We focus on the convergence analysis of averaged relaxations of cutters, specifically for variants that-depending upon how parameters are chosen-resemble alternating projections, the Douglas-Rachford method, relaxed reflect-reflect, or the Peaceman-Rachford method. Such methods are frequently used to solve convex feasibility problems. The standard convergence analyses of projection algorithms are based on the firm nonexpansivity property of the relevant operators. However if the projections onto the constraint sets are replaced by cutters (projections onto separating hyperplanes), the firm nonexpansivity is lost. We provide a proof of convergence for a family of related averaged relaxed cutter methods under reasonable assumptions, relying on a simple geometric argument. This allows us to clarify fine details related to the allowable choice of the relaxation parameters, highlighting the distinction between the exact (firmly nonexpansive) and approximate (strongly quasi-nonexpansive) settings. We provide illustrative examples and discuss practical implementations of the method.
IntroductionProjection and reflection methods are used for solving the feasibility problem of finding a point in the intersection of a finite collection of closed, convex sets in a Hilbert space. Such problems have a wide range of application in variational analysis, optimisation, physics and mathematics in general. One of the most successful methods from this class is the Douglas-Rachford method that uses a combination of reflections and averaging on each iteration. The idea first appeared in [33] as a numerical scheme for solving differential equations, and the convergence of a more general scheme for finding a zero of the sum of two maximally monotone operators was framed in [43] (also see [12, Chapter 26] for a modern treatment). Aragón Artacho and Campoy have recently introduced a modification of the Douglas-Rachford method for finding closest feasible points [7].Convergence rates for such methods are the subject of extensive research; we provide a brief sampling. Under appropriate conditions, the Douglas-Rachford method converges in finitely many steps [12]. Convergence rates may frequently be obtained through analysis of regularity conditions [38]. Additionally, semialgebraic structure admits further bounds on convergence rates for projection methods more generally [20,21,34] and for the Douglas-Rachford method in particular [40]. For a recent survey on the Douglas-Rachford method, see [41].The idea of replacing projections with their approximations, and specifically with the approximations constructed from the subdifferentials of the convex functions that describe the sets, was introduced by Fukushima [36]. It has been used in various contexts recently, including the numerical solution of variational arXiv:1810.02463v1 [math.OC] 4 Oct 2018inequalities; see, for example, [16,17]. In particular, Combettes has used relaxation parameters together with subgradient projections in the construction of his extrapolation method of parallel sub...