On the range of accretive operators

The solvability of the nonlinear operator equationw=x+Bx, whereB is accretive in a general Banach spaceX is studied by means of discrete approximations. In particular, ifB is continuous and everywhere defined an algorithm is given for solving the equation. Sponsored by the United States Army under Contract No. DAAG29-75-C-0024. Supported in part by NSF grant MCS 76-10227     

Maximality of sums of two maximal monotone operators

We use methods from convex analysis, relying on an ingenious function of Simon Fitzpatrick, to prove maximality of the sum of two maximal monotone operators on reflexive Banach space under weak transversality conditions.

     

Resolvent splitting for sums of monotone operators with minimal lifting

Mathematical Programming - In this work, we study fixed point algorithms for finding a zero in the sum of $$nge 2$$ maximally monotone operators by using their resolvents. More precisely, we...     

Natural closures, natural compositions and natural sums of monotone operators

We introduce new methods for defining generalized sums of monotone operators and generalized compositions of monotone operators with linear maps. Under asymptotic conditions we show these operations coincide with the usual ones. When the monotone operators are subdifferentials of convex functions, a similar conclusion holds. We compare these generalized operations with previous constructions by Attouch–Baillon–Théra, Revalski–Théra and Pennanen–Revalski–Théra. The constructions we present are motivated by fuzzy calculus rules in nonsmooth analysis. We also introduce a convergence and a closure operation for operators which may be of independent interest.     

Commutators and accretive operators

In this paper, singular values of commutators of Hilbert space operators are estimated. To this aim the accretivity of a transform of the operators is applied. Some recent results of Kittaneh [F. Kittaneh, Singular value inequalities for commutators of Hilbert space operators, Linear Algebra Appl. 430 (2009) 2362-2367] are extended.     

On maximal monotone operators with relatively compact range

It is shown that every maximal monotone operator on a real Banach space with relatively compact range is of type NI. Moreover, if the space has a separable dual space then every maximally monotone operator T can be approximated by a sequence of maximal monotone operators of type NI, which converge to T in a reasonable sense (in the sense of Kuratowski-Painleve convergence).     

Maximality of sums of two maximal monotone operators in general Banach space

We combine methods from convex analysis, based on a function of Simon Fitzpatrick, with a fine recent idea due to Voisei, to prove maximality of the sum of two maximal monotone operators in Banach space under various natural domain and transversality conditions.

     

Zeros of accretive operators

In the investigation of accretive operators in Banach spaces X, the existence of zeros plays an important role, since it yields surjectivity results as well as fixed point theorems for operators S such that I-S is accretive. Let D?X and T: D→X an operator such that the initial value problems (1) u′(t)=-Tu(t), u(0)=x εD are solvable. Then T has a zero iff (1) has a constant solution for some xεD. Under certain assumptions on D and T it is possible to show that (1) has a unique solution u(t,x) on [0,∞), for every xεD. In this case, define U(t): D→D by U(t)x=u(t,x). If T is accretive it turns out that U(t) is nonexpansive for every t≥0. This fact constitutes the basis for several authors concerned with this subject. They proceed with assumptions on D and X ensuring either that the U(t) must have a common fixed point x_o or that U(_p) has a fixed point x_p for every p≥0. In the first case, U(t)x_o is a constant solution of (1), whence Tx_o=0. In the second case, U(t)x_p is a p-periodic solution of (1). Hence, one has to impose additional conditions on T which imply that a p-periodic solution must be constant, for some p>0. The main purpose of the present paper is to show that, in certain situations, either the operators U(t) are actually strict contractions or T may be approximated by operators T_n such that the corresponding U_n(t) are strict contractions. Thus, we obtain several results in general Banach spaces and a unification of some results in special spaces.     

Constructing zeros of accretive operators

We prove a convergence theorem for an implicit iterative scheme and then apply it to an explicit one.     

Constructing zeros of accretive operators II

In this paper we study a homogenization problem for a time periodic boundary value problem concerning the quasi-stationary Maxwell equations with a non linear monotone magne tic characteristic. The main features of the problem are related to the vanishing of the conductivity inside each period so that the type of the equations is rapidly oscillating. The unknowns are a vector potential and a scalar potential. We show that the first one converges to zero up to terms of second order, while the second one converges to the solution of a suitable homogenized stationary equation (with time as a parameter). We show moreover that when the magnetic characteristic is linear and symmetric the second order terms in the asymptotic expansion of the vector potential can be identified and related to the time derivative of the limit scalar potential.     

A note on the existence of zeroes of convexly regularized sums of maximal monotone operators

Many algorithms for solving the problem of finding zeroes of a sum of two maximal monotone operators T₁ and T₂, have regularized subproblems of the kind 0T₁(x)+T₂(x)+∂D(x), where D is a convex function. We develop an unified analysis for existence of solutions of these subproblems, through the introduction of the concept of convex regularization, which includes several well-known cases in the literature. Finally, we establish conditions, either on D or on the operators, which assure solvability of the subproblems.     

Pictures of monotone operators

Let E be a real Banach space with dual E ^*. We associate with any nonempty subset H of E×E ^* a certain compact convex subset of the first quadrant in ², which we call the picture of H, (H). In general, (H) may be empty, but (M) is nonempty if M is a nonempty monotone subset of E×E ^*. If E is reflexive and M is maximal monotone then (M) is a single point on the diagonal of the first quadrant of ². On the other hand, we give an example (for E the nonreflexive space L ₁[0,1]) of a maximal monotone subset M of E×E ^* such that (0,1)(M) and (1,1)(M) but (1,0)(M). We show that the results for reflexive spaces can be recovered for general Banach spaces by using monotone operator of type (NI) — a class of multifunctions from E into E ^* which includes the subdifferentials of all proper, convex, lower semicontinuous functions on E, all surjective operators and, if E is reflexive, all maximal monotone operators. Our results lead to a simple proof of Rockafellar's result that if E is reflexive and S is maximal monotone on E then S+J is surjective. Our main tool is a classical minimax theorem.     

Pairs of monotone operators

This note is an addendum to Sum theorems for monotone operators and convex functions. In it, we prove some new results on convex functions and monotone operators, and use them to show that several of the constraint qualifications considered in the preceding paper are, in fact, equivalent.

     

Viscosity methods for zeroes of accretive operators

This paper deals with the general iteration method , for calculating a particular zero of A, an m-accretive operator in a Banach space X, T_n being a sequence of nonexpansive self-mappings in X. Under suitable conditions on the parameters and X, we state strong and weak convergence results of (x_n). We also show how to compute a common zero of two m-accretive operators in X.