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A relaxed alternating CQ-algorithm for convex feasibility problems. (English) Zbl 1256.49044

Summary: Let H 1 ,H 2 ,H 3 be real Hilbert spaces, let CH 1 , QH 2 be two nonempty closed convex level sets, let A:H 1 H 3 , B:H 2 H 3 be two bounded linear operators. Our interest is in solving the following new convex feasibility problem

FindxC,yQsuchthatAx=By,

which allows asymmetric and partial relations between the variables x and y. In this paper, we present and study the convergence of a relaxed alternating CQ-algorithm (RACQA) and show that the sequences generated by such an algorithm weakly converge to a solution of the above problem. The interest of RACQA is that we just need projections onto half-spaces, thus making the relaxed CQ-algorithm implementable. Note that by taking B=I we recover the split convex feasibility problem originally introduced by Y. Censor and J. Elfving [Numer. Algorithms 8, No. 2–4, 221–239 (1994; Zbl 0828.65065)] and used later in intensity-modulated radiation therapy [Y. Censor et al., “A unified approach for inversion problems in intensity-modulated radiation therapy”, Physics in Medicine and Biology 51, 2353–2365 (2006)]. We also recover the relaxed CQ-algorithm introduced by Q. Yang [Inverse Probl. 20, No. 4, 1261–1266 (2004; Zbl 1066.65047)] by particularizing both B and a given parameter.

MSC:
49M37Methods of nonlinear programming type in calculus of variations
49J53Set-valued and variational analysis
65K10Optimization techniques (numerical methods)
90C25Convex programming