Analysis and Probability on Infinite-Dimensional Spaces
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Path Regularity and Tightness of Capac- Ities
Applications of Compact Superharmonic Func- tions: Path Regularity and Tightness of Capac- ities Lucian Beznea and Michael R¨ockner Abstract. We establish relations between the existence of the L-superharmonic functions that have compact level sets (L being the generator of a right Markov process), the path regularity of the process, and the tightness of the induced capacities. We present several examples in infinite dimensional situations, like the case when L is the Gross-Laplace operator on an abstract Wiener space and a class of measure-valued branching process associated with a nonlinear perturbation of L. Mathematics Subject Classification (2000). Primary 60J45, 31C15; Secondary 47D07, 60J40, 60J35. Keywords. resolvent of kernels, tight capacity, Lyapunov function, right pro- cess, standard process. 1. Introduction A major difficulty in the stochastic analysis in infinite dimensions is the lack of the local compactness of the state space. The developments from the last two decades, in particular the theory of quasi-regular Dirichlet forms (cf. [14]), indicated, as an adequate substitute, the existence of a nest of compact sets, or equivalently, the tightness property of the associated capacity. However, in applications it is sometimes not easy to verify such a property, for example, because the capacity is not expressed directly in terms of the infinitesimal generator L of the theory. It happens that a more convenient property is the existence of an L-superharmonic function having compact level sets; such a function was called compact Lyapunov. The aim of this paper is to give an overview on the relations between the existence of a compact Lyapunov function, the path regularity of the process, and the tightness of the induced capacities. -
On Two Dimensional Markov Processes with Branching Property^)
ON TWO DIMENSIONAL MARKOV PROCESSES WITH BRANCHING PROPERTY^) BY SHINZO WATANABE Introduction. A continuous state branching Markov process (C.B.P.) was introduced by Jirina [8] and recently Lamperti [10] determined all such processes on the half line. (A quite similar result was obtained independently by the author.) This class of Markov processes contains as a special case the diffusion processes (which we shall call Feller's diffusions) studied by Feller [2]. The main objective of the present paper is to extend Lamperti's result to multi-dimensional case. For simplicity we shall consider the case of 2-dimensions though many arguments can be carried over to the case of higher dimensions(2). In Theorem 2 below we shall characterize all C.B.P.'s in the first quadrant of a plane and construct them. Our construction is in an analytic way, by a similar construction given in Ikeda, Nagasawa and Watanabe [5], through backward equations (or in the terminology of [5] through S-equations) for a simpler case and then in the general case by a limiting procedure. A special attention will be paid to the case of diffusions. We shall show that these diffusions can be obtained as a unique solution of a stochastic equation of Ito (Theorem 3). This fact may be of some interest since the solutions of a stochastic equation with coefficients Holder continuous of exponent 1/2 (which is our case) are not known to be unique in general. Next we shall examine the behavior of sample functions near the boundaries (xj-axis or x2-axis). -
A. the Bochner Integral
A. The Bochner Integral This chapter is a slight modification of Chap. A in [FK01]. Let X, be a Banach space, B(X) the Borel σ-field of X and (Ω, F,µ) a measure space with finite measure µ. A.1. Definition of the Bochner integral Step 1: As first step we want to define the integral for simple functions which are defined as follows. Set n E → ∈ ∈F ∈ N := f :Ω X f = xk1Ak ,xk X, Ak , 1 k n, n k=1 and define a semi-norm E on the vector space E by f E := f dµ, f ∈E. To get that E, E is a normed vector space we consider equivalence classes with respect to E . For simplicity we will not change the notations. ∈E n For f , f = k=1 xk1Ak , Ak’s pairwise disjoint (such a representation n is called normal and always exists, because f = k=1 xk1Ak , where f(Ω) = {x1,...,xk}, xi = xj,andAk := {f = xk}) and we now define the Bochner integral to be n f dµ := xkµ(Ak). k=1 (Exercise: This definition is independent of representations, and hence linear.) In this way we get a mapping E → int : , E X, f → f dµ which is linear and uniformly continuous since f dµ f dµ for all f ∈E. Therefore we can extend the mapping int to the abstract completion of E with respect to E which we denote by E. 105 106 A. The Bochner Integral Step 2: We give an explicit representation of E. Definition A.1.1. -
Patterns in Random Walks and Brownian Motion
Patterns in Random Walks and Brownian Motion Jim Pitman and Wenpin Tang Abstract We ask if it is possible to find some particular continuous paths of unit length in linear Brownian motion. Beginning with a discrete version of the problem, we derive the asymptotics of the expected waiting time for several interesting patterns. These suggest corresponding results on the existence/non-existence of continuous paths embedded in Brownian motion. With further effort we are able to prove some of these existence and non-existence results by various stochastic analysis arguments. A list of open problems is presented. AMS 2010 Mathematics Subject Classification: 60C05, 60G17, 60J65. 1 Introduction and Main Results We are interested in the question of embedding some continuous-time stochastic processes .Zu;0Ä u Ä 1/ into a Brownian path .BtI t 0/, without time-change or scaling, just by a random translation of origin in spacetime. More precisely, we ask the following: Question 1 Given some distribution of a process Z with continuous paths, does there exist a random time T such that .BTCu BT I 0 Ä u Ä 1/ has the same distribution as .Zu;0Ä u Ä 1/? The question of whether external randomization is allowed to construct such a random time T, is of no importance here. In fact, we can simply ignore Brownian J. Pitman ()•W.Tang Department of Statistics, University of California, 367 Evans Hall, Berkeley, CA 94720-3860, USA e-mail: [email protected]; [email protected] © Springer International Publishing Switzerland 2015 49 C. Donati-Martin et al. -
Gaussian Measures in Traditional and Not So Traditional Settings
BULLETIN (New Series) OF THE AMERICAN MATHEMATICAL SOCIETY Volume 33, Number 2, April 1996 GAUSSIAN MEASURES IN TRADITIONAL AND NOT SO TRADITIONAL SETTINGS DANIEL W. STROOCK Abstract. This article is intended to provide non-specialists with an intro- duction to integration theory on pathspace. 0: Introduction § Let Q1 denote the set of rational numbers q [0, 1], and, for a Q1, define the ∈ ∈ translation map τa on Q1 by addition modulo 1: a + b if a + b 1 τab = ≤ for b Q1. a + b 1ifa+b>1 ∈ − Is there a probability measure λQ1 on Q1 which is translation invariant in the sense that 1 λQ1 (Γ) = λQ1 τa− Γ for every a Q1? To answer∈ this question, it is necessary to be precise about what it is that one is seeking. Indeed, the answer will depend on the level of ambition at which the question is asked. For example, if denotes the algebra of subsets Γ Q1 generated by intervals A ⊆ [a, b] 1 q Q1 : a q b Q ≡ ∈ ≤ ≤ for a b from Q1, then it is easy to check that there is a unique, finitely additive way to≤ define λ on .Infact, Q1 A λ 1 [a, b] 1 = b a, for a b from Q1. Q Q − ≤ On the other hand, if one is more ambitious and asks for a countably additive λQ1 on the σ-algebra generated by (equivalently, all subsets of Q1), then one is asking A for too much. In fact, since λQ1 ( q ) = 0 for every q Q1, countable additivity forces { } ∈ λ 1 (Q1)= λ 1 q =0, Q Q { } q 1 X∈Q which is obviously irreconcilable with λQ1 (Q1) = 1. -
Arxiv:Math/0409479V1
EXCEPTIONAL TIMES AND INVARIANCE FOR DYNAMICAL RANDOM WALKS DAVAR KHOSHNEVISAN, DAVID A. LEVIN, AND PEDRO J. MENDEZ-HERN´ ANDEZ´ n ABSTRACT. Consider a sequence Xi(0) of i.i.d. random variables. As- { }i=1 sociate to each Xi(0) an independent mean-one Poisson clock. Every time a clock rings replace that X-variable by an independent copy and restart the clock. In this way, we obtain i.i.d. stationary processes Xi(t) t 0 { } ≥ (i = 1, 2, ) whose invariant distribution is the law ν of X1(0). ··· Benjamini et al. (2003) introduced the dynamical walk Sn(t) = X1(t)+ + Xn(t), and proved among other things that the LIL holds for n ··· 7→ Sn(t) for all t. In other words, the LIL is dynamically stable. Subse- quently (2004b), we showed that in the case that the Xi(0)’s are standard normal, the classical integral test is not dynamically stable. Presently, we study the set of times t when n Sn(t) exceeds a given 7→ envelope infinitely often. Our analysis is made possible thanks to a con- nection to the Kolmogorov ε-entropy. When used in conjunction with the invariance principle of this paper, this connection has other interesting by-products some of which we relate. We prove also that the infinite-dimensional process t S n (t)/√n 7→ ⌊ •⌋ converges weakly in D(D([0, 1])) to the Ornstein–Uhlenbeck process in C ([0, 1]). For this we assume only that the increments have mean zero and variance one. In addition, we extend a result of Benjamini et al. -
Functions in the Fresnel Class of an Abstract Wiener Space
J. Korean Math. Soc. 28(1991), No. 2, pp. 245-265 FUNCTIONS IN THE FRESNEL CLASS OF AN ABSTRACT WIENER SPACE J.M. AHN*, G.W. JOHNSON AND D.L. SKOUG o. Introduction Abstract Wiener spaces have been of interest since the work of Gross in the early 1960's (see [18]) and are currently being used as a frame work for the Malliavin calculus (see the first half of [20]) and in the study of the Fresnel and Feynman integrals. It is this last topic which concerns us in this paper. The Feynman integral arose in nonrelativistic quantum mechanics and has been studied, with a variety of different definitions, by math ematicians and, in recent years, an increasing number of theoretical physicists. The Fresnel integral has been defined in Hilbert space [1], classical Wiener space [2], and abstract Wiener space [13] settings and used as an approach to the Feynman integral. Such approaches have their shortcomings; for example, the class offunctions that can be dealt with is somewhat limited. However, these approaches also have some distinct advantages including the fact, seen most clearly in [14], that several different definitions of the Feynman integral exist and agree. A key hypothesis in most of the theorems about the Fresnel and Feynman integrals is that the functions involved are in the 'Fresnel class' of the Hilbert space being used. Theorem 2 below is a general result insuring membership in :F(B), the Fresnel class of an abstract Wiener space B. :F(B) consists of all 'stochastic Fourier transforms' of certain Borel measures of finite variation. -
A Divergence Theorem for Hilbert Space
TRANSACTIONS OF THE AMERICAN MATHEMATICAL SOCIETY Volume 164, February 1972 A DIVERGENCE THEOREM FOR HILBERT SPACE BY VICTOR GOODMAN« Abstract. Let B be a real separable Banach space. A suitable linear imbedding of a real separable Hubert space into B with dense range determines a probability measure on B which is known as abstract Wiener measure. In this paper it is shown that certain submanifolds of B carry a surface measure uniquely defined in terms of abstract Wiener measure. In addition, an identity is obtained which relates surface integrals to abstract Wiener integrals of functions associated with vector fields on regions in B. The identity is equivalent to the classical divergence theorem if the Hubert space is finite dimensional. This identity is used to estimate the total measure of certain surfaces, and it is established that in any space B there exist regions whose boundaries have finite surface measure. 1. Introduction. Let B be a real separable Banach space with norm || • ||. Con- sider a linear imbedding i of a real separable Hubert space into B with dense range. Gross [5] has shown that if the function \¿- || is a measurable norm on the Hubert space, then the space B carries a Borel probability measure px uniquely characterized by the identity Px({xeB : iy,xy < r}) = (2Tr\¿*y\2)-1'2 f exp [-(2\¿*y\2Y1s2]ds. J —00 Here, y is any element of the topological dual space of B, ¿* is the adjoint of the imbedding map, and | • | is the norm on the dual Hubert space. The space B, to- gether with the Hubert space and the imbedding, is said to be an abstract Wiener space, andpx is said to be abstract Wiener measure on B. -
Infinite Dimension
Tel Aviv University, 2006 Gaussian random vectors 10 2 Basic notions: infinite dimension 2a Random elements of measurable spaces: formu- lations ......................... 10 2b Random elements of measurable spaces: proofs, andmorefacts .................... 14 2c Randomvectors ................... 17 2d Gaussian random vectors . 20 2e Gaussian conditioning . 24 foreword Up to isomorphism, we have for each n only one n-dimensional Gaussian measure (like a Euclidean space). What about only one infinite-dimensional Gaussian measure (like a Hilbert space)? Probability in infinite-dimensional spaces is usually overloaded with topo- logical technicalities; spaces are required to be Hilbert or Banach or locally convex (or not), separable (or not), complete (or not) etc. A fuss! We need measurability, not continuity; a σ-field, not a topology. This is the way to a single infinite-dimensional Gaussian measure (up to isomorphism). Naturally, the infinite-dimensional case is more technical than finite- dimensional. The crucial technique is compactness. Back to topology? Not quite so. The art is, to borrow compactness without paying dearly to topol- ogy. Look, how to do it. 2a Random elements of measurable spaces: formulations Before turning to Gaussian measures on infinite-dimensional spaces we need some more general theory. First, recall that ∗ a measurable space (in other words, Borel space) is a set S endowed with a σ-field B; ∗ B ⊂ 2S is a σ-field (in other words, σ-algebra, or tribe) if it is closed under finite and countable operations (union, intersection and comple- ment); ∗ a collection closed under finite operations is called a (Boolean) algebra (of sets); Tel Aviv University, 2006 Gaussian random vectors 11 ∗ the σ-field σ(C) generated by a given set C ⊂ 2S is the least σ-field that contains C; the algebra α(C) generated by C is defined similarly; ∗ a measurable map from (S1, B1) to (S2, B2) is f : S1 → S2 such that −1 f (B) ∈ B1 for all B ∈ B2. -
Long Term Risk: a Martingale Approach
Long Term Risk: A Martingale Approach Likuan Qin∗ and Vadim Linetskyy Department of Industrial Engineering and Management Sciences McCormick School of Engineering and Applied Sciences Northwestern University Abstract This paper extends the long-term factorization of the pricing kernel due to Alvarez and Jermann (2005) in discrete time ergodic environments and Hansen and Scheinkman (2009) in continuous ergodic Markovian environments to general semimartingale environments, with- out assuming the Markov property. An explicit and easy to verify sufficient condition is given that guarantees convergence in Emery's semimartingale topology of the trading strate- gies that invest in T -maturity zero-coupon bonds to the long bond and convergence in total variation of T -maturity forward measures to the long forward measure. As applications, we explicitly construct long-term factorizations in generally non-Markovian Heath-Jarrow- Morton (1992) models evolving the forward curve in a suitable Hilbert space and in the non-Markovian model of social discount of rates of Brody and Hughston (2013). As a fur- ther application, we extend Hansen and Jagannathan (1991), Alvarez and Jermann (2005) and Bakshi and Chabi-Yo (2012) bounds to general semimartingale environments. When Markovian and ergodicity assumptions are added to our framework, we recover the long- term factorization of Hansen and Scheinkman (2009) and explicitly identify their distorted probability measure with the long forward measure. Finally, we give an economic interpre- tation of the recovery theorem of Ross (2013) in non-Markovian economies as a structural restriction on the pricing kernel leading to the growth optimality of the long bond and iden- tification of the physical measure with the long forward measure. -
A Short History of Stochastic Integration and Mathematical Finance
A Festschrift for Herman Rubin Institute of Mathematical Statistics Lecture Notes – Monograph Series Vol. 45 (2004) 75–91 c Institute of Mathematical Statistics, 2004 A short history of stochastic integration and mathematical finance: The early years, 1880–1970 Robert Jarrow1 and Philip Protter∗1 Cornell University Abstract: We present a history of the development of the theory of Stochastic Integration, starting from its roots with Brownian motion, up to the introduc- tion of semimartingales and the independence of the theory from an underlying Markov process framework. We show how the development has influenced and in turn been influenced by the development of Mathematical Finance Theory. The calendar period is from 1880 to 1970. The history of stochastic integration and the modelling of risky asset prices both begin with Brownian motion, so let us begin there too. The earliest attempts to model Brownian motion mathematically can be traced to three sources, each of which knew nothing about the others: the first was that of T. N. Thiele of Copen- hagen, who effectively created a model of Brownian motion while studying time series in 1880 [81].2; the second was that of L. Bachelier of Paris, who created a model of Brownian motion while deriving the dynamic behavior of the Paris stock market, in 1900 (see, [1, 2, 11]); and the third was that of A. Einstein, who proposed a model of the motion of small particles suspended in a liquid, in an attempt to convince other physicists of the molecular nature of matter, in 1905 [21](See [64] for a discussion of Einstein’s model and his motivations.) Of these three models, those of Thiele and Bachelier had little impact for a long time, while that of Einstein was immediately influential. -
Probability Measures on Infinite-Dimensional Stiefel Manifolds
JOURNAL OF GEOMETRIC MECHANICS doi:10.3934/jgm.2017012 ©American Institute of Mathematical Sciences Volume 9, Number 3, September 2017 pp. 291–316 PROBABILITY MEASURES ON INFINITE-DIMENSIONAL STIEFEL MANIFOLDS Eleonora Bardelli‡ Microsoft Deutschland GmbH, Germany Andrea Carlo Giuseppe Mennucci∗ Scuola Normale Superiore Piazza dei Cavalieri 7, 56126, Pisa, Italia Abstract. An interest in infinite-dimensional manifolds has recently appeared in Shape Theory. An example is the Stiefel manifold, that has been proposed as a model for the space of immersed curves in the plane. It may be useful to define probabilities on such manifolds. Suppose that H is an infinite-dimensional separable Hilbert space. Let S ⊂ H be the sphere, p ∈ S. Let µ be the push forward of a Gaussian measure γ from TpS onto S using the exponential map. Let v ∈ TpS be a Cameron–Martin vector for γ; let R be a rotation of S in the direction v, and ν = R#µ be the rotated measure. Then µ, ν are mutually singular. This is counterintuitive, since the translation of a Gaussian measure in a Cameron– Martin direction produces equivalent measures. Let γ be a Gaussian measure on H; then there exists a smooth closed manifold M ⊂ H such that the projection of H to the nearest point on M is not well defined for points in a set of positive γ measure. Instead it is possible to project a Gaussian measure to a Stiefel manifold to define a probability. 1. Introduction. Probability theory has been widely studied for almost four cen- turies. Large corpuses have been written on the theoretical aspects.