We present quasi-diagonal preconditioners for piecewise polynomial discretizations of pseudodifferential operators of order minus two in any space dimension. Here, quasi-diagonal means diagonal up to a sparse transformation. ...

We study the existence and characterization properties of compact Hermitian operators C on a Hilbert space H such that the norm of C is less or equal to that of C + D , for all the real and compact diagonals D in a fixed ...

We study the C* -algebra D+K which consists of sums of a diagonal plus a compact operator. We describe the structure of the unitary group, the sets of ideals, automorphisms and projections.

In the present work, numerical methods are used to analyze random one-dimensional tight binding models. In physical terms, these models describe a quantum particle (\electron") which moves along a one-dimensional chain ...

Let H be a separable infinite-dimensional complex Hilbert space, B(H) the algebra of bounded linear operators acting on H and J a proper two-sided ideal of B(H). Denote by UJ(H) the group of all unitary operators of the ...