DOCSLIB.ORG

Differentiability Problems in Banach Spaces

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Differentiability Problems in Banach Spaces Derivatives Differentiability problems in Banach spaces For vector valued functions there are two main version of derivatives: Gâteaux (or weak) derivatives and Fréchet (or strong) derivatives. For a function f from a Banach space X 1 David Preiss into a Banach space Y the Gâteaux derivative at a point x0 2 X is by definition a bounded linear operator T : X −! Y so that Expanded notes of a talk based on a nearly finished research monograph “Fréchet differentiability of Lipschitz functions and porous sets in Banach for every u 2 X, spaces” f (x + tu) − f (x ) written jointly with lim 0 0 = Tu t!0 t Joram Lindenstrauss2 and Jaroslav Tišer3 The operator T is called the Fréchet derivative of f at x if it is a with some results coming from a joint work with 0 4 5 Gâteaux derivative of f at x0 and the limit above holds uniformly Giovanni Alberti and Marianna Csörnyei in u in the unit ball (or unit sphere) in X. So T is the Fréchet derivative of f at x0 if 1Warwick 2Jerusalem 3Prague 4Pisa 5London f (x0 + u) = f (x0) + Tu + o(kuk) as kuk ! 0: 1 / 24 2 / 24 Existence of derivatives Sharpness of Lebesgue’s result The first continuous nowhere differentiable f : R ! R was constructed by Bolzano about 1820 (unpublished), who Lebesgue’s result is sharp in the sense that for every A ⊂ of however did not give a full proof. Around 1850, Riemann R measure zero there is a Lipschitz (and monotone) function mentioned such an example, which was later found slightly f : ! which fails to have a derivative at any point of A. incorrect. The first published example with a valid proof is by R R Weierstrass in 1875. A more precise result was proved by Zahorski in 1946. The first general result on existence of derivatives for functions Theorem. A set A ⊂ R is a Gδσ set of Lebesgue measure zero f : R −! R was found by Lebesgue (around 1900). He proved if and only if there is a Lipschitz function f : R −! R which is that a monotone function f : R −! R is differentiable almost differentiable exactly at points of R n A. everywhere. As a consequence it follows that every Lipschitz Explanation. A set A ⊂ R is function f : R −! R i.e., a function which satisfies T1 Gδ if there are open sets Gi so that A = i=1 Gi . G if there are G sets G so that A = S1 G . jf (s) − f (t)j ≤ Cjs − tj δσ δ i i=1 i for some constant C and every s; t 2 R, has a derivative a.e. 3 / 24 4 / 24 Rademacher’s Theorem Infinitely many dimensions Lebesgue’s theorem was extended to Lipschitz functions f : n ! by Rademacher in 1919 who showed that in this R R The notion of a Lipschitz function makes sense for functions case f is also differentiable a.e. f : M ! N between metric spaces, However, this result is not as sharp as Lebesgue’s: in Rn, n ≥ 2 there are sets of measure zero containing points of dist(f (x); f (y)) ≤ const · dist(x; y): differentiability of all Lipschitz f : R2 ! R (Preiss 1990). Doré and Maleva (in preparation) found a compact set in R2 of This gives rise to the study of derivatives of Lipschitz functions Hausdorff dimension one that contains points of differentiability between Banach spaces X and Y . 2 of all Lipschitz functions f : R ! R. If dim(X) < 1 and f is Lipschitz, the two notions of derivative With Alberti and Csörnyei (in preparation) we proved that coincide. However, if dim X = 1 easy examples show that Rademacher’s theorem is sharp for maps from R2 to R2. there is a big difference between Gâteaux and Fréchet Many higher dimensional results are also known, but the differentiability even for simple Lipschitz functions. question of sharpness of Rademacher’s theorem for maps from R3 to R3 is still open. 5 / 24 6 / 24 Almost everywhere Further obstacles It is easy to find nowhere differentiable Lipschitz maps f : R ! Y , Y a Banach space. For example, for Y = c0 (the Recall that in infinite dimensional spaces there is no Lebesgue space of sequences converging to zero with maximum norm), measure. If we wish to extend Lebesgue’s theorem to infinite dimensional setting, we have to extend the notion of a.e. sin(t) sin(2t) sin(3t) f (t) = ; ; ;::: (almost everywhere, except for a null set) to such spaces. 1 2 3 So we have to define in a reasonable way a family of negligible Spaces Y for which this pathology does not happen were sets on such spaces. They should form a proper σ-ideal of characterized in various ways by many authors (including subsets of the given space X, i.e., be closed under subsets and Walter Schachermayer). They are called spaces with the RNP countable unions, and should not contain all subsets of X. (Radon-Nikodým property) and include all reflexive spaces. It turns out that there are infinitely many non-equivalent natural There are more obstacles to Fréchet differentiability; e.g. the σ-ideals, some of which are suitable for some differentiability norm on ` is nowhere Fréchet differentiable. Spaces for which questions. 1 this behaviour does not happen are called Asplund spaces. Among separable spaces they are precisely those with separable dual. 7 / 24 8 / 24 Gâteaux differentiability Fréchet differentiability The unsolved questions about Gâteaux differentiability pale in Starting from about 1970, the theorem of Lebesgue has been comparison with those concerning Fréchet differentiability. extended to Gâteaux differentiability, with various notions of The only general positive result is negligible sets, independently by a number of authors: Mankiewicz, Christensen, Aronszajn, Phelps,. Theorem (Preiss 1990). Every real-valued Lipschitz function on an Asplund space has points of Fréchet differentiability. Theorem. Every Lipschitz map from a separable Banach space X into a space Y with the RNP is Gâteaux differentiable However, no “almost everywhere” result is known. almost everywhere. In other words, we do not whether every countable collection of The situation concerning the existence of Gâteaux derivatives real-valued Lipschitz functions on an Asplund space has a is generally deemed to be quite satisfactory. However, once one common point of Fréchet differentiability. goes a bit deeper, fundamental questions remain unanswered. We do not even know whether three Lipschitz functions on a Hilbert space have a common point of Fréchet differentiability. 9 / 24 10 / 24 Γ-null sets What about Hilbert spaces Lindenstrauss and Preiss (2003) defined a new σ-ideal of In Hilbert spaces (as well as in all `p, 1 < p < 1) there are negligible sets: A set N ⊂ X is γ-null if it is null on residually porous sets that are not Γ-null. The best we can do is many infinite dimensional C1 surfaces. The differentiability result with these sets is rather curious. Theorem (Lindenstrauss, Preiss, Tišer). Every pair of real-valued Lipschitz function on a Hilbert space has a common Every real-valued Lipschitz function on an Asplund Theorem. point of Fréchet differentiability. space X is Fréchet differentiable Γ-almost everywhere if and only if every set porous in X is Γ-null. Theorem (Lindenstrauss, Preiss, Tišer). Every collection Porous sets are special sets of Fréchet nondifferentiability: a of n real-valued Lipschitz functions on `p, 1 < p < 1, p ≥ n set E ⊂ X is porous if an only if the function x ! dist(x; E) is has a common point of Fréchet differentiability. Fréchet nondifferentiable at any point of E. These results are special cases of a more general result. Notice that the class of porous sets is considerably smaller than Theorem (Lindenstrauss, Preiss, Tišer). If a Banach the class of Fréchet nondifferentiability sets, already on . R space X has modulus of asymptotic smoothness − Originally only asymptotically c0 spaces were known to satisfy o(tn logn 1(1=t)) then every collection of n real-valued Lipschitz the assumption of this Theorem. We now know that it holds for functions on X has a common point of Fréchet differentiability. spaces asymptotically smooth with modulus o(tn) for any n. 11 / 24 12 / 24 Why are 2 and 3 so different? Asymptotic smoothness The modulus of asymptotic uniform smoothness of a Banach Theorem (Preiss, Tišer). There are three real-valued space X is defined by Lipschitz functions f1; f2; f3 on a Hilbert space so that ρ¯X (t) = sup inf sup kx + yk − 1; t > 0: k k= ( = )<1 y2Y @f1(x) @f2(x) @f2(x) x 1 dim X Y + + = 0 kyk≤t @x1 @x2 @x3 The moduli of asymptotic uniform smoothness of ` , at every point at which they are all Fréchet differentiable but not p 1 ≤ p < 1, and c are at every point at which they are all Gâteaux differentiable. 0 p 1=p We call this phenomenon a (rather strong) failure of the ρ¯`p (t) = (1 + t ) − 1 multi-dimensional mean value estimate. In our positive results ρ¯c0 (t) = max(t − 1; 0) we can actually prove that the multi-dimensional mean value estimate holds. So the above is impossible with two functions. Unlike the usual moduli of uniform smoothness which cannot go to zero faster that t2, there is no limit on behaviour of these moduli as t & 0. 13 / 24 14 / 24 "-differentiability Mean value estimates So called "-Fréchet differentiability results for Lipschitz functions in asymptotically uniformly smooth spaces say that for Mean value estimates are inequalities controlling increment of every Lipschitz map f of such a space X into a finite the function with the help of its derivative.
Load more

Recommended publications
  • Math 346 Lecture #3 6.3 the General Fréchet Derivative
    Math 346 Lecture #3 6.3 The General Fr´echet Derivative We now extend the notion of the Fr´echet derivative to the Banach space setting. Throughout let (X; k · kX ) and (Y; k · kY ) be Banach spaces, and U an open set in X. 6.3.1 The Fr´echet Derivative Definition 6.3.1. A function f : U ! Y is Fr´echet differentiable at x 2 U if there exists A 2 B(X; Y ) such that kf(x + h) − f(x) − A(h)k lim Y = 0; h!0 khkX and we write Df(x) = A. We say f is Fr´echet differentiable on U if f is Fr´echet differ- entiable at each x 2 U. We often refer to Fr´echet differentiable simply as differentiable. Note. When f is Fr´echet differentiable at x 2 U, the derivative Df(x) is unique. This follows from Proposition 6.2.10 (uniqueness of the derivative for finite dimensional X and Y ) whose proof carries over without change to the general Banach space setting (see Remark 6.3.9). Remark 6.3.2. The existence of the Fr´echet derivative does not change when the norm on X is replace by a topologically equivalent one and/or the norm on Y is replaced by a topologically equivalent one. Example 6.3.3. Any L 2 B(X; Y ) is Fr´echet differentiable with DL(x)(v) = L(v) for all x 2 X and all v 2 X. The proof of this is exactly the same as that given in Example 6.2.5 for finite-dimensional Banach spaces.
    [Show full text]
  • FROM CLASSICAL MECHANICS to QUANTUM FIELD THEORY, a TUTORIAL Copyright © 2020 by World Scientific Publishing Co
    FROM CLASSICAL MECHANICS TO QUANTUM FIELD THEORY A TUTORIAL 11556_9789811210488_TP.indd 1 29/11/19 2:30 PM This page intentionally left blank FROM CLASSICAL MECHANICS TO QUANTUM FIELD THEORY A TUTORIAL Manuel Asorey Universidad de Zaragoza, Spain Elisa Ercolessi University of Bologna & INFN-Sezione di Bologna, Italy Valter Moretti University of Trento & INFN-TIFPA, Italy World Scientific NEW JERSEY • LONDON • SINGAPORE • BEIJING • SHANGHAI • HONG KONG • TAIPEI • CHENNAI • TOKYO 11556_9789811210488_TP.indd 2 29/11/19 2:30 PM Published by World Scientific Publishing Co. Pte. Ltd. 5 Toh Tuck Link, Singapore 596224 USA office: 27 Warren Street, Suite 401-402, Hackensack, NJ 07601 UK office: 57 Shelton Street, Covent Garden, London WC2H 9HE British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. FROM CLASSICAL MECHANICS TO QUANTUM FIELD THEORY, A TUTORIAL Copyright © 2020 by World Scientific Publishing Co. Pte. Ltd. All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the publisher. For photocopying of material in this volume, please pay a copying fee through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA. In this case permission to photocopy is not required from the publisher. ISBN 978-981-121-048-8 For any available supplementary material, please visit https://www.worldscientific.com/worldscibooks/10.1142/11556#t=suppl Desk Editor: Nur Syarfeena Binte Mohd Fauzi Typeset by Stallion Press Email: [email protected] Printed in Singapore Syarfeena - 11556 - From Classical Mechanics.indd 1 02-12-19 3:03:23 PM January 3, 2020 9:10 From Classical Mechanics to Quantum Field Theory 9in x 6in b3742-main page v Preface This book grew out of the mini courses delivered at the Fall Workshop on Geometry and Physics, in Granada, Zaragoza and Madrid.
    [Show full text]
  • Value Functions and Optimality Conditions for Nonconvex
    Value Functions and Optimality Conditions for Nonconvex Variational Problems with an Infinite Horizon in Banach Spaces∗ H´el`ene Frankowska† CNRS Institut de Math´ematiques de Jussieu – Paris Rive Gauche Sorbonne Universit´e, Campus Pierre et Marie Curie Case 247, 4 Place Jussieu, 75252 Paris, France e-mail: [email protected] Nobusumi Sagara‡ Department of Economics, Hosei University 4342, Aihara, Machida, Tokyo, 194–0298, Japan e-mail: [email protected] February 11, 2020 arXiv:1801.00400v3 [math.OC] 10 Feb 2020 ∗The authors are grateful to the two anonymous referees for their helpful comments and suggestions on the earlier version of this manuscript. †The research of this author was supported by the Air Force Office of Scientific Re- search, USA under award number FA9550-18-1-0254. It also partially benefited from the FJMH Program PGMO and from the support to this program from EDF-THALES- ORANGE-CRITEO under grant PGMO 2018-0047H. ‡The research of this author benefited from the support of JSPS KAKENHI Grant Number JP18K01518 from the Ministry of Education, Culture, Sports, Science and Tech- nology, Japan. Abstract We investigate the value function of an infinite horizon variational problem in the infinite-dimensional setting. Firstly, we provide an upper estimate of its Dini–Hadamard subdifferential in terms of the Clarke subdifferential of the Lipschitz continuous integrand and the Clarke normal cone to the graph of the set-valued mapping describing dynamics. Secondly, we derive a necessary condition for optimality in the form of an adjoint inclusion that grasps a connection between the Euler–Lagrange condition and the maximum principle.
    [Show full text]
  • On Fréchet Differentiability of Lipschitz Maps
    Annals of Mathematics, 157 (2003), 257–288 On Fr´echet differentiability of Lipschitz maps between Banach spaces By Joram Lindenstrauss and David Preiss Abstract Awell-known open question is whether every countable collection of Lipschitz functions on a Banach space X with separable dual has a common point ofFr´echet differentiability. We show that the answer is positive for some infinite-dimensional X. Previously, even for collections consisting of two functions this has been known for finite-dimensional X only (although for one function the answer is known to be affirmative in full generality). Our aims are achieved by introducing a new class of null sets in Banach spaces (called Γ-null sets), whose definition involves both the notions of category and mea- sure, and showing that the required differentiability holds almost everywhere with respect to it. We even obtain existence of Fr´echet derivatives of Lipschitz functions between certain infinite-dimensional Banach spaces; no such results have been known previously. Our main result states that a Lipschitz map between separable Banach spaces is Fr´echet differentiable Γ-almost everywhere provided that it is reg- ularly Gˆateaux differentiable Γ-almost everywhere and the Gˆateaux deriva- tives stay within a norm separable space of operators. It is easy to see that Lipschitz maps of X to spaces with the Radon-Nikod´ym property are Gˆateaux differentiable Γ-almost everywhere. Moreover, Gˆateaux differentiability im- plies regular Gˆateaux differentiability with exception of another kind of neg- ligible sets, so-called σ-porous sets. The answer to the question is therefore positive in every space in which every σ-porous set is Γ-null.
    [Show full text]
  • Gateaux Differentiability Revisited Malek Abbasi, Alexander Kruger, Michel Théra
    Gateaux differentiability revisited Malek Abbasi, Alexander Kruger, Michel Théra To cite this version: Malek Abbasi, Alexander Kruger, Michel Théra. Gateaux differentiability revisited. 2020. hal- 02963967 HAL Id: hal-02963967 https://hal.archives-ouvertes.fr/hal-02963967 Preprint submitted on 12 Oct 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Noname manuscript No. (will be inserted by the editor) Gateaux differentiability revisited Malek Abbasi · Alexander Y. Kruger · Michel Thera´ Received: date / Accepted: date Abstract We revisit some basic concepts and ideas of the classical differential calculus and convex analysis extending them to a broader frame. We reformulate and generalize the notion of Gateaux differentiability and propose new notions of generalized derivative and general- ized subdifferential in an arbitrary topological vector space. Meaningful examples preserving the key properties of the original notion of derivative are provided. Keywords Gateaux differentiability · Moreau–Rockafellar subdifferential · Convex function · Directional derivative. Mathematics Subject Classification (2000) 49J52 · 49J53 · 90C30 1 Introduction Gateaux derivatives are widely used in the calculus of variations, optimization and physics. According to Laurent Mazliak [7,8], Gateaux differentiability first appeared in Gateaux’s notes [3,4] under the name variation premiere` .
    [Show full text]
  • ECE 821 Optimal Control and Variational Methods Lecture Notes
    ECE 821 Optimal Control and Variational Methods Lecture Notes Prof. Dan Cobb Contents 1 Introduction 3 2 Finite-Dimensional Optimization 5 2.1 Background ........................................ 5 2.1.1 EuclideanSpaces ................................. 5 2.1.2 Norms....................................... 6 2.1.3 MatrixNorms................................... 8 2.2 Unconstrained Optimization in Rn ........................... 9 2.2.1 Extrema...................................... 9 2.2.2 Jacobians ..................................... 9 2.2.3 CriticalPoints................................... 10 2.2.4 Hessians...................................... 11 2.2.5 Definite Matrices . 11 2.2.6 Continuity and Continuous Differentiability . 14 2.2.7 Second Derivative Conditions . 15 2.3 Constrained Optimization in Rn ............................. 16 2.3.1 Constrained Extrema . 16 2.3.2 OpenSets..................................... 17 2.3.3 Strict Inequality Constraints . 18 2.3.4 Equality Constraints and Lagrange Multipliers . 19 2.3.5 Second Derivative Conditions . 21 2.3.6 Non-Strict Inequality Constraints . 23 2.3.7 Mixed Constraints . 27 3 Calculus of Variations 27 3.1 Background ........................................ 27 3.1.1 VectorSpaces ................................... 27 3.1.2 Norms....................................... 28 3.1.3 Functionals .................................... 30 3.2 Unconstrained Optimization in X ............................ 32 3.2.1 Extrema...................................... 32 3.2.2 Differentiation of Functionals .
    [Show full text]
  • An Inverse Function Theorem Converse
    AN INVERSE FUNCTION THEOREM CONVERSE JIMMIE LAWSON Abstract. We establish the following converse of the well-known inverse function theorem. Let g : U → V and f : V → U be inverse homeomorphisms between open subsets of Banach spaces. If g is differentiable of class Cp and f if locally Lipschitz, then the Fr´echet derivative of g at each point of U is invertible and f must be differentiable of class Cp. Primary 58C20; Secondary 46B07, 46T20, 46G05, 58C25 Key words and phrases. Inverse function theorem, Lipschitz map, Banach space, chain rule 1. Introduction A general form of the well-known inverse function theorem asserts that if g is a differentiable function of class Cp, p ≥ 1, between two open subsets of Banach spaces and if the Fr´echet derivative of g at some point x is invertible, then locally around x, there exists a differentiable inverse map f of g that is also of class Cp. But in various settings, one may have the inverse function f readily at hand and want to know about the invertibility of the Fr´echet derivative of g at x and whether f is of class Cp. Our purpose in this paper is to present a relatively elementary proof of this arXiv:1812.03561v1 [math.FA] 9 Dec 2018 converse result under the general hypothesis that the inverse f is (locally) Lipschitz. Simple examples like g(x)= x3 at x = 0 on the real line show that the assumption of continuity alone is not enough. Thus it is a bit surprising that the mild strengthening to the assumption that the inverse is locally Lipschitz suffices.
    [Show full text]
  • A Generalization of the Implicit Function Theorem
    Applied Mathematical Sciences, Vol. 4, 2010, no. 26, 1289 - 1298 A Generalization of the Implicit Function Theorem Elvio Accinelli Facultad de Economia de la UASLP Av. Pintores S/N, Fraccionamiento Burocratas del Estado CP 78263 San Luis Potosi, SLP Mexico [email protected] Abstract In this work, we generalize the classical theorem of the implicit func- tion, for functions whose domains are open subset of Banach spaces in to a Banach space, to the case of functions whose domains are convex subset, not necessarily open, of Banach spaces in to a Banach space. We apply this theorem to show that the excess utility function of an economy with infinitely many commodities, is a differentiable mapping. Keywords: Implicit Function Theorem, convex subsets, Banach spaces 1 Introduction The purpose of this work is to show that the implicit function theorem can be generalized to the case of functions whose domains are defined as a cartesian product of two convex subsets S, and W not necessarily open, of a cartesian product X × Y of Banach spaces, in to a Banach space Z. To prove our main theorem we introduce the concept of Gateaux derivative. However, it is enough the existence of the Gateaux derivatives of a given function, only in admissible directions. Let S ⊂ X be a subset of a Banach space X, and x ∈ S. So, we will say that the vector h ∈ X is admissible for x ∈ S if and only if x + h ∈ S. In order to define the Gateaux derivative of a function in admissible directions in a point x, the only necessary condition is the convexity of the domain of the function.
    [Show full text]
  • DIFFERENTIABILITY PROPERTIES of UTILITY FUNCTIONS Freddy
    DIFFERENTIABILITY PROPERTIES OF UTILITY FUNCTIONS Freddy Delbaen Department of Mathematics, ETH Z¨urich Abstract. We investigate differentiability properties of monetary utility functions. At the same time we give a counter-example – important in finance – to automatic continuity for concave functions. Notation and Preliminaries We use standard notation. The triple (Ω, F, P) denotes an atomless probability space. In practice this is not a restriction since the property of being atomless is equivalent to the fact that on (Ω, F, P), we can define a random variable that has a continuous distribution function. We use the usual – unacceptable – convention to identify a random variable with its class (defined by a.s. equality). By Lp we denote the standard spaces, i.e. for 0 <p<∞, X ∈ Lp if and only if E[|X|p] < ∞. L0 stands for the space of all random variables and L∞ is the space of bounded random variables. The topological dual of L∞ is denoted by ba, the space of bounded finitely additive measures µ defined on F with the property that P[A]=0 implies µ(A)=0. The subset of normalised non-negative finitely additive measures – the so called finitely additive probability measures – is denoted by Pba. The subset of countably additive elements of Pba (a subset of L1)isdenoted by P. Definition. A function u: L∞ → R is called a monetary utility function if (1) ξ ∈ L∞ and ξ ≥ 0 implies u(ξ) ≥ 0, (2) u is concave, (3) for a ∈ R and ξ ∈ L∞ we have u(ξ + a)=u(ξ)+a. If u moreover satisfies u(λξ)=λu(ξ) for λ ≥ 0 (positive homogeniety), we say that u is coherent.
    [Show full text]
  • Computation of a Bouligand Generalized Derivative for the Solution Operator of the Obstacle Problem∗
    COMPUTATION OF A BOULIGAND GENERALIZED DERIVATIVE FOR THE SOLUTION OPERATOR OF THE OBSTACLE PROBLEM∗ ANNE-THERESE RAULSy AND STEFAN ULBRICHy Abstract. The non-differentiability of the solution operator of the obstacle problem is the main challenge in tackling optimization problems with an obstacle problem as a constraint. Therefore, the structure of the Bouligand generalized differential of this operator is interesting from a theoretical and from a numerical point of view. The goal of this article is to characterize and compute a specific element of the Bouligand generalized differential for a wide class of obstacle problems. We allow right-hand sides that can be relatively sparse in H−1(Ω) and do not need to be distributed among all of H−1(Ω). Under assumptions on the involved order structures, we investigate the relevant set-valued maps and characterize the limit of a sequence of G^ateauxderivatives. With the help of generalizations of Rademacher's theorem, this limit is shown to be in the considered Bouligand generalized differential. The resulting generalized derivative is the solution operator of a Dirichlet problem on a quasi-open domain. Key words. optimal control, non-smooth optimization, Bouligand generalized differential, gen- eralized derivatives, obstacle problem, variational inequalities, Mosco convergence AMS subject classifications. 49J40, 35J86, 47H04, 49J52, 58C06, 58E35 1. Introduction. This article is concerned with the derivation of a generalized gradient for the reduced objective associated with the optimal control of an obstacle problem min J (y; u) y;u subject to y 2 K ; (1.1) hLy − f(u); z − yi −1 1 ≥ 0 for all z 2 K : H (Ω);H0 (Ω) d 1 Here, Ω ⊂ R is open and bounded, J : H0 (Ω)×U ! R is a continuously differentiable 1 −1 objective function and L 2 L H0 (Ω);H (Ω) is a coercive and strictly T-monotone operator.
    [Show full text]
  • Arxiv:1802.07633V1 [Math.FA]
    GATEAUX-DIFFERENTIABILITYˆ OF CONVEX FUNCTIONS IN INFINITE DIMENSION. MOHAMMED BACHIR AND ADRIEN FABRE Abstract. It is well known that in Rn, Gˆateaux (hence Fr´echet) differ- entiability of a convex continuous function at some point is equivalent to the existence of the partial derivatives at this point. We prove that this result extends naturally to certain infinite dimensional vector spaces, in particular to Banach spaces having a Schauder basis. 1. Introduction Recall that if E is a topological vector space and E∗ its topological dual, the subdifferential of a function f : E −→ R at some point x∗ ∈ E is the following subset of E∗: ∂f(x∗) := {p ∈ E∗ : hp, x − x∗i≤ f(x) − f(x∗); ∀x ∈ E}. Let U be an open subset of E and f : E −→ R a function. We say that f is differentiable at x∗ ∈ U in the direction of h ∈ E if the following limit exists 1 f ′(x∗; h) := lim f(x∗ + th) − f(x∗) . t→0 t6=0 t We say that f is Gˆateaux-differentiable at x∗ ∈ U, if there exists F ∈ E∗ (called the Gˆateaux-derivative of f at x∗ and generally denoted by df(x∗)) such that f ′(x∗; h)= F (h), for all h ∈ E. It is well known that if E is a Hausdorff locally convex topological vector space and f is a convex continuous function then, f is Gˆateaux-differentiable at x∗ ∈ E if and only if, ∂f(x∗) is a singleton (see [3, Corollary 10.g, p. 66]). arXiv:1802.07633v1 [math.FA] 21 Feb 2018 In this case ∂f(x∗) = {df(x∗)}, where df(x∗) is the Gˆateaux-derivative of f at x∗.
    [Show full text]
  • Numerical Solutions to Partial Differential Equations
    Numerical Solutions to Partial Differential Equations Zhiping Li LMAM and School of Mathematical Sciences Peking University Finite Element Method | a Method Based on Variational Problems Finite Difference Method: 1 Based on PDE problem. 2 Introduce a grid (or mesh) on Ω. 3 Define grid function spaces. 4 Approximate differential operators by difference operators. 5 PDE discretized into a finite algebraic equation. Finite Element Method: 1 Based on variational problem, say F (u) = infv2X F (v). 2 Introduce a grid (or mesh) on Ω. 3 Establish finite dimensional subspaces Xh of X. 4 Restrict the original problem on the subspaces, say F (Uh) = infVh2Xh F (Vh). 5 PDE discretized into a finite algebraic equation. Variational Form of Elliptic Boundary Value Problems Abstract Variational Problems Functional Minimization Problem An Abstract Variational Form of Energy Minimization Problem Many physics problems, such as minimum potential energy principle in elasticity, lead to an abstract variational problemµ 8 <Find u 2 U such that J(u) = inf J(v); : v2U where U is a nonempty closed subset of a Banach space V, and J : v 2 U ! R is a functional. In many practical linear problems, V is a Hilbert space, U a closed linear subspace of V; the functional J often has the form 1 J(v) = a(v; v) − f (v); 2 a(·; ·) and f are continuous bilinear and linear functionals. 3 / 33 Variational Form of Elliptic Boundary Value Problems Abstract Variational Problems Functional Minimization Problem Find Solutions to a Functional Minimization Problem Method 1 | Direct method of calculus of variations: 1 Find a minimizing sequence, say, by gradient type methods; 2 Find a convergent subsequence of the minimizing sequence, say, by certain kind of compactness; 3 Show the limit is a minimizer, say, by lower semi-continuity of the functional.
    [Show full text]