A note on “Extensional PERs”

In the paper “Extensional PERs” by P. Freyd, P. Mulry, G. Rosolini and D. Scott, a category C of “pointed complete extensional PERs” and computable maps is introduced to provide an instance of an algebraically compact category relative to a restricted class of functors. Algebraic compactness is a synthetic condition on a category which ensures solutions of recursive equations involving endofunctors of the category. We extend that result to include all internal functors on C when C is viewed as a full internal category of the effective topos. This is done using two general results: one about internal functors in general, and one about internal functors in the effective topos.     

Normal <Emphasis Type="Italic">CR</Emphasis> structures on S

We classify the normal CR structures on S³ and their automorphism groups. Together with [2], this closes the classification of normal CR structures on compact 3-manifolds. We give a criterion to compare two normal CR structures, and we show that the underlying contact structure is, up to diffeomorphism, unique. Mathematics Subject Classification (2000): 53A40, 53C25, 53D10. Received: 9 April 2002; in final form: 8 May 2002 // Published online: 20 March 2003     

Phragmén-Lindelöf principles on algebraic varieties

Estimates of Phragmén-Lindelöf (PL) type for plurisubharmonic functions on algebraic varieties in have been of interest for a number of years because of their equivalence with certain properties of constant coefficient partial differential operators; e.g. surjectivity, continuation properties of solutions and existence of continuous linear right inverses. Besides intrinsic interest, their importance lies in the fact that, in many cases, verification of the relevant PL-condition is the only method to check whether a given operator has the property in question.

In the present paper the property which characterizes the existence of continuous linear right inverses is investigated. It is also the one closest in spirit to the classical Phragmén-Lindelöf Theorem as various equivalent formulations for homogeneous varieties show. These also clarify the relation between and the PL-condition used by Hörmander to characterize the surjectivity of differential operators on real-analytic functions.

We prove the property for an algebraic variety implies that , the tangent cone of at infinity, also has this property. The converse implication fails in general. However, if is a manifold outside the origin, then satisfies if and only if the real points in have maximal dimension and if the distance of to is bounded by as tends to infinity. In the general case, no geometric characterization of the algebraic varieties which satisfy is known, nor any of the other PL-conditions alluded to above.

Besides these main results the paper contains several auxiliary necessary conditions and sufficient conditions which make it possible to treat interesting examples completely. Since it was submitted they have been applied by several authors to achieve further progress on questions left open here.

     

Monge–Ampère measures on pluripolar sets

In this article we solve the complex Monge–Ampère problem for measures with large singular part. This result generalizes classical results by Demailly, Lelong and Lempert a.o., who considered singular parts carried on discrete sets. By using our result we obtain a generalization of Ko?odziej's subsolution theorem. More precisely, we prove that if a non-negative Borel measure is dominated by a complex Monge–Ampère measure, then it is a complex Monge–Ampère measure.     

Image Projection and Representation on S<Superscript>n</Superscript>

In signal processing, communications, and other branches of information technologies, it is often desirable to map the higher-dimensional signals on Sⁿ. In this article we introduce a novel method of representing signals on Sⁿ. This approach is based on geometric function theory, in particular on the theory of quasiregular mappings. The importance of sampling is underlined, and new geometric sampling theorems for general manifolds are presented.     

Nikulin involutions on <Emphasis Type="Italic">K</Emphasis>3 surfaces

We study the maps induced on cohomology by a Nikulin (i.e. a symplectic) involution on a K3 surface. We parametrize the 11-dimensional irreducible components of the moduli space of algebraic K3 surfaces with a Nikulin involution and we give examples of the general K3 surface in various components. We conclude with some remarks on Morrison–Nikulin involutions, these are Nikulin involutions which interchange two copies of E ₈(−1) in the Néron Severi group. The second author is supported by DFG Research Grant SA 1380/1-1.     

Some remarks on <Emphasis Type="Italic">S</Emphasis>-paracompact spaces

Some characterizations of S-paracompact spaces are given. We introduce a class of S-expandable spaces and study topological properties of S-expandable spaces. The second author acknowledges the support from the NSFC (No. 10571081) and the Natural Science Foundation of Jingsu Higher Education Institutions of China (Grant No. 07KJB110055).     

A note on infinite-dimensional <Emphasis Type="Italic">M</Emphasis>-matrices

A definition of an infinite-dimensional M-matrix is given. It is proved that the set of all such infinite-dimensional matrices composes a topological group, and moreover, it is contractible.     

A Note on Asymptotic Normality of Kernel Estimation for Linear Random Fields on <Emphasis Type="Italic">Z</Emphasis><Superscript>2</Superscript>

This note considers the kernel estimation of a linear random field on Z ². Instead of imposing certain mixing conditions on the random fields, it is assumed that the weights of the innovations satisfy a summability property. By building a martingale decomposition based on a suitable filtration, asymptotic normality is proven for the kernel estimator of the marginal density of the random field. T.-L. Cheng’s research is supported in part by NSC 94-2118-M-018-001, Taiwan. Also, he is indebted to Department of Mathematics and Statistics, University of Calgary, for their hospitality during his visit. X. Lu’s research is supported in part by NSERC Discovery Grant of Canada.     

Multi-positive linear functionals on locally<Emphasis Type="Italic">C</Emphasis>*-algebras

We will show that ann×n matrix of continuous linear functionals on a locallyC*-algebraA, which satisfies the generalized positivity condition induces a continuous *-representation ofA on a Hilbert space. This generalizes the classical GNS-representation. Also, we give a necessary and sufficient condition such that this representation is irreducible, and determine a certain class of extreme points in the set of all continuous completely positive linear maps fromA toM _n (ℂ) that preserve identity.     

ℱ<Emphasis Type="Italic">K</Emphasis>-convex functions on metric spaces

 By an ℱK-convex function on a length metric space, we mean one that satisfies f ⁿ ≥ −Kf on all unitspeed geodesics. We show that natural ℱK-convex (-concave) functions occur in abundance on metric spaces of curvature bounded above (below) by K in the sense of Alexandrov. We prove Lipschitz extension and approximation theorems for ℱK-convex functions on CAT(K) spaces. Received: 10 May 2002 Mathematics Subject Classification (2000): 53C70, 52A41     

Orthogonal separable Hamiltonian systems on <Emphasis Type="Italic">T</Emphasis><Superscript>2</Superscript>

In this paper we characterize the Liouvillian integrable orthogonal separable Hamiltonian systems on T ² for a given metric, and prove that the Hamiltonian flow on any compact level hypersurface has zero topological entropy. Furthermore, by examples we show that the integrable Hamiltonian systems on T ² can have complicated dynamical phenomena. For instance they can have several families of invariant tori, each family is bounded by the homoclinic-loop-like cylinders and heteroclinic-loop-like cylinders. As we know, it is the first concrete example to present the families of invariant tori at the same time appearing in such a complicated way. This work is partially supported by the National Natural Science Foundation of China (Grant Nos. 10671123, 10231020), “Dawn” Program of Shanghai Education Comission of China (Grant No. 03SG10) and Program for New Century Excellent Tatents in University of China (Grant No. 050391)     

Polynomial clones on groups of order <Emphasis Type="Italic">pq</Emphasis>

For two distinct primes p, q, we describe those clones on a set of size pq that contain a given group operation and all constant operations. We show that each such clone is determined by congruences and commutator relations. Thus we obtain that there is only a finite number of such clones on a fixed set. Supported by the Austrian Science Fund (FWF), grant P15691.     

Covariance information based on the <Emphasis Type="Italic">t</Emphasis> prior

The exact distribution of the covariance between two populations is derived. It is assumed that one of the population means has a Student's t prior while the other is taken to come from one of normal, Student's t, Laplace, logistic or Bessel families (the five well-known symmetric distributions). The exact distribution is given in terms of the characteristic function. The calculations involve several special functions.     

<Emphasis Type="Italic">L</Emphasis><Superscript>2</Superscript>-invariants and morse-smale flows on manifolds

We study the homotopy invariants of free cochain and Hilbert complexes. These L ²-invariants are applied to the calculation of exact values of minimum numbers of closed orbits of some indices of nonsingular Morse-Smale flows on manifolds of large dimensions. Published in Ukrains’kyi Matematychnyi Zhurnal, Vol. 59, No. 4, pp. 522–533, April, 2007.     

Comments on ‘A finite extensibility nonlinear oscillator’

The aim of this comment is to provide more information about the study of the dynamics of a finite extensibility nonlinear oscillator conducted by Febbo [M. Febbo, A finite extensibility nonlinear oscillator, Applied Mathematics and Computation 217 (2011) 6464-6475]. We show that the linearized harmonic balance method is not sufficiently adequate for this oscillator and that the harmonic balance method (HBM) without linearization provides better results. We also discuss what happens when the oscillation amplitude approaches 1 and why the harmonic balance method does not give optimum results. For these values of the oscillation amplitude the periodic solution becomes markedly anharmonic and is almost straight between x = +A and x = −A (with negative slope) and between x = −A and x = +A (with positive slope). Finally, a ‘heuristic’ solution is proposed which is adequate for the whole amplitude range 0 < A < 1, which is consistent with the approximate solution obtained previously for A < 0.9 using the HBM.     

Higher‐order Sobolev‐type embeddings on Carnot–Carathéodory spaces

A sufficient condition for higher‐order Sobolev‐type embeddings on bounded domains of Carnot–Carathéodory spaces is established for the class of rearrangement‐invariant function spaces. The condition takes form of a one‐dimensional inequality for suitable integral operators depending on the isoperimetric function relative to the Carnot–Carathéodory structure of the relevant sets. General results are then applied to particular Sobolev spaces built upon Lebesgue, Lorentz and Orlicz spaces on John domains in the Heisenberg group. In the case of the Heisenberg group, the condition is shown to be necessary as well.     

“Localized” self-adjointness of Schrödinger type operators on Riemannian manifolds

We prove self-adjointness of the Schrödinger type operator , where ∇ is a Hermitian connection on a Hermitian vector bundle E over a complete Riemannian manifold M with positive smooth measure dμ which is fixed independently of the metric, and V∈L_loc¹(EndE) is a Hermitian bundle endomorphism. Self-adjointness of H_V is deduced from the self-adjointness of the corresponding “localized” operator. This is an extension of a result by Cycon. The proof uses the scheme of Cycon, but requires a refined integration by parts technique as well as the use of a family of cut-off functions which are constructed by a non-trivial smoothing procedure due to Karcher.