The multiplicative Weyl functional calculus

The Weyl calculus discussed in the author's previous papers starts with a fixed set of n noncommuting self-adjoint operators and associates an operator to a real function of n variables. The calculus is not multiplicative with respect to point-wise multiplication of functions. However, if the n self-adjoint operators generate a unitary Lie group representation, a “skew product” of functions can be defined which yields multiplicativity. This skew product depends only on the Lie group, not on the particular representation. In the case of the Heisenberg group, this skew product makes it possible to write the Schrödinger equation as an integro-differential equation on the phase plane. Strong convergence of the dynamical group, as Planck's constant goes to zero, to the classical Hamiltonian flow is proved under various conditions on the Hamiltonian.     

Greedy and randomized versions of the multiplicative Schwarz method

We consider sequential, i.e., Gauss–Seidel type, subspace correction methods for the iterative solution of symmetric positive definite variational problems, where the order of subspace correction steps is not deterministically fixed as in standard multiplicative Schwarz methods. Here, we greedily choose the subspace with the largest (or at least a relatively large) residual norm for the next update step, which is also known as the Gauss–Southwell method. We prove exponential convergence in the energy norm, with a reduction factor per iteration step directly related to the spectral properties, e.g., the condition number, of the underlying space splitting. To avoid the additional computational cost associated with the greedy pick, we alternatively consider choosing the next subspace randomly, and show similar estimates for the expected error reduction. We give some numerical examples, in particular applications to a Toeplitz system and to multilevel discretizations of an elliptic boundary value problem, which illustrate the theoretical estimates.     

Matrix versions of the Cauchy and Kantorovich inequalities

Summary A version of Cauchy's inequality is obtained which relates two matrices by an inequality in the sense of the Loewner ordering. In that ordering a symmetric idempotent matrix is dominated by the identity matrix and this fact yields a simple proof.A consequence of this matrix Cauchy inequality leads to a matrix version of the Kantorovich inequality, again in the sense of Loewner.     

On additive and multiplicative Hilbert cubes

Given subset E of natural numbers FS(E) is defined as the collection of all sums of elements of finite subsets of E and any translation of FS(E) is said to be Hilbert cube. We can define the multiplicative analog of Hilbert cube as well. E.G. Strauss proved that for every ε>0 there exists a sequence with density >1−ε which does not contain an infinite Hilbert cube. On the other hand, Nathanson showed that any set of density 1 contains an infinite Hilbert cube. In the present note we estimate the density of Hilbert cubes which can be found avoiding sufficiently sparse (in particular, zero density) sequences. As a consequence we derive a result in which we ensure a dense additive Hilbert cube which avoids a multiplicative one.     

Generalized matrix versions of the Cauchy-Schwarz and Kantorovich inequalities

Summary A recent note by Marshall and Olkin (1990), in which the Cauchy-Schwarz and Kantorovich inequalities are considered in matrix versions expressed in terms of the Loewner partial ordering, is extended to cover positive semidefinite matrices in addition to positive definite ones.Dedicated to the memory of Alexander M. Ostrowski on the occasion of the 100th anniversary of his birth     

Newton additive and multiplicative Schwarz iterative methods

Daniel B. Szyld^¶Department of Mathematics, Temple University, Philadelphia, PA 19122, USA ^{Convergence properties are presented for Newton additive andmultiplicative Schwarz (AS and MS) iterative methods for thesolution of nonlinear systems in several variables. These methodsconsist of approximate solutions of the linear Newton step usingeither AS or MS iterations, where overlap between subdomainscan be used. Restricted versions of these methods are also considered.These Schwarz methods can also be used to precondition a Krylovsubspace method for the solution of the linear Newton steps.Numerical experiments on parallel computers are presented, indicatingthe effectiveness of these methods.}     

On the abstract theory of additive and multiplicative Schwarz algorithms

Summary. In recent years, it has been shown that many modern iterative algorithms (multigrid schemes, multilevel preconditioners, domain decomposition methods etc.) for solving problems resulting from the discretization of PDEs can be interpreted as additive (Jacobi-like) or multiplicative (Gauss-Seidel-like) subspace correction methods. The key to their analysis is the study of certain metric properties of the underlying splitting of the discretization space into a sum of subspaces and the splitting of the variational problem on into auxiliary problems on these subspaces. In this paper, we propose a modification of the abstract convergence theory of the additive and multiplicative Schwarz methods, that makes the relation to traditional iteration methods more explicit. The analysis of the additive and multiplicative Schwarz iterations can be carried out in almost the same spirit as in the traditional block-matrix situation, making convergence proofs of multilevel and domain decomposition methods clearer, or, at least, more classical. In addition, we present a new bound for the convergence rate of the appropriately scaled multiplicative Schwarz method directly in terms of the condition number of the corresponding additive Schwarz operator. These results may be viewed as an appendix to the recent surveys [X], [Ys]. Received February 1, 1994 / Revised version received August 1, 1994     

An additive analysis of multiplicative Schwarz methods

We present an analysis of multiplicative Schwarz methods for symmetric positive definite problems that is based on the theory of additive Schwarz preconditioners and discuss applications to multigrid methods and domain decomposition methods.     

Random Clarkson inequalities and LP versions of Grothendieck's inequality

In a recent paper KATO [3] used the LITTLEWOOD matrices to generalise CLARKSON'S inequalities. Our first aim is to indicate how KATO'S result can be deduced from a neglected version of the HAUSDORFF -YOUNG inequality which was proved by WELLS and WILLIAMS [12]. We next establish ?random CLARKSON inequalities”?. These show that the expected behaviour of matrices whose coefficients are random ± 1′s is, as one might expect, the same as the behaviour that KATO observed in the LITTLEWOOD matrices. Finally we show how sharp Lp versions of GROTHENDIECK'S inequality can be obtained by combining a KATO -like result with a theorem of BENNETT [1] on SCHUR multipliers.