An example in the theory of -operators

-operators are a generalization in the context of well-boundedness of normal operators on Hilbert space. It was shown by Doust and Walden that compact -operators have a representation as a conditionally convergent sum reminiscent of the spectral representations for compact normal operators. In this representation, the eigenvalues must be taken in a particular order to ensure convergence of the sum. Here we show that one cannot replace the ordering given by Doust and Walden by the more natural one suggested in their paper.

     

Linear Sections of the Finite Veronese Varieties and Authentication Systems Defined Using Geometry

In this paper we deal with authentication systems in which one key is used to authenticate many source states. We answer a related question on the cardinalities of the intersections of quadrics in PG (d,q). We first generalize a class of geometric authentication systems, which has been introduced by Beutelspacher, Tallini and Zanella4. The source states are the lines through a special point N of PG (d,q) (the d-dimensional projective space over GF (q)). The keys are some hypersurfaces which have N as a nucleus ( N is a nucleus of if every line through N meets in exactly one point). The message belonging to a source state and a key is the unique point of intersection of the line with the hypersurface . We give the values of s for which the constructed authentication systems have a security which is comparable to the best allowed by a theoretical bound. In case the hypersurfaces are quadrics, we give further results on the security. To this end, we determine the greatest cardinality for the intersections of the finite Veronese varieties with the projective subspaces of any given dimension. Finally, we discuss a possible implementation.     

Optimal control and relaxation of nonlinear elliptic systems

In this paper we study the optimal control of systems driven by nonlinear elliptic partial differential equations. First, with the aid of an appropriate convexity hypothesis we establish the existence of optimal admissible pairs. Then we drop the convexity hypothesis and we pass to the larger relaxed system. First we consider a relaxed system based on the Gamkrelidze-Warga approach, in which the controls are transition probabilities. We show that this relaxed problem has always had a solution and the value of the problem is that of the original one. We also introduce two alternative formulations of the relaxed problem (one of them control free), which we show that they are both equivalent to the first one. Then we compare those relaxed problems, with that of Buttazzo which is based on the -regularization of the extended cost functional. Finally, using a powerful multiplier rule of Ioffe-Tichomirov, we derive necessary conditions for optimality in systems with inequality state constraints.Research supported by NSF Grant DMS-8802688     

Solution of a question of Knarr

The Moufang condition is one of the central group theoretical conditions in Incidence Geometry, and was introduced by Jacques Tits in his famous lecture notes (1974).

About ten years ago, Norbert Knarr studied generalized quadrangles (buildings of Type ) which satisfy one of the Moufang conditions locally at one point. He then posed the fundamental question whether the group generated by the root-elations with its root containing that point is always a sharply transitive group on the points opposite this point, that is, whether this group is an elation group.

In this paper, we solve the question and a more general version affirmatively for finite generalized quadrangles.

Moreover, we show that this group is necessarily nilpotent (which was only known up till now when both Moufang conditions are satisfied for all points and lines).

In fact, as a corollary, we will prove that these groups always have to be -groups for some prime .

     

Approximation and Prediction of the Numerical Solution of Some Burgers Problems

The Aitken's ²-prediction of Brezinski has already been used by Morandi Cecchi et al. in order to compute a numerical approximation of the solution of a parabolic initial-boundary value problem. This method consists in two consecutive steps: the first one is the approximation with a finite elements method, where the solution of the involved nonlinear system is computed by Gauss–Seidel method; the second one is a prediction of further terms with Aitken's ²-process. By comparison with this method, we use other methods of prediction in another way. First, we consider a generalization of ²-prediction, the so-called -prediction. In this paper, we only use vector prediction which is more stable than the scalar one. Then, the methods of prediction presented can be used in order to predict the starting vector of the Gauss–Seidel method.     

Abstract Differential Geometry, Differential Algebras of Generalized Functions, and de Rham Cohomology

Abstract differential geometry is a recent extension of classical differential geometry on smooth manifolds which, however, does no longer use any notion of Calculus. Instead of smooth functions, one starts with a sheaf of algebras, i.e., the structure sheaf, considered on an arbitrary topological space, which is the base space of all the sheaves subsequently involved. Further, one deals with a sequence of sheaves of modules, interrelated with appropriate differentials, i.e., suitable Leibniz sheaf morphisms, which will constitute the differential complex. This abstract approach captures much of the essence of classical differential geometry, since it places a powerful apparatus at our disposal which can reproduce and, therefore, extend fundamental classical results. The aim of this paper is to give an indication of the extent to which this apparatus can go beyond the classical framework by including the largest class of singularities dealt with so far. Thus, it is shown that, instead of the classical structure sheaf of algebras of smooth functions, one can start with a significantly larger, and nonsmooth, sheaf of so-called nowhere dense differential algebras of generalized functions. These latter algebras, which contain the Schwartz distributions, also provide global solutions for arbitrary analytic nonlinear PDEs. Moreover, unlike the distributions, and as a matter of physical interest, these algebras can deal with the vastly larger class of singularities which are concentrated on arbitrary closed, nowhere dense subsets and, hence, can have an arbitrary large positive Lebesgue measure. Within the abstract differential geometric context, it is shown that, starting with these nowhere dense differential algebras as a structure sheaf, one can recapture the exactness of the corresponding de Rham complex, and also obtain the short exponential sequence. These results are the two fundamental ingredients in developing differential geometry along classical, as well as abstract lines. Although the commutative framework is used here, one can easily deal with a class of singularities which is far larger than any other one dealt with so far, including in noncommutative theories.     

Connectivity properties of Mandelbrot's percolation process

Summary In 1974, Mandelbrot introduced a process in [0, 1]² which he called canonical curdling and later used in this book(s) on fractals to generate self-similar random sets with Hausdorff dimension D(0,2). In this paper we will study the connectivity or percolation properties of these sets, proving all of the claims he made in Sect. 23 of the Fractal Geometry of Nature and a new one that he did not anticipate: There is a probability p _c(0,1) so that if p<p _c then the set is duslike i.e., the largest connected component is a point, whereas if pp _c (notice the =) opposing sides are connected with positive probability and furthermore if we tile the plane with independent copies of the system then there is with probability one a unique unbounded connected component which intersects a positive fraction of the tiles. More succinctly put the system has a first order phase transition.Work supported by the NSF under Grant #DMR-83-14625Work supported by the DOE under Grant #DE-AC02-83-ER13044Work supported by the NSF under Grant #DMS-85-05020     

Equivalence of metric homomorphisms

Suppose K is an algebra with involution overk and A, B are K-modules on which are defined -Hermitian K-invariant forms with values ink. Metric homomorphisms of the module A into the module are called equivalent in the broad sense if one can be obtained from the other by multiplying by automorphisms of both modules, and equivalent in the narrow sense if one can be obtained from the other by multiplying by an automorphism of B. Necessary and sufficient conditions are given for the broad and narrow equivalence of two metric homomorphisms of one semisimple module of finite length into another. As a consequence, a classification of representations of one quadratic form by means of another is obtained.Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova AN SSSR, Vol. 114, pp. 205–210, 1982.In conclusion, the author would like to express her sincere gratitude to A. V. Yakovlev, under whose guidance this paper was written, for many valuable discussions and for his useful advice.     

Systems of Roots and Topology of Complex Flag Manifolds

In this paper we study the topology of a complex homogeneous space M = G/H of complex dimension n, with non vanishing Euler characteristic and G of type A, D, E by means of a topological invariant ₂, which is related to the Poincaré polynomial of M. We introduce the function Q = ₂/n and we examine how it varies as one passes from a principal orbit of the adjoint representation of a compact Lie group G to a more singular one. Moreover, it is proved that if M is a principal orbit G/T then Q depends only on the Weyl group of G.     

Elements with generalized bounded conjugation orbits

For a pair of linear bounded operators and on a complex Banach space , if commutes with then the orbits of under are uniformly bounded. The study of the converse implication was started in the 1970s by J. A. Deddens. In this paper, we present a new approach to this type of question using two localization theorems; one is an operator version of a theorem of tauberian type given by Katznelson-Tzafriri and the second one is on power-bounded operators by Gelfand-Hille. This improves former results of Deddens-Stampfli-Williams.     

An application of Shoenfield's absoluteness theorem to the theory of uniform distribution

IfC is a Polish probability space, a Borel set whose sectionsW _x ( have measure one and are decreasing , then we show that the set _x W _x has measure one. We give two proofs of this theorem—one in the language of set theory, the other in the language of probability theory, and we apply the theorem to a question on completely uniformly distributed sequences.Supported by DFG grant Ko 490/7-1.     

Global optimization of univariate Lipschitz functions: II. New algorithms and computational comparison

We consider the following global optimization problems for a Lipschitz functionf implicitly defined on an interval [a, b]. Problem P: find a globally-optimal value off and a corresponding point; Problem Q: find a set of disjoint subintervals of [a, b] containing only points with a globally-optimal value and the union of which contains all globally optimal points. A two-phase algorithm is proposed for Problem P. In phase I, this algorithm obtains rapidly a solution which is often globally-optimal. Moreover, a sufficient condition onf for this to be the case is given. In phase II, the algorithm proves the-optimality of the solution obtained in phase I or finds a sequence of points of increasing value containing one with a globally-optimal value. The new algorithm is empirically compared (on twenty problems from the literature) with a best possible algorithm (for which the optimal value is assumed to be known), with a passive algorithm and with the algorithms of Evtushenko, Galperin, Shen and Zhu, Piyavskii, Timonov and Schoen. For small, the new algorithm requires only a few percent more function evaluations than the best possible one. An extended version of Piyavskii's algorithm is proposed for problem Q. A sufficient condition onf is given for the globally optimal points to be in one-to-one correspondance with the obtained intervals. This result is achieved for all twenty test problems.The research of the authors has been supported by AFOSR grants 0271 and 0066 to Rutgers University. Research of the second author has been also supported by NSERC grant GP0036426, FCAR grant 89EQ4144 and partially by AFOSR grant 0066. We thank Nicole Paradis for her help in drawing the figures.     

Geometric Properties of W-Algebras and the Toda model

The W-algebra minimal models on hyperelliptic Riemann surfaces are constructed. Using a proposal by Polyakov, we reduce the partition function of the Toda field theory on the hyperelliptic surface to a product of partition functions: one of a free field theory on the sphere with inserted Toda vertex operators and one of a free scalar field theory with antiperiodic boundary conditions with inserted twist fields.     

The Inversion of Multiscale Convolution Approximation and Average of Distributions

We develop two kinds of inversion formulas of the multiscale convolution approximation which is defined by a convolution kernel . The inversion formulas are constructed by a convolution kernel which is defined in terms of and has a vanishing moment of order one. A large class of generalized moving average approximations with B-splines, Box-splines and exponential Box-splines (EB-splines) as convolution kernels is included in the theory formulated in this paper. The average of distributions is considered, and correspondingly, the formulas related to the EB-splines are obtained from the -average.     

Solvability of the general boundary-value problem for the linearized nonstationary system of Navier-Stokes equations

On the bounded cylindrical domain Q_T = x(0,T), ³, one considers the initialboundary-value problem for the system of Navier-Stokes equations in which the boundary conditions are given by an arbitrary matrix differential operator of dimension 3×4. Under certain restrictions on this operator and on the data of the problem one shows the existence of a solution in the spaces W _p ^2l,l (Q_T) for any finite T.Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova AN SSSR, Vol. 110, pp. 105–119, 1981.     

A method of centers algorithm for certain minimax problems

An algorithm is presented for the numerical solution of nonlinear programming problems in which the objective function is to be minimized over feasiblex after having been maximized over feasibley. The vectorsx andy are subjected to separate nonlinear constraints. The algorithm is obtained as follows: One starts with an outer algorithm for the minimization overx, that algorithm being taken here to be a method of centers; then, one inserts into this algorithm an adaptive inner procedure, which is designed to compute a suitable approximation to the maximizery in a finite number of steps. The outer and inner algorithms are blended in such a way as to cause the inner one to converge more rapidly. The results on convergence and rate of convergence for the outer algorithm continue to hold (essentially) for the composite algorithm. Thus, what is considered here, for the first time for this type of problem, is the question of how one inserts an approximation procedure into an algorithm so as to preserve its convergence and its rate of convergence.     

Group Actions on the Cubic Tree

It is known that every group which acts transitively on the ordered edges of the cubic tree ₃, with finite vertex stabilizer, is isomorphic to one of seven finitely presented subgroups of the full automorphism group of ₃–one of which is the modular group. In this paper a complete answer is given for the question (raised by Djokovi and Miller) as to whether two such subgroups which intersect in the modular group generate their free product with the modular group amalgamated.     

Thermodynamical approach to the traveling salesman problem: An efficient simulation algorithm

We present a Monte Carlo algorithm to find approximate solutions of the traveling salesman problem. The algorithm generates randomly the permutations of the stations of the traveling salesman trip, with probability depending on the length of the corresponding route. Reasoning by analogy with statistical thermodynamics, we use the probability given by the Boltzmann-Gibbs distribution. Surprisingly enough, using this simple algorithm, one can get very close to the optimal solution of the problem or even find the true optimum. We demonstrate this on several examples.We conjecture that the analogy with thermodynamics can offer a new insight into optimization problems and can suggest efficient algorithms for solving them.The author acknowledges stimulating discussions with J. Piút concerning the main ideas of the present paper. The author is also indebted to P. Brunovský, J. erný, M. Hamala, . Peko, . Znám, and R. Zajac for useful comments.