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The aim of this work is to develop an S-iteration technique for finding common fixed points for nonself quasi-nonexpansive mappings in the framework of a uniformly convex Banach space. Convergence properties of the proposed algorithm are analyzed in the setting of uniformly convex Banach spaces. To prove the usability of our results, some novel applications are provided, focused on zeros of accretive operators, convex programming, and feasibility problems. Some numerical experiments with real datasets for Lasso problems are provided. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to introduce a new class of generalized nonexpansive mappings and establish a fixed point existence result in a uniformly convex Banach space. The convergence analysis of Mann iteration and S-iteration process for this class of mappings has been studied in Banach spaces. By some examples, it is shown that S-iteration process is faster than that of Mann and Ishikawa iteration processes. Our result extends and improves various existing results in literature in the context of S-iteration process. [ABSTRACT FROM AUTHOR]

The Mann iterates behave well for nonexpansive mappings for any initial guess in the domain. Our aim in this article is to extend this method to a broad class of inexact fixed point algorithms generated by nearly nonexpansive sequences in Banach spaces and to locate the weak limit of the iterates by its initial guesses. Due to the inexactness, our algorithms become efficiently applicable for a wider class of problems. As applications, we give convergence theorems for finding solutions of variational inclusion problems and constrained multiple-sets split feasibility problems. Our results are significant refinements and improvements of the corresponding results in the literature. [ABSTRACT FROM AUTHOR]

In this paper, we are concerned with the semilocal convergence analysis of a Newton-like method discussed by Bartle (Amer Math Soc 6: 827-831, ) to solve the generalized operator equations containing nondifferentiatble term in Banach spaces. This method has also been studied by Rheinboldt (SIAM J Numer Anal 5: 42-63, ). The aim of the paper is to discuss the convergence analysis under local Lipschitz condition $\|F'_{x}-F'_{x_{0}}\|\le \omega (\|x-x_{0}\|)$ for a given point $x_{0}$. Our results extend and improve the previous ones in the sense of local Lipschitz conditions. We apply our results to solve the Fredholm-type operator equations. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to discuss some fundamental properties of Bregman distance, generalized projection operators, firmly nonexpansive mappings, and resolvent operators of set-valued monotone operators corresponding to a functional Φ(|| · ||). We further study some proximal point algorithms for finding zeros of monotone operators and solving generalized mixed equilibrium problems in Banach spaces. Our results improve and extend some recent results concerning generalized projection operators corresponding to Bregman distance. [ABSTRACT FROM AUTHOR]

Let X be a real reflexive Banach space with a weakly continuous duality mapping Jφ. Let C be a nonemptyweakly closed star-shaped (with respect to u) subset of X. Let F= {T(t) : t ∊ [0, + ∞)} be a uniformly continuous semigroup of asymptotically nonexpansive self-mappings of C, which is uniformly continuous at zero. We will showthat the implicit iteration scheme: yn = αnu+(1-αn)T(tn)yn, for all n ∊ N, converges strongly to a common fixed point of the semigroup F for some suitably chosen parameters {αn} and {tn}. Our results extend and improve corresponding ones of Suzuki (2002), Xu (2005), and Zegeye and Shahzad (2009). [ABSTRACT FROM AUTHOR]

We study in this paper the existence and the approximation of solutions of variational inequalities involving generalized pseudocontractive mappings in Banach spaces. The convergence analysis of a proposed hybrid iterative method for approximating common zeros or fixed points of a possibly infinitely countable or uncountable family of such operators will be conducted within the conceptual framework of the "viscosity approximation technique" in reflexive Banach spaces with uniform Gâteaux differentiable norms. This technique should make existing or new results in solving variational inequalities more applicable. [ABSTRACT FROM AUTHOR]

The hybrid steepest-descent method introduced by Yamada (2001) is an algorithmic solution to the variational inequality problem over the fixed point set of nonlinear mapping and applicable to a broad range of convexly constrained nonlinear inverse problems in real Hilbert spaces. Lehdili and Moudafi (1996) introduced the new prox-Tikhonov regularization method for proximal point algorithm to generate a strongly convergent sequence and established a convergence property for it by using the technique of variational distance in Hilbert spaces. In this paper, motivated by Yamada's hybrid steepest-descent and Lehdili and Moudafi's algorithms, a generalized hybrid steepest-descent algorithm for computing the solutions of the variational inequality problem over the common fixed point set of sequence of nonexpansive-type mappings in the framework of Banach space is proposed. The strong convergence for the proposed algorithm to the solution is guaranteed under some assumptions. Our strong convergence theorems extend and improve certain corresponding results in the recent literature. [ABSTRACT FROM AUTHOR]

In this paper, we deal with the strong convergence of the iteration scheme given by Akutsah et al., for mean nonexpansive mappings in uniformly convex Banach space endowed with directed graph. Also, we show fastness of Akutsah et al. iteration scheme with some well-known iteration schemes with the help of numerical examples. We, also present an application of fixed point theory in solution of integral equation. [ABSTRACT FROM AUTHOR]

The purpose of this study is to provide a generalized CR−iteration algorithm for finding common fixed points (CF Ps) for nonself quasi-nonexpansive mappings (QNEMs) in a uniformly convex Banach space. The suggested algorithm’s convergence analysis is analyzed in uniformly convex Banach spaces. [ABSTRACT FROM AUTHOR]

In this paper, we design a new third order Newton-like method and establish its convergence theory for finding the approximate solutions of nonlinear operator equations in the setting of Banach spaces. First, we discuss the convergence analysis of our third order Newton-like method under the ω -continuity condition. Then we apply our approach to solve nonlinear fixed point problems and Fredholm integral equations, where the first derivative of an involved operator does not necessarily satisfy the Hölder and Lipschitz continuity conditions. Several numerical examples are given, which compare the applicability of our convergence theory with the ones in the literature. [ABSTRACT FROM AUTHOR]

In this paper we introduce a new projection-type algorithm in a reflexive Banach space. Then, using generalized resolvents operators and generalized projections, we prove a strong convergence theorem for computing a common element of the set of fixed points of a Bregman relatively nonexpansive mapping, solutions of an equilibrium problem, fixed points of a resolvent operator and fixed points of an infinite family of Bregman W-mappings. [ABSTRACT FROM AUTHOR]

We consider the class of generalized α-nonexpansive mappings in a setting of Banach spaces. We prove existence of fixed point and convergence results for these mappings under the K∗-iterative process. The weak convergence is obtained with the help of Opial's property while strong convergence results are obtained under various assumptions. Finally, we construct two numerical examples and connect our K∗-iterative process with them. An application to solve a fractional differential equation (FDE) is also provided. It has been eventually shown that the K∗- iterative process of this example gives more accurate numerical results corresponding to some other iterative processes of the literature. The main outcome is new and improves some known results of the literature. [ABSTRACT FROM AUTHOR]

In this manuscript, using (H-η-accretive mappings we study the existence of common solution of a system of generalized mixed variational-like inclusion problems and the set of fixed point problems in q-uniformly smooth Banach spaces. The method used in this paper can be considered as an extension of methods for studying the existence of common solution for various classes of variational inclusions considered and studied by many authors in q-uniformly smooth Banach spaces. [ABSTRACT FROM AUTHOR]

The purpose of this study is to introduce an inertial algorithm for approximating a solution of the split equality Hammerstein type equation problem in general reflexive real Banach spaces. Strong convergence results are established under the assumption that the associated mappings are monotone and uniformly continuous. The results in this paper generalize and improve many of the existing results in the literature in the sense that the underlying mappings are relaxed from Lipschitz continuous to uniformly continuous and the spaces under consideration are extended from Hilbert spaces to reflexive real Banach spaces with a more general problem which includes the Hammerstein type equation problems. [ABSTRACT FROM AUTHOR]

In this paper, we introduce a new extragradient algorithm by using generalized metric projection. We prove a strong convergence theorem for finding a common element of the solution set of split feasibility problem and the set of fixed points of relatively nonexpansive mapping and a finite family of resolvent operator and the set of solutions of an equilibrium problem. [ABSTRACT FROM AUTHOR]

In this paper, we introduce a new scheme and prove convergence results for nonexpansive mappings as well as for weak contractions in the frame of Banach spaces. Moreover, we prove analytically and numerically that the proposed scheme converges to a fixed point of a weak contraction faster than some known and leading schemes. Further, we prove that the new scheme is almost stable with respect to weak contraction. For supporting the main results, we give a couple of nontrivial numerical examples, and the visualization is shown by using the Matlab program. Finally, the solution of a nonlinear fractional differential equation is approximated by operating the main result of the paper. [ABSTRACT FROM AUTHOR]

We give some extensions of the beautiful 1968 fixed point theorem of Maia [Maia, M. G. Un'osservazione sulle contrazioni metriche. (Italian) Rend. Sem. Mat. Univ. Padova 40 (1968), 139-143] to three classes of enriched contractive mappings in Banach spaces: enriched contractions, Kannan enriched contractions and Ć irić-Reich-Rus contractions. [ABSTRACT FROM AUTHOR]

For q > 1, let E be a q-uniformly smooth real Banach space with dual space E Let A : E → E be a Lipschitz and strongly monotone mapping such that A-1(0) = For given x1 E, let {xn} be generated iteratively by the algorithm ... where J is the normalized duality mapping from E into E and λ is a positive real number choosen in a suitable interval. Then it is proved that the sequence {xn} converges strongly to x, the unique point of A-1(0). Our theorems are applied to the convex minimization problem. Futhermore, our technique of proof is of independent interest. [ABSTRACT FROM AUTHOR]

New fixed point results are presented for multivalued maps defined on subsets of a Fréchet space E. The proof relies on the notion of a pseudo open set, degree and index theory, and on viewing E as the projective limit of a sequence of Banach spaces. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to introduce and analyze two inertial algorithms with self-adaptive stepsizes for solving variational inequalities in reflexive Banach spaces. Our algorithms are based on inertial hybrid and shrinking projection methods. Knowledge of the Lipschitz constant of the cost operator is not required. Under appropriate conditions, the strong convergence of the algorithms is established. We also present several numerical experiments which bring out the efficiency and the advantages of the proposed algorithms. Our work provides extensions of many known results from Hilbert spaces to reflexive Banach spaces. [ABSTRACT FROM AUTHOR]

Let A and B be two nonempty subsets of a normed linear space X. A mapping T : A ∪ B → A ∪ B is said to be noncyclic if T (A) ⊆ A and T (B) ⊆ B. In the current paper, we consider the problem of finding the best proximity pair for the noncyclic mapping T, that is, two fixed points of T which achieve the minimum distance between the sets A and B. We do it from some different approaches. The common condition on these results is relatively nonexpansivity of the mapping T. At the first conclusion, we obtain the existence of best proximity pairs in the setting of uniformly convex in every direction Banach spaces where the pair (A, B) is nonconvex. Then we conclude a similar result by replacing the geometric property of Opial's property of the Banach space and adding another assumption on the mapping T, called condition (C). We also show that the same result is true when X is a 2-uniformly convex Banach space. In the setting of k-uniformly convex Banach spaces, we prove that every nonempty, and convex pair of subsets has a geometric notion of proximal normal structure and then, we deduce the existence of best proximity pairs for relatively nonexpansive mappings in such spaces. [ABSTRACT FROM AUTHOR]

In this work, we introduce and study a new class of weak enriched nonexpasive mappings which is a generalization of enriched nonexpansive mappings provided by Berinde [Approximating fixed points of enriched nonexpansive mappings by Krasnoselskij iteration in Hilbert spaces, Carpathian J. Math., 35 (2019), No. 3, 293-304]. This class of mappings generalizes several important classes of nonlinear mappings. We prove some fixed point theorems regarding this kind of mappings which extend some important results in [Berinde, V., Approximating fixed points of enriched nonexpansive mappings by Krasnoselskij iteration in Hilbert spaces, Carpathian J. Math., 35 (2019), No. 3, 293-304]. Moreover, some examples, to ensure the existence of these mappings and support our main theorems, are also given. [ABSTRACT FROM AUTHOR]

Themain aimof this paper is to establish some fixed point theorems for enriched Ćiriū-Reich-Rus contractions in Banach spaces and convex metric spaces, respectively. [ABSTRACT FROM AUTHOR]

This article aims to introduce and analyze the viscosity method for hierarchical variational inequalities involving a ϕ-contraction mapping defined over a common solution set of variational inclusion and fixed points of a nonexpansive mapping on Hadamard manifolds. Several consequences of the composed method and its convergence theorem are presented. The convergence results of this article generalize and extend some existing results from Hilbert/Banach spaces and from Hadamard manifolds. We also present an application to a nonsmooth optimization problem. Finally, we clarify the convergence analysis of the proposed method by some computational numerical experiments in Hadamard manifold. [ABSTRACT FROM AUTHOR]

Let K be a nonempty subset of a Banach space E. A mapping T : K → K is said to satisfy (RCSC) condition if each a , b ∈ K , 1 / 2 a − F a ≤ a − b ⇒ F a − F b ≤ 1 / 3 a − b + a − F b + b − F a . In this paper, we study, under some appropriate conditions, weak and strong convergence for this class of maps through M iterates in uniformly convex Banach space. We also present a new example of mappings with condition (RCSC). We connect M iteration and other well-known processes with this example to show the numerical efficiency of our results. The presented results improve and extend the corresponding results of the literature. [ABSTRACT FROM AUTHOR]

In this article, we prove some weak and strong convergence theorems for mappings satisfying condition (E) using the A K iterative scheme in the setting of Banach spaces. We offer a new example of mapping with condition (E) in support of our main result. Our results extend and improve many well-known corresponding results of the current literature. [ABSTRACT FROM AUTHOR]

In the present article, we establish weak and strong convergence theorems of a new modified three-step iteration method for three G-nonexpansive mappings in a uniformly convex Banach space with a directed graph. Moreover, weak convergence theorem without making use of the Opial's condition is proved. We also show the numerical experiment for supporting our main results and comparing rate of convergence of the new modified three-step iteration with the three-step Noor iteration and the SP iteration. We also provide some numerical examples to illustrate the convergence behavior and advantages of the proposed method. Furthermore, we apply our results to find solutions of constrained minimization problems and split feasibility problems. [ABSTRACT FROM AUTHOR]

In this paper, we extend the study of an iteration process introduced by Thakur-Thakur-Postolache [J. Inequal. Appl., (2014), 147–155] from the class of nonexpansive operators to the more general class of Suzuki operators. We prove weak and strong convergence theorems regarding the iteration method for mappings satisfying the condition (C) of Suzuki in uniformly convex Banach spaces. We study the stability of this iteration process, and introduce a data dependency result for the subclass of contractive operators. An example illustrates the theoretical outcome. [ABSTRACT FROM AUTHOR]

In this paper, we propose the generalized M-iteration process for approximating the fixed points from Banach spaces to hyperbolic spaces. Using our new iteration process, we prove Δ -convergence and strong convergence theorems for the class of mappings satisfying the condition C λ and the condition E which is the generalization of Suzuki generalized nonexpansive mappings in the setting of hyperbolic spaces. Moreover, a numerical example is given to present the capability of our iteration process and the solution of the integral equation is also illustrated using our main result. [ABSTRACT FROM AUTHOR]

In this article, we introduce a novel iterative algorithm to approximate fixed point of mappings with B γ , μ condition. We establish some strong and weak convergence results in a uniformly convex Banach space. Using a numerical example, we compare the speed of the proposed algorithm with some leading iterative algorithms. [ABSTRACT FROM AUTHOR]

We prove Browder's convergence theorem for multivalued mappings in a uniformly convex Banach space with a uniformly Gâteaux differentiable norm by using the notion of diametrically regular mappings. Our results are significant improvement on results of Jung (2007) and Panyanak and Suantai (2020). [ABSTRACT FROM AUTHOR]

For finding a zero of an m-accretive operator in a real Banach space, we consider an implicit hybrid viscosity approximation method and show that its explicit variant converges strongly to a zero under weaker assumptions than the ones used recently by Ceng et al. (Numer. Func. Anal. Opt. 35(2), 142–165, 2012). First, we examine the case of spaces which are uniformly smooth or reflexive and strictly convex with a uniformly Gâteaux differentiable norm. Afterwards, we improve our convergence results when the space is uniformly convex. Furthermore, we show that the strong convergence of some modified Halpern and Krasnosel'skii–Mann type methods can also be deduced from our results. Finally, a numerical example is also given to illustrate the convergence analysis of the considered method. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to introduce the modified Agarwal-O'Regan-Sahu iteration process (S-iteration) for finding endpoints of multivalued nonexpansive mappings in the setting of Banach spaces. Under suitable conditions, some weak and strong convergence results of the iterative sequence generated by the proposed process are proved. Our results especially improve and unify some recent results of Panyanak (J. Fixed Point Theory Appl. (2018)). At the end of the paper, we offer an example to illustrate the main results. [ABSTRACT FROM AUTHOR]

In this article, we introduce a new class of contractive mappings and study analytical and computational aspects of a special case of Jungck-Khan iterative algorithm generated by this class of mappings. In particular, we improve upon strong convergence, rate of convergence and data dependence results existing in the current literature. Analytical as well numerical illustrative examples are given to support the new results. [ABSTRACT FROM AUTHOR]

In this study, it is proposed a new general fixed point iteration method for approximation of fixed point of contraction mappings in Banach spaces. Next, it has been proven that this iteration method is faster than the AK, S-Picard, Thakur, Vatan twosteps fixed point iterative methods recently described in the literature. Moreover, it is shown that the convergence of Mann fixed point iteration method is equivalent to the convergence of newly defined fixed point iteration method. Also, numerical examples are given to support the analytic proofs. Finally, the data dependence of this fixed point iteration method has been proven. [ABSTRACT FROM AUTHOR]

In this paper, we introduce a new discontinuous operator and investigate the existence and uniqueness of fixed points for the operators in complete metric spaces. We also provide rate of convergence and data dependency of S-iterative scheme for a fixed point of the discontinuous operators in Banach spaces. Moreover, we prove the estimation Collage theorems and compare error estimate between data dependency and Collage theorems. Numerical examples are provided to support our results. [ABSTRACT FROM AUTHOR]

In this paper, we study the feedback stabilisation of homogeneous bilinear systems on a Banach state-space. Sufficient conditions for weak and strong stabilisation are given. A decay rate of the stabilised state is explicitly estimated. Applications to heat and transport equations are provided. [ABSTRACT FROM AUTHOR]

In this paper, we introduce a new iterative scheme to approximate fixed point of generalized α-nonexpansive mappings and then, we prove that the proposed iteration process is faster than all of Picard, Mann, Ishikawa, Noor, Agarwal, Abbas, and Thakur processes for contractive mappings. We also obtain some weak and strong convergence theorems for generalized α-nonexpansive mappings. At the end, by using an example for generalized α-nonexpansive mappings, we compare the convergence behavior of new iterative process with other iterative processes. [ABSTRACT FROM AUTHOR]

In this paper, in order to solve the split common null point problem, we investigate a new explicit iteration method, base on the shrinking projection method and ε-enlargement of a maximal monotone operator. We also give some applications of our main results for the problem of split minimum point, multiple-sets split feasibility, and split variational inequality. Two numerical examples also are given to illustrate the effectiveness of the proposed algorithm. [ABSTRACT FROM AUTHOR]

Let C be a closed, convex and nonempty subset of a q-uniformly smooth Banach space X. Let be a strict pseducontractive inward mapping. We consider the boundary point map depending on C and T defined by , for all. Then for a suitable step-by-step construction of the control coefficients by using the function , we show the convergence of the Mann-Dotson algorithm to a fixed point of T. We obtain strong convergence if and weak convergence if. [ABSTRACT FROM AUTHOR]

In this paper, we introduce a new accelerated iteration for finding a fixed point of monotone generalized α-nonexpansive mapping in an ordered Banach space. We establish some weak and strong convergence theorems of fixed point for monotone generalized α-nonexpansive mapping in a uniformly convex Banach space with a partial order. Further, we provide a numerical example to illustrate the convergence behavior and effectiveness of the proposed iteration process. [ABSTRACT FROM AUTHOR]

Throughout this work, we introduce some results of convergence, summable almost stability for s-iteration and Picard-Manniterationin Banach space with respect to an integral condition. Also, we study the equivalence between stability of these types of schemes. [ABSTRACT FROM AUTHOR]

In this article, we introduce a faster iteration for finding a fixed point of G-monotone nonexpansive mapping in a uniformly convex Banach space with a directed graph. We establish weak and strong convergence theorems of fixed point for G-monotone nonexpansive mapping with a more convenient G-convex interval instead of the previous fixed point dominated conditions in convergence analysis. Moreover, we provide two numerical examples to illustrate the convergence behavior and advantages of the proposed method. [ABSTRACT FROM AUTHOR]

In this paper, using sunny generalized nonexpansive retractions which are different from the metric projection and generalized metric projection in Banach spaces, we present new extragradient and line search algorithms for finding the solution of a J-variational inequality whose constraint set is the common elements of the set of fixed points of a family of generalized nonexpansive mappings and the set of solutions of a pseudomonotone J-equilibrium problem for a J -α-inverse-strongly monotone operator in a Banach space. To prove strong convergence of generated iterates in the extragradient method, we introduce a ϕ∗-Lipschitz-type condition and assume that the equilibrium bifunction satisfies this condition. This condition is unnecessary when the line search method is used instead of the extragradient method. Using FMINCON optimization toolbox in MATLAB, we give some numerical examples and compare them with several existence results in literature to illustrate the usability of our results. [ABSTRACT FROM AUTHOR]

In this work, we introduce implicit and explicit iterations for solving the variational inclusion problem for the sum of two operators and the fixed point problem of nonexpansive mappings. We then prove its strong convergence theorems in the framework of Banach spaces. We finally provide some applications of the main results. [ABSTRACT FROM AUTHOR]

In this paper, we introduce a three step iteration scheme and establish that this iterative method can be used to approximate fixed point of weak contraction mappings in the frame work of Banach spaces. We also establish that our newly proposed iterative scheme is faster than some existing iterative processes in literature and stability of this iterative scheme is established. Furthermore, we prove that this iterative method is equivalent to M iterative scheme introduced by Ullah et al. in [19], the iterative scheme introduced by Karakaya et al. in [11] and the Mann iterative iteration process. Finally, we established that the rate of convergence of our newly proposed iterative scheme is the same as that of M iteration scheme introduced by Ullah et al. in [19], the iterative scheme introduced by Karakaya et al. in [11] and we present an analytic proof and also a numerical example to support our claim. [ABSTRACT FROM AUTHOR]

In a real uniformly convex and uniformly smooth Banach space, some new monotone projection iterative algorithms for countable maximal monotone mappings and countable weakly relatively non-expansive mappings are presented. Under mild assumptions, some strong convergence theorems are obtained. Compared to corresponding previous work, a new projection set involves projection instead of generalized projection, which needs calculating a Lyapunov functional. This may reduce the computational labor theoretically. Meanwhile, a new technique for finding the limit of the iterative sequence is employed by examining the relationship between the monotone projection sets and their projections. To check the effectiveness of the new iterative algorithms, a specific iterative formula for a special example is proved and its computational experiment is conducted by codes of Visual Basic Six. Finally, the application of the new algorithms to a minimization problem is exemplified. [ABSTRACT FROM AUTHOR]

Let K be a nonempty convex subset of a real Banach space X. Let T be a nearly weak uniformly L-Lipschitzian mapping. A modified Mann-type iteration scheme is proved to converge strongly to the unique fixed point of T. Our result is a significant improvement and generalization of several known results in this area of research. We give a specific example to support our result. Furthermore, an interesting equivalence of T-stability result between the convergence of modified Mann-type and modified Mann iterations is included. [ABSTRACT FROM AUTHOR]

This paper studies the problem of approximating a function f in a Banach space $$\mathcal{X}$$ from measurements $$l_j(f)$$ , $$j=1,\ldots ,m$$ , where the $$l_j$$ are linear functionals from $$\mathcal{X}^*$$ . Quantitative results for such recovery problems require additional information about the sought after function f. These additional assumptions take the form of assuming that f is in a certain model class $$K\subset \mathcal{X}$$ . Since there are generally infinitely many functions in K which share these same measurements, the best approximation is the center of the smallest ball B, called the Chebyshev ball, which contains the set $$\bar{K}$$ of all f in K with these measurements. Therefore, the problem is reduced to analytically or numerically approximating this Chebyshev ball. Most results study this problem for classical Banach spaces $$\mathcal{X}$$ such as the $$L_p$$ spaces, $$1\le p\le \infty $$ , and for K the unit ball of a smoothness space in $$\mathcal{X}$$ . Our interest in this paper is in the model classes $$K=\mathcal{K}(\varepsilon ,V)$$ , with $$\varepsilon >0$$ and V a finite dimensional subspace of $$\mathcal{X}$$ , which consists of all $$f\in \mathcal{X}$$ such that $$\mathrm{dist}(f,V)_\mathcal{X}\le \varepsilon $$ . These model classes, called approximation sets, arise naturally in application domains such as parametric partial differential equations, uncertainty quantification, and signal processing. A general theory for the recovery of approximation sets in a Banach space is given. This theory includes tight a priori bounds on optimal performance and algorithms for finding near optimal approximations. It builds on the initial analysis given in Maday et al. (Int J Numer Method Eng 102:933-965, 2015) for the case when $$\mathcal{X}$$ is a Hilbert space, and further studied in Binev et al. (SIAM UQ, 2015). It is shown how the recovery problem for approximation sets is connected with well-studied concepts in Banach space theory such as liftings and the angle between spaces. Examples are given that show how this theory can be used to recover several recent results on sampling and data assimilation. [ABSTRACT FROM AUTHOR]