Iterative Approximation of Statistical Distributions and Relation to Information Geometry

The optimal control of stochastic processes through sensor estimation of probability density functions is given a geometric setting via information theory and the information metric. Information theory identifies the exponential distribution as the maximum entropy distribution if only the mean is known and the Γ distribution if also the mean logarithm is known. The surface representing Γ models has a natural Riemannian information metric. The exponential distributions form a one-dimensional subspace of the two-dimensional space of all Γ distributions, so we have an isometric embedding of the random model as a subspace of the Γ models. This geometry provides an appropriate structure on which to represent the dynamics of a process and algorithms to control it. This short paper presents a comparative study on the parameter estimation performance between the geodesic equation and the B-spline function approximations when they are used to optimize the parameters of the Γ family distributions. In this case, the B-spline functions are first used to approximate the Γ probability density function on a fixed length interval; then the coefficients of the approximation are related, through mean and variance calculations, to the two parameters (i.e. μ and β) in Γ distributions. A gradient based parameter tuning method has been used to produce the trajectories for (μ, β) when B-spline functions are used, and desired results have been obtained which are comparable to the trajectories obtained from the geodesic equation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Harmonicity of Quasiconformal Measures and Poisson Boundaries of Hyperbolic Spaces

We consider a group Γ of isometries acting on a proper (not necessarily geodesic) δ -hyperbolic space X. For any continuous α-quasiconformal measure ν on ∂X assigning full measure to Λ _r , the radial limit set of Γ, we produce a (nontrivial) measure μ on Γ for which ν is stationary. This means that the limit set together with ν forms a μ-boundary and ν is harmonic with respect to the random walk induced by μ. As a basic example, take and Γ to be any geometrically finite Kleinian group with ν a Patterson-Sullivan measure for Γ. In the case when X is a CAT(−1) space and Γ is discrete with quasiconvex action, we show that (Λ _r , ν) is the Poisson boundary for μ. In the course of the proofs, we establish sufficient conditions for a set of continuous functions to form a positive basis, either in the L ¹ or L ^∞ norm, for the space of uniformly positive lower-semicontinuous functions on a general metric measure space. The first author was supported in part by an NSF postdoctoral fellowship and DMS-0420432. The second author was supported in part by an NSF postdoctoral fellowship.     

Weyl chamber flow on irreducible quotients of PSL(2, ℝ) × PSL(2, ℝ)

We study the topological dynamics of the action of the diagonal subgroup on quotients Γ\PSL(2, ℝ) × PSL(2, ℝ), where Γ is an irreducible lattice. Closed orbits are described and a set of points of dense orbit is explicitly given. Such properties are expressed using the Furstenberg boundary of the associated symmetric space ℍ × ℍ.     

Monotone Nonincreasing Random Fields on Partially Ordered Sets. II. Probability Distributions on Polyhedral Cones

In this part of the paper, we investigate the structure of an arbitrary measure μ supported by a polyhedral cone C in R ^d in the case where the decumulative distribution function g_μ of the measure μ satisfies certain continuity conditions. If a face Γ of the cone C satisfies appropriate conditions, the restriction μ|_Γint of the measure μ to the interior part of Γ is proved to be absolutely continuous with respect to the Lebesgue measure λ_Γ on the face Γ. Besides, the density of the measure μ|_Γint is expressed as the derivative of the function g_μ multipied by a constant. This result was used in the first part of the paper to find the finite-dimensional distributions of a monotone random field on a poset. Bibliography: 6 titles. __________ Translated from Zapiski Nauchnykh Seminarov POMI, Vol. 307, 2004, pp. 5–56.     

Random weighting,asymptotic counting,and inverse isoperimetry

For a family X of k-subsets of the set {1, …, n}, let |X| be the cardinality of X and let Γ(X, μ) be the expected maximum weight of a subset from X when the weights of 1, …, n are chosen independently at random from a symmetric probability distribution μ on ℝ. We consider the inverse isoperimetric problem of finding μ for which Γ(X, μ) gives the best estimate of ln |X|. We prove that the optimal choice of μ is the logistic distribution, in which case Γ(X, μ) provides an asymptotically tight estimate of ln |X| as k ⁻¹ ln |X} grows. Since in many important cases Γ(X, μ) can be easily computed, we obtain computationally efficient approximation algorithms for a variety of counting problems. Given μ, we describe families X of a given cardinality with the minimum value of Γ(X, μ), thus extending and sharpening various isoperimetric inequalities in the Boolean cube. The research of the first author was partially supported by NSF Grants DMS 9734138 and DMS 0400617. The research of the second author was partially supported by ISF Grant 039-7165 and by GIF grant I-2052.     

Critical exponents for groups of isometries

Let Γ be a convex co-compact group of isometries of a CAT(−1) space X and let Γ₀ be a normal subgroup of Γ. We show that, provided Γ is a free group, a sufficient condition for Γ and Γ₀ to have the same critical exponent is that Γ / Γ₀ is amenable.      

On order structure and operators in L ∞(μ)

It is known that there is a continuous linear functional on L _∞ which is not narrow. On the other hand, every order-to-norm continuous AM-compact operator from L _∞(μ) to a Banach space is narrow. We study order-to-norm continuous operators acting from L _∞(μ) with a finite atomless measure μ to a Banach space. One of our main results asserts that every order-to-norm continuous operator from L _∞(μ) to c ₀(Γ) is narrow while not every such an operator is AM-compact.     

The common curve of quadrics sharing a self-polar simplex

Summary When n-1 quadrics in projective space [n] of n dimensions have a common self-polar simplex their common curve Γ admits a group of2 ⁿ self-projectivities. The consequent properties of Γ are investigated, and further specialisations are imposed which amplify the the group and endow Γ with further properties. There is some reference to the osculating spaces and principal chords of Γ, and some properties of particular curves in four and five dimensions are described. Entrata in Redazione il 23 giugno 1976.     

Maximizing a congruence with respect to its partition of idempotents

For a congruence σ on a semigroupS a congruence μ(σ) onS, containing σ, is defined such that the semigroupS/σ is fundamental if and only if σ=μ(σ). The congruence μ(σ) is shown to possess maximality properties and for idempotent-surjective semigroups, μ(σ) is the maximum congruence with respect to the partition of the idempotents determined by σ. Thus μ is the maximum idempotent-separating congruence on any idempotent-surjective semigroup. It is shown that μ(μ(σ))=μ(σ). If ρ is another congruence onS, possibly with the same partition of the idempotents as σ, then it is of interest to know when ρ⊆σ (or ρ⊆μ(σ)) implies μ(ρ)⊆μ(σ) or even μ(ρ)=μ(σ). These implications are not true in general but if σ⊆ρ⊆μ(σ) then μ(ρ)⊆μ(σ). IfS is an idempotent-surjective semigroup and ρ and σ have the same partition of the idempotents then μ(ρ)=μ(σ).     

Symbolic dynamics for triangle groups

We generalize E. Artin’s continued fraction coding of the geodesics on the modular surface to any finite index subgroup Θ of a nonuniform hyperbolic triangle group Γ. D. Mayer’s study of the Selberg zeta function of PSl (2, Z ) is extended to Θ and its group representations. We give representatives for Γ-primitive conjugacy classes and derive a Markov system of interval maps for Γ and a Markov partition for the billiard flow on Γ\ SH ² . This leads to identities for values of the dilogarithm function at algebraic numbers. We also find the Γ-analogues of Gauss measure on [0,1]. Oblatum 16-VIII-1993 & 15-VIII-1994 & 2-I-1996     

An isomorphism theorem for Teichmüller curves

It is proved that for any Fuchsian group Γ such that ℍ/Γ is a hyperbolic Riemann surface, the Teichmüller curve V(Γ) has a unique complex manifold structure so that the natural projection of the Bers fiber space F(Γ) onto V(Γ) is holomorphic with local holomorphic sections. An isomorphism theorem for Teichmüller curves is deduced, which generalizes a classical result that the Teichmüller curve V(Γ) depends only on the type of Γ and not on the orders of the elliptic elements of Γ when ℍ/Γ is a compact hyperbolic Riemann surface.     

Adic realizations of ergodic actions by homeomorphisms of Markov compacta and ordered Bratteli diagrams

For any ergodic transformation T of a Lebesgue space (X, μ), it is possible to introduce a topology τ on X such that (a) X becomes a totally disconnected compactum (a Cantor set) with a Markov structure, and μ becomes a Borel Markov measure; (b) T becomes a minimal strictly ergodic homeomorphism of (X, τ); (c) the orbit partition of T is the tail partition of the Markov compactum (up to two classes of the partition). The Markov compactum structure is the same as the path structure of the Bratteli diagram for some AF-algebra. Bibliography: 19 titles. Published inZapiski Nauchnykh Seminarov POMI, Vol. 223, 1995, pp. 120–126.     

Group Representations with Empty Residual Spectrum

Let X be a Banach space on which a discrete group Γ acts by isometries. For certain natural choices of X, every element of the group algebra, when regarded as an operator on X, has empty residual spectrum. We show, for instance, that this occurs if X is ℓ ²(Γ) or the group von Neumann algebra VN(Γ). In our approach, we introduce the notion of a surjunctive pair, and develop some of the basic properties of this construction. The cases X = ℓ ^p (Γ) for 1 ≤ p < 2 or 2 < p < ∞ are more difficult. If Γ is amenable we can obtain partial results, using a majorization result of Herz; an example of Willis shows that some condition on Γ is necessary.     

Universal minimal topological dynamical systems

Rokhlin (1963) showed that any aperiodic dynamical system with finite entropy admits a countable generating partition. Krieger (1970) showed that aperiodic ergodic systems with entropy < log a, admit a generating partition with no more than a sets. In Symbolic Dynamics terminology, these results can be phrased— ℕ^ℤ is a universal system in the category of aperiodic systems, and [a]^ℤ is a universal system for aperiodic ergodic systems with entropy < log a. Weiss ([We89], 1989) presented a Minimal system, on a Compact space (a subshift of ) which is universal for aperiodic systems. In this work we present a joint generalization of both results: given ɛ, there exists a minimal subshift of [a]^ℤ, universal for aperiodic ergodic systems with entropy < log a − ɛ.     

Integrable structure of the field theory of open superstrings

We find asymptotic expansions in the “string mass” μ for μ-deformed Γ-functions and Neumann coefficients characterizing the three-string vertex in the light-cone gauge of the field theory of open superstrings on a maximally supersymmetric pp-wave background. Dedicated to the 80th birthday of Academician Anatolii Alekseevich Logunov __________ Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 149, No. 3, pp. 381–385, December, 2006.     

Un critère de rationalité provenant de la géométrie non commutative

We prove a conjecture of A. Connes, which gives a rationality criterion for elements of the closure of ℂΓ (Γ a free group) in the space of bounded operators in l ² (Γ). We show that this criterion applies also to the ring of Malcev–Neumann series on Γ.

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Oblatum 2-VIII-1996 & 20-IX-1996     

Characterization of algebraic curves by Chebyshev quadrature

Complex potential theory is used to show that Chebyshev-type quadrature works particularly well on algebraic Jordan curves Γ in ℝ ^d , supplied with normalized arc length or a similar probability measure μ. Evaluating the integral ∫_Γ fdμ by the arithmetic mean of the value off on any cycle ofN equally spaced nodes on Γ (relative to μ), the quadrature error will, be bounded byAe ^−bN sup_Γ|f| for allN and all polynomialsf(x) of degree ≤cN. It is plausible that small shifts of the nodes would give quadrature error zero for such polynomials. There are related results for algebraic Jordan arcs and certain algebraic surfaces. The situation is completely different for nonalgebraic curves and surfaces, where corresponding quadrature remainders are at least of order 1/N.     

On equivalent strictly G-convex renormings of Banach spaces

We study strictly G-convex renormings and extensions of strictly G-convex norms on Banach spaces. We prove that ℓ_ω(Γ) space cannot be strictly G-convex renormed given Γ is uncountable and G is bounded and separable.     

Plane curves of genus p and degree 2p as projections of space curves of the same degree

We characterize plane curves Γ of genus p and degree 2p with respect to the possibility of obtaining them as projections of space curves C′ of the same degree. When Γ is hyperelliptic, we link this characterization with the configuration of the singularities of Γ and with the position of C′ on certain scrolls. Supported by the M.U.R.S.T. of the Italian Government