Beyond submodular maximization via one-sided smoothness

In the paper, we propose a deterministic approximation algorithm for maximizing a generalized monotone submodular function subject to a matroid constraint. The function is generalized through a curvature parameter \(c\in [0,1]\), and essentially reduces to a submodular function when \(c=1\). Our algorithm employs the deterministic approximation devised by Buchbinder et al. [3] for the \(c=1\) case of the problem as a building block, and eventually attains an approximation ratio of \(\frac{1+g_c(x)+\Delta \cdot \left[ 3+c-(2+c)x-(1+c)g_c(x)\right] }{2+c+(1+c)(1-x)}\) for the curvature parameter \(c\in [0,1]\) and for a calibrating parameter that is any \(x\in [0,1]\). For \(c=1\), the ratio attains 0.5008 by setting \(x=0.9\), coinciding with the renowned performance guarantee of the problem. Moreover, when the submodular set function degenerates to a linear function, our generalized algorithm always produces optimum solutions and thus achieves an approximation ratio 1.

Machine learning can provide powerful tools for the design and analysis of multi-agent systems, as well as a novel lens on important questions in the area. In this talk I will focus on the use of machine learning for understanding submodular functions, an important class of discrete functions that model laws of diminishing returns and enjoy many important applications in multi-agent settings. For example, submodular functions are commonly used to model valuation functions for bidders in auctions, the influence of various subsets of agents in social networks, and the benefits of performing different actions in a variety of situations. Traditionally it is assumed that these functions are known to the decision maker; however, for large scale systems, in the age of big data, it is often the case they must be learned from observations. In this talk, I will discuss a recent line of work on studying the learnability of submodular functions, and highlight its applications to the analysis of multi-agent systems. I will discuss both general algorithms for learning such functions, as well as even better guarantees that can be achieved for important classes appearing in multi-agent scenarios that exhibit additional structure. These classes include: probabilistic coverage functions that can be used to model the influence function in classic models of information diffusion in networks; functions with bounded complexity used in modeling bidder valuation functions in auctions, including XOS and gross-substitutes; and classes of functions appearing in cooperative game theory for expressing the values of various types of coalitions. I will additionally discuss a large scale application of our algorithms for learning the influence functions in social networks, that significantly outperforms existing approaches empirically in both synthetic and real world data.

In recent years, the issue of maximizing submodular functions has attracted much interest from research communities. However, most submodular functions are specified in a set function. Meanwhile, recent advancements have been studied for maximizing a diminishing return submodular (DR-submodular) function on the integer lattice. Because plenty of publications show that the DR-submodular function has wide applications in optimization problems such as sensor placement impose problems, optimal budget allocation, social network, and especially machine learning. In this research, we propose two main streaming algorithms for the problem of maximizing a monotone DR-submodular function under cardinality constraints. Our two algorithms, which are called StrDRS1 and StrDRS2, have (1/2−ϵ), (1−1/e−ϵ) of approximation ratios and O(nϵlog(logBϵ)logk), O(nϵlogB), respectively. We conducted several experiments to investigate the performance of our algorithms based on the budget allocation problem over the bipartite influence model, an instance of the monotone submodular function maximization problem over the integer lattice. The experimental results indicate that our proposed algorithms not only provide solutions with a high value of the objective function, but also outperform the state-of-the-art algorithms in terms of both the number of queries and the running time.

We study the problem of maximizing a monotone submodular set function subject to linear packing constraints. An instance of this problem consists of a matrix A∈[0,1]m ×n, a vector b∈[1,∞)m, and a monotone submodular set function f: 2[n]→ℝ+. The objective is to find a set S that maximizes f(S) subject to AxS≤b, where xS stands for the characteristic vector of the set S. A well-studied special case of this problem is when f is linear. This special linear case captures the class of packing integer programs. Our main contribution is an efficient combinatorial algorithm that achieves an approximation ratio of Ω(1 / m1/W), where W= min {bi / Aij : Aij>0} is the width of the packing constraints. This result matches the best known performance guarantee for the linear case. One immediate corollary of this result is that the algorithm under consideration achieves constant factor approximation when the number of constraints is constant or when the width of the constraints is sufficiently large. This motivates us to study the large width setting, trying to determine its exact approximability. We develop an algorithm that has an approximation ratio of (1−e)(1−1/e) when W=Ω(ln m / e2). This result essentially matches the theoretical lower bound of 1−1/e. We also study the special setting in which the matrix A is binary and k-column sparse. A k-column sparse matrix has at most k non-zero entries in each of its column. We design a fast combinatorial algorithm that achieves an approximation ratio of Ω(1 / (Wk1/W)), that is, its performance guarantee only depends on the sparsity and width parameters.

A new class of functions, called slightly generalized β-continuous functions is introduced.Basic properties of slightly generalized β-continuous functions are studied.The class of slightly generalized βcontinuous functions properly includes the class of slightly β-continuous functions and generalized β-continuous functions.Also, by using slightly generalized β-continuous functions, some properties of domain/range of functions are characterized.

A new class of closed sets, known as semi-regular weekly closed sets, is introduced in ideal topological space. In addition, we define a new class of continuous functions known as semi-regular weekly -continuous functions is introduced and studied. Some of their characteristics have also been examined and discussed interesting properties of semi-regular weekly -irresolute functions, strongly semi-regular weekly -continuous functions, and semi- regular weekly -continuous functions.

4 - Recursive Enumerability

The theory of evolutionary computation (EC) has experienced rapid and productive growth in recent years. New proof techniques and novel theoretical frameworks have allowed advances in our understanding of the processes and structures inherent in evolutionary optimization. As a result, the frontiers of our knowledge have been expanded further than ever before. Some recent trends in this field, which are covered in this issue, include developments in the understanding of the behavior of evolutionary algorithms (EAs) in dynamic environments rather than just static settings, a theoretical appreciation of the advantages arising from the parallelization of evolutionary algorithms through a greater comprehension of the underlying dynamics, and an understanding of algorithm behavior on broad function classes, including -hard problems.The primary goal of this special issue is to provide extended and polished versions of diverse examples of the best theoretical work presented at conferences in 2014, and to serve as a forum for researchers to advance the theoretical understanding of evolutionary computation methods. The papers included in this special issue span a plurality of topics and offer the reader a cross section of recent outstanding work in EC theory.In dynamic optimization the objective function changes over time, and optimization algorithms face the additional challenge of tracking these changes to be successful. The article “Analysis of Randomised Search Heuristics for Dynamic Optimisation,” by Thomas Jansen and Christine Zarges, presents a novel analytical framework for the analysis of randomized search heuristics on dynamic problems inspired by the fixed-budget computations perspective. The authors introduce a new interesting class of bi-stable dynamic functions where the optimum oscillates between two complementary strings, and apply the framework to analyze and compare the performance of evolutionary algorithms and artificial immune systems on the novel class of functions.Over three decades ago, László Lovász observed that in discrete optimization, submodularity is the counterpart to convexity. However, in contrast to the focus on convex functions in continuous evolutionary optimization, so far submodular functions have received comparatively little attention from EC theoreticians studying discrete functions. The article “Maximizing Submodular Functions under Matroid Constraints by Evolutionary Algorithms,” by Tobias Friedrich and Frank Neumann, addresses this gap by analyzing the performance of evolutionary algorithms on different classes of submodular functions. The maximization of submodular functions is -hard in general, and the authors present several approximation results for monotone submodular and nonmonotone symmetric submodular functions under different kinds of matroid constraints.The idea behind parallel evolutionary algorithms is to evolve multiple subpopulations in parallel and allow interprocess communication at given time intervals. During these migration phases, fractions of each subpopulation can be shared among the subpopulations. There is very little understanding of how the migration frequency affects algorithmic performance, so setting the migration interval parameter appropriately may be difficult. In the article “Design and Analysis of Schemes for Adapting Migration Intervals in Parallel Evolutionary Algorithms,” Andrea Mambrini and Dirk Sudholt propose two schemes to automatically adapt the migration interval of parallel EAs during execution and provide a rigorous analytical framework that yields upper bounds on the expected runtime and expected communication effort of the parallel EAs with different migration topologies for various function classes.In the field of genetic programming, a long-standing open problem is how to address the issue of bloat, the emergence during evolution of solution elements that do not contribute significantly or at all to program fitness or semantics but increase program complexity. The article “On the Performance of Different Genetic Programming Approaches for the SORTING Problem,” by Markus Wagner, Frank Neumann, and Tommaso Urli, tackles the issue of bloat control in the context of sorting. As a basis for their study, they consider program trees and use some measure of sortedness of an in-order traversal to evaluate their fitness. The authors investigate single- and multiobjective variants of genetic programming algorithms with and without bloat control mechanisms, give rigorous upper bounds on their running times, and complement the study with experiments.The topic of constraint handling has recently gained traction in the continuous domain. The article “Markov Chain Analysis of Cumulative Step-Size Adaptation on a Linear Constrained Problem,” by Alexandre Chotard, Anne Auger, and Nikolaus Hansen, presents a rigorous analysis of a (1,)-Evolution Strategy using resampling on a linear function with a linear constraint. The authors prove the previously assumed property that a Markov chain, describing the behavior of the algorithm, exhibits stability in cases with constant step-size and with cumulative step-size adaptation with cumulation parameter equal to 1. This property characterizes the divergence of the algorithm with constant step-size and the geometric divergence or convergence with step-size adaptation, implying fast convergence of Monte Carlo simulations of the divergence rate.In their seminal 2006 paper, Droste, Jansen, and Wegener introduced the concept of black box complexity in order to establish a complexity theory for general-purpose randomized search heuristics. Generally speaking, the black box complexity of a problem is a lower bound on the number of function evaluations needed by any black box algorithm to solve it. Recently, black box models have been refined and developed extensively, allowing the hardness of objective function classes to be more precisely understood. The article “Unbiased Black Box Complexities of Jump Functions,” by Benjamin Doerr, Carola Doerr, and Timo Kötzing, analyzes the unbiased black box complexity of a Jump function class where in each function a local optimum is k bits in distance from the global optimum. The authors provide polynomial upper bounds on the black box complexity of Jump for different sizes of the gap. In particular, they show that an unbiased polynomial-time black box algorithm exists even when almost all of the search space is a plateau of constant fitness.The guest editors would like to thank the authors for their contributions, the referees for their careful reviewing and constructive comments, and the editor-in-chief, Hans-Georg Beyer, for his support in preparing this special issue.

Sensitivity of a general class of shape functionals to topological changes

We consider a simple and well-studied model for procurement problems and solve it to optimality. A buyer with a fixed budget wants to procure, from a set of available workers, a budget feasible subset that maximizes her utility: Any worker has a private reservation price and provides a publicly known utility to the buyer in case of being procured. The buyer’s utility function is additive over items. The goal is designing a direct revelation mechanism that solicits workers’ reservation prices and decides which workers to recruit and how much to pay them. Moreover, the mechanism has to maximize the buyer’s utility without violating her budget constraint. We study this problem in the prior-free setting; our main contribution is finding the optimal mechanism in this setting, under the “small bidders” assumption. This assumption, also known as the “small bid to budget ratio assumption,” states that the bid of each seller is small compared with the buyer’s budget. We also study a more general class of utility functions: submodular utility functions. For this class, we improve the existing mechanisms significantly under our assumption. The online appendix is available at https://doi.org/10.1287/opre.2017.1693 .

We develop a framework for the distributed minimization of submodular functions. Submodular functions are a discrete analog of convex functions and are extensively used in large-scale combinatorial optimization problems. While there has been a significant interest in the distributed formulations of convex optimization problems, distributed minimization of submodular functions has received relatively little research attention. Our framework relies on an equivalent convex reformulation of a submodular minimization problem, which is efficiently computable. We then use this relaxation to exploit methods for the distributed optimization of convex functions. The proposed framework is applicable to submodular set functions as well as to a wider class of submodular functions defined over certain lattices. We also propose an approach for solving distributed motion-planning problems in discrete state space based on submodular function minimization. We establish through a challenging setup of the capture the flag game that submodular functions over lattices can be used to design artificial potential fields for multiagent systems with discrete inputs. These potential fields are designed such that their minima correspond to desired behaviors, that is, agents are attracted toward their goals and are repulsed from obstacles and from each other for collision avoidance. Finally, we demonstrate that the proposed distributed framework can be employed effectively for generating feasible trajectories in such motion coordination problems.

Our starting point are several general classes of real functions defined on the unit square satisfying some basic properties such as a boundary condition or several types of monotonicity and continuity. Applying to these functions some parameterized transformations and other constructions such as the transpose and flipping (which describe different aspects of symmetry) and truncation, we ask for conditions yielding (again) a bivariate copula. Some of these transformations are involutive (on one or more classes of functions), others are not even injective, and occasionally they induce additional properties, yielding, e.g., a (quasi-)copula. For several typical scenarios we identify the (not necessarily convex) sets of parameters leading to a copula and conditions imposing a minimal set of parameters.

We introduced and studied some new classes of functions called (e*, s)-continuous functions, (e, s)-continuous functions and (a, s)-continuous functions. These new notions of functions generalize the class of almost contra-super-continuous functions. Some properties and several characterizations of these types of functions are obtained. We investigate the relationships between these classes of functions and other classes of non-continuous functions. .

Submodularity is one of the most well-studied properties of problem classes in combinatorial optimization and many applications of machine learning and data mining, with strong implications for guaranteed optimization. In this thesis, we investigate the role of submodularity in provable non-convex optimization and validation of algorithms. A profound understanding which classes of functions can be tractably optimized remains a central challenge for non-convex optimization. By advancing the notion of submodularity to domains (termed continuous submodularity), we characterize a class of generally non-convex and non-concave functions -- submodular functions, and derive algorithms for approximately maximizing them with strong approximation guarantees. Meanwhile, submodularity captures a wide spectrum of applications, ranging from revenue maximization with general marketing strategies, MAP inference for DPPs to mean field inference for probabilistic log-submodular models, which renders it as a valuable domain knowledge in optimizing this class of objectives. Validation of algorithms is an information-theoretic framework to investigate the robustness of algorithms to fluctuations in the input/observations and their generalization ability. We investigate various algorithms for one of the paradigmatic unconstrained submodular maximization problem: MaxCut. Due to submodularity of the MaxCut objective, we are able to present efficient approaches to calculate the algorithmic information content of MaxCut algorithms. The results provide insights into the robustness of different algorithmic techniques for MaxCut.

We consider classes of objective functions of cardinality-constrained maximization problems for which the greedy algorithm guarantees a constant approximation. We propose the new class of \(\gamma \)-\(\alpha \)-augmentable functions and prove that it encompasses several important subclasses, such as functions of bounded submodularity ratio, \(\alpha \)-augmentable functions, and weighted rank functions of an independence system of bounded rank quotient – as well as additional objective functions for which the greedy algorithm yields an approximation. For this general class of functions, we show a tight bound of \(\frac{\alpha }{\gamma }\cdot \frac{\textrm{e}^{\alpha }}{\textrm{e}^{\alpha }-1}\) on the approximation ratio of the greedy algorithm that tightly interpolates between bounds from the literature for functions of bounded submodularity ratio and for \(\alpha \)-augmentable functions. In particular, as a by-product, we close a gap in [Math.Prog., 2020] by obtaining a tight lower bound for \(\alpha \)-augmentable functions for all \(\alpha \ge 1\). For weighted rank functions of independence systems, our tight bound becomes \(\frac{\alpha }{\gamma }\), which recovers the known bound of 1/q for independence systems of rank quotient at least q.KeywordsGreedy algorithmCardinality-constrained maximizationApproximation ratioIndependence systemSubmodularity ratioAugmentability