View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by AIR Universita degli studi di Milano G. Crespi D. La Torre M.Rocca Second-order mollified derivatives and optimization 2002/9 UNIVERSITÀ DELL'INSUBRIA FACOLTÀ DI ECONOMIA http://eco.uninsubria.it In questi quaderni vengono pubblicati i lavori dei docenti della Facoltà di Economia dell’Università dell’Insubria. La pubblicazione di contributi di altri studiosi, che abbiano un rapporto didattico o scientifico stabile con la Facoltà, può essere proposta da un professore della Facoltà, dopo che il contributo sia stato discusso pubblicamente. Il nome del proponente è riportato in nota all'articolo. I punti di vista espressi nei quaderni della Facoltà di Economia riflettono unicamente le opinioni degli autori, e non rispecchiano necessariamente quelli della Facoltà di Economia dell'Università dell'Insubria. These Working papers collect the work of the Faculty of Economics of the University of Insubria. The publication of work by other Authors can be proposed by a member of the Faculty, provided that the paper has been presented in public. The name of the proposer is reported in a footnote. The views expressed in the Working papers reflect the opinions of the Authors only, and not necessarily the ones of the Economics Faculty of the University of Insubria. © Copyright G.Crespi D. La Torre M.Rocca Printed in Italy in March 2002 Università degli Studi dell'Insubria Via Ravasi 2, 21100 Varese, Italy All rights reserved. No part of this paper may be reproduced in any form without permission of the Author. Second–order mollified derivatives and optimization∗ Giovanni P. Crespi† Davide La Torre‡ Matteo Rocca§ Abstract The class of strongly semicontinuous functions is considered. For these func- tions the notion of mollified derivatives, introduced by Ermoliev, Norkin and Wets, is extended to the second order. By means of a generalized Taylor’s formula, second order necessary and sufficient conditions are proved for both unconstrained and constrained optimization. Keywords: Mollifiers, Optimization, Smooth approximations, Strong semicontinuity . 1 Introduction In this paper we extend to the second-order the approach introduced by Ermoliev, Norkin and Wets [8] to define generalized derivatives even for discontinuous func- tions, which often arise in applications (see [8] for references). To deal with such applications a number of approaches have been proposed to develop a subdifferen- tial calculus for nonsmooth and even discontinuous functions. Among the many possibilities, let us remember the notions due to Aubin [2], Clarke [5], Ioffe [13], Michel and Penot [20], Rockafellar [21], in the context of Variational Analysis. The previous approaches are based on the introduction of first-order generalized deriva- tives. Extensions to higher-order derivatives have been provided for instance by Hiriart-Hurruty, Strodiot and Hien Nguyen [12], Jeyakumar and Luc [14], Klatte and Tammer [15], Michel and Penot [19], Yang and Jeyakumar [26] , Yang [27]. Most of these higher-order approaches assume that the functions involved are of class C1,1, that is once differentiable with locally Lipschitz gradient, or at least of class C1. Anyway, another possibility, concerning the differentiation of nonsmooth functions dates back to the 30’s and is related to the names of Sobolev [25], who introduced the concept of “weak derivative” and later of Schwartz [24] who gener- alized Sobolev’s approach with the “theory of distributions”. These tecniques are ∗This work has been supportted by the F.A.R. 2000, of the University of Insubria. †Universit`aBocconi, I.M.Q., v.le Isonzo 25, 20137 Milano, Italia. e–mail: giovanni.crespi@uni- bocconi.it ‡Universit`adi Milano, Dipartimento di Economia Politica e Aziendale, via Conservatorio 7, 20122 Milano, Italia. e–mail: [email protected] §Universit`adell’Insubria, Dipartimento di Economia, via Ravasi 2, 21100 Varese, Italia. e–mail: [email protected] 1 widely used in the theory of partial differential equations, in Mathematical Physics and in related problems, but they have not been applied to deal with optimization problems involving nonsmooth functions, until the work of Ermoliev, Norkin and Wets. The tools which allow to link the “modern” and the “ancient” approaches to Non- smooth Analysis are those of “mollifier” and of “mollified functions”. More specifi- cally, the approach followed by Ermoliev, Norkin and Wets appeals to some of the results of the theory of distributions. They associate with a point x ∈ Rn a family of mollifiers (density functions) whose support tends toward x and converges to the Dirac function. Given such a family, say {ψ, > 0}, one can define a family of mollified functions associated to a function f : Rn → R as the convolution of f and ψ (mollified functions will be denoted by f). Hence a mollified function can be viewed as an averaged function. The mollified functions possess the same regularity 2 of the mollifiers ψ and hence, if they are at least of class C , one can define first and second-order generalized derivatives as the cluster points of all possible values of first and second-order derivatives of f. For more details one can see [8]. In this paper, section 2 recalls the notions of mollifier, of epi-convergence of a se- quence of functions and some definitions introduced in [8]. Section 3 is devoted to the introduction of second-order derivatives by means of mollified functions; sec- tions 4 and 5 deal, respectively, with second-order necessary and sufficient optimality conditions for unconstrained and constrained problems. 2 Preliminaries To follow the approach presented in [8] we first need to introduce the notion of mollifier (see e.g. [4]): Definition 1 A sequence of mollifiers is any sequence of functions ψ := {ψ} : m R → R+, ↓ 0, such that: n i) supp ψ := {x ∈ R | ψ(x) > 0} ⊆ ρclB, ρ ↓ 0, Z ii) ψ(x)dx = 1, n R where B is the unit ball in Rn and clX means the closure of the set X. Although in the sequel we may consider general families of mollifiers, some ex- amples may be useful: Example 1 Let be a positive number. i) The functions: ( 1 m , max1,...,m |xi| ≤ 2 ψ(x) = 0, otherwise are called Steklov mollifiers. 2 ii) The functions: ( C 2 m exp kxk2−2 , if kxk < ψ(x) = 0, if kxk ≥ R with C ∈ such that m ψ(x)dx = 1, are called standard mollifiers. R R It is easy to check that both the previous families of functions are of class C∞. Definition 2 ([4]) Given a locally integrable function f : Rm → R and a sequence of bounded mollifiers, define the functions f(x) through the convolution: Z f(x) := f(x − z)ψ(z)dz. m R The sequence f(x) is said a sequence of mollified functions. In the following all the functions considered will be assumed to be locally integrable. Remark 1 There is no loss of generality in considering f : Rm → R. The results in this paper remain true also if f is defined on an open subset of Rm. Some properties of the mollified functions can be considered classical: m Theorem 1 ([4]) Let f ∈ C (R ). Then f converges continuosly to f, i.e. f(x) → f(x) for all x → x. In fact f converges uniformly to f on every compact subset of Rm as ↓ 0. The previous convergence property can be generalized. m Definition 3 ([1], [23]) A sequence of functions {fn : R → R} epi–converges to f : Rm → R at x, if: i) lim infn→+∞ fn(xn) ≥ f(x) for all xn → x; ii) limn→+∞ fn(xn) = f(x) for some sequence xn → x. m The sequence {fn} epi–converges to f if this holds for all x ∈ R , in which case we write f = e − lim fn. Remark 2 It can be easily checked that when f is the epi–limit of some sequence fn then f is lower semicontinuous. Moreover if fn converges continuously, then also epi–converges. Definition 4 ([8]) A function f : Rm → R is said strongly lower semicontinuous (s.l.s.c.) at x if it is lower semicontinuous at x and there exists a sequence xn → x with f continuous at xn (for all n) such that f(xn) → f(x). The function f is strongly lower semicontinuous if this holds at all x. The function f is said strongly upper semicontinuous (s.u.s.c.) at x if it is upper semicontinuous at x and there exists a sequence xn → x with f continuous at xn (for all n) such that f(xn) → f(x). The function f is strongly lower semicontinuous if this holds at all x. Proposition 1 If f : Rm → R is s.l.s.c., then −f is s.u.s.c. Proof: It follows directly from the definitions. 2 3 m Theorem 2 ([8]) Let εn ↓ 0. For any s.l.s.c. funcion f : R → R, and any associated sequence fn of mollified functions we have f = e − lim fn . Remark 3 It can be seen that, according to Remark 2, Theorem 1 follows from Theorem 2. m Theorem 3 Let εn ↓ 0. For any s.u.s.c. function f : R → R and any associated m sequence fn of mollified functions, we have for any x ∈ R : i) lim supn→+∞ fn (xn) ≤ f(x) for any sequence xn → x; ii) limn→+∞ fn (xn) = f(x) for some sequence xn → x. Proof: Since f is s.u.s.c., we have −f s.l.s.c. and thus Theorem 2 applies: i) for any sequence xn → x, lim infn→+∞ ( − fn (xn)) ≥ −f(x), which implies: lim sup fn (xn) = − lim inf ( − fn (xn)) ≤ f(x); n→+∞ n→+∞ ii) for some sequence xn → x, limn→+∞ ( − fn (xn)) = −f(x), from which we conclude: lim f (xn) = f(x).