few results in the general context of normed linear spaces. It is well known that strong convergence implies weak convergence but weak convergence does not imply strong convergence. We also show that an operator T \(\epsilon\) B(H) is compact if and only if T maps every weakly convergent sequence in H to a strongly convergent sequence.
Keywords: Compact operator, Hilbert space Normed linear spaces, strong convergence, weak conver- gence