The K-theory and the invertibility of almost periodic Toeplitz operators

We use K-theory to study the invertibility of systems of almost periodic Toeplitz operators.Research supported in part by NSF under Grant No. DMS-8717185.     

A note on solutions of orientor fields for periodic systems

It is shown that the orientor field problem can be solved for control problems governed by ordinary differential equations where periodic controls give periodic trajectories.This research was partially supported by NSF Grant No. GP-27973.     

On Doubly Degenerate Quasilinear Parabolic Equations of Higher Order

We deal with the existence of periodic solutions for doubly degenerate quasilinear parabolic equations of higher order, which can degenerate, on a part of the boundary, on a segment in the interior of the domain and in time.     

Existence and stability of large scale nonlinear oscillations in suspension bridges

A nonlinear model of a suspension bridge is considered in which large-scale, stable oscillatory motions can be produced by constant loading and a small-scale, external oscillatory force. Loud's implicit-function theoretic method for determining existence and stability of periodic solutions or nonlinear differential equations is extended to a case of a non-differentiable nonlinearity.Author partially supported by NSF under Grant DMS 8318204 and AFOSR Grant 85-0330.Author partially supported by NSF under Grant DMS 9519882.Author partially supported by NSF under Grant DMS 8519776.     

The subgaussian constant and concentration inequalities

We study concentration inequalities for Lipschitz functions on graphs by estimating the optimal constant in exponential moments of subgaussian type. This is illustrated on various graphs and related to various graph constants. We also settle, in the affirmative, a question of Talagrand on a deviation inequality for the discrete cube. Research supported in part by NSF Grant No. DMS-0405587 and by EPSRC Visiting Fellowship. Research supported in part by NSF Grant No. DMS-9803239, DMS-0100289. Research supported in part by NSF Grant No. DMS-0401239.     

Maximum principle of optimal control for functional differential systems

We show that the maximum principle holds for optimal periodic control problems governed by functional differential equations under a Lipschitz condition on the value functional. Generalizations to other boundary conditions are also considered.This research was partially supported by NSF Grant No. DMS-84-01719.The first author was partially supported by the Science Fund of the Chinese Academy of Sciences, Beijing, China.     

A note on a theorem of Yamamuro

Conditions are presented which are necessary for the existence of a regular fixed point of aC ¹ map.Work supported in part by NSF Grant No. MCS 77-15509.Work supported in part by ONR Grant No. N0014-75-C-0495 and NSF Grant No. Eng. 76-81058.     

On the product formulas for <Emphasis Type="Italic">C</Emphasis>-semigroups

In this paper we obtain a Chernoff-type approximation theorem for C-semigroups, from which we derive the Trotter product formula and the Post-Widder inversion formula for C-semigroups. The first author was supported by the NSF of China (Grant No. 10501032) and the NSFC-RFBR Programm (Grant No. 108011120015), and the second author by the NSF of China (Grant No. 10671079).     

Exponential sums on (G m ) n

Partially supported by NSF Grant No. DMS-8803085     

Semifield planes with a transitive autotopism group

Research supported in part by NSF Grant No. DMS-9107372     

Bifurcation and symmetry-breaking

Research supported in part by the NSF under Grant No. MCS-830123     

Groups admitting ergodic actions with generalized discrete spectrum

The class of groups admitting an effective ergodic action with generalized discrete spectrum is a natural generalization of the class of maximally almost periodic groups. H. Freudenthal has given a complete characterization of the connected maximally almost periodic groups, and here we give a complete characterization of the almost connected groups admitting an effective ergodic action with generalized discrete spectrum. Namely, we show that an almost connected group is in this class if and only if it is typeR. It is known that this is equivalent to the group being of polynomial growth, and for Lie groups is just the condition that all eigenvalues of the adjoint representation lie on the unit circle.Research partially supported by NSF Grant MCS-77-13070 and the Miller InstituteResearch partially supported by NSF Grant MCS 76-06626     

Scalar labelings for homotopy paths

Two scalar labelings are introduced for obtaining approximate solutions to systems of nonlinear equations by simplicial approximation. Under reasonable assumptions the new scalar-labeling algorithms are shown to follow, in a limiting sense, homotopy paths which can also be tracked by piecewise linear vector labeling algorithms. Though the new algorithms eliminate the need to pivot on a system of linear equations, the question of relative computational efficiency is unresolved.The work of this author was supported in part by NSF Grant No. MCS-77-15509 and by ARO Grant No. DAAG-29-78-G-0160.The work of this author was supported in part by ONR Grant No. N00014-75-C-0495 and NSF Grant No. 81058.     

On optimal control of functional differential systems

We use relaxed controls to derive optimality condition for control problems governed by functional differential systems. We include general boundary conditions, and the case of periodic trajectories is a special one.This research was supported by NSF Grant HRD-91-54077.     

Sarason interpolation and Toeplitz corona theorem for almost periodic matrix functions

Sarason interpolation and Toeplitz corona problems are studied for almost periodic matrix functions. Recent results on almost periodic factorization and related generalized Toeplitz operators are the main tools in the study.Supported in part by NSF Grant DMS 9500912Supported in part by NATO Collaborative Research Grant 950332Supported by NSF Grant DMS 9500924     

Geometrically finite groups,Patterson-Sullivan measures and Ratner's ridigity theorem

Summary The rigidity properties of the horospherical foliations of geometrically finite hyperbolic manifolds are investigated. Ratner's theorem generalizes to these foliations with respect to the Patterson-Sullivan measure. In the spirit of Mostow, we prove the nonexistence of invariant measurable distributions on the boundary of hyperbolic space for geometrically finite groups. Finally, we show that the frame flow on geometrically finite hyperbolic manifolds is Bernoulli.partially supported by NSF Grant No. DMS-820-04024partially supported by NSF Grant No. DMS-85-02319     

Robust solutions of uncertain complex-valued quadratically constrained programs

In this paper, we discuss complex convex quadratically constrained optimization with uncertain data. Using S-Lemma, we show that the robust counterpart of complex convex quadratically constrained optimization with ellipsoidal or intersection-of-two-ellipsoids uncertainty set leads to a complex semidefinite program. By exploring the approximate S-Lemma, we give a complex semidefinite program which approximates the NP-hard robust counterpart of complex convex quadratic optimization with intersection-of-ellipsoids uncertainty set.     

Solving nonlinear inequalities in a finite number of iterations

This paper describes a modified Newton algorithm for solving a finite system of inequalities in a finite number of iterations.This research was supported by NSF Grant No. ENG-73-08214-AO1, by NSF-RANN Grant No. ENV-76-04264, and by the United Kingdom Science Research Council.     

Homogeneous linear differential equations with only periodic solutions

We determine all third order homogeneous linear differential equations with periodic coefficients and only periodic solutions. The method extends tonth order equations. As an application, we show that the Laguerre-Forsyth canonical form cannot be used for global investigations i projective differential geometry. Research supported by NSF Grant GP-8176. To the Memory of Eri Jabotinsky     

Pointwise quasi-Newton method for unconstrained optimal control problems,II

In this paper, the necessary optimality conditions for an unconstrained optimal control problem are used to derive a quasi-Newton method where the update involves only second-order derivative terms. A pointwise update which was presented in a previous paper by the authors is changed to allow for more general second-order sufficiency conditions in the control problem. In particular, pointwise versions of the Broyden, PSB, and SR1 update are considered. A convergence rate theorem is given for the Broyden and PSB versions.This research was supported by NSF Grant No. DMS-89-00410, by NSF Grant No. INT-88-00560, by AFOSR Grant No. AFOSR-89-0044, and by the Deutsche Forschungsgemeinschaft.