Covering problems

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In combinatorics and computer science, covering problems are computational problems that ask whether a certain combinatorial structure 'covers' another, or how large the structure has to be to do that. Covering problems are minimization problems and usually integer linear programs, whose dual problems are called packing problems.

The most prominent examples of covering problems are the set cover problem, which is equivalent to the hitting set problem, and its special cases, the vertex cover problem and the edge cover problem.

Covering problems allow the covering primitives to overlap; the process of covering something with non-overlapping primitives is called decomposition.

General linear programming formulation

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In the context of linear programming, one can think of any minimization linear program as a covering problem if the coefficients in the constraint matrix, the objective function, and right-hand side are nonnegative.[1] More precisely, consider the following general integer linear program:

minimize  
subject to  
 .

Such an integer linear program is called a covering problem if   for all   and  .

Intuition: Assume having   types of object and each object of type   has an associated cost of  . The number   indicates how many objects of type   we buy. If the constraints   are satisfied, it is said that   is a covering (the structures that are covered depend on the combinatorial context). Finally, an optimal solution to the above integer linear program is a covering of minimal cost.

Kinds of covering problems

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There are various kinds of covering problems in graph theory, computational geometry and more; see Category:Covering problems. Other stochastic related versions of the problem can be found.[2]

Covering in Petri nets

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For Petri nets, the covering problem is defined as the question if for a given marking, there exists a run of the net, such that some larger (or equal) marking can be reached. Larger means here that all components are at least as large as the ones of the given marking and at least one is properly larger.

Rainbow covering

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In some covering problems, the covering should satisfy some additional requirements. In particular, in the rainbow covering problem, each of the original objects has a "color", and it is required that the covering contains exactly one (or at most one) object of each color. Rainbow covering was studied e.g. for covering points by intervals:[3]

  • There is a set J of n colored intervals on the real line, and a set P of points on the real line.
  • A subset Q of J is called a rainbow set if it contains at most a single interval of each color.
  • A set of intervals J is called a covering of P if each point in P is contained in at least one interval of Q.
  • The Rainbow covering problem is the problem of finding a rainbow set Q that is a covering of P.

The problem is NP-hard (by reduction from linear SAT).

Conflict-free covering

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A more general notion is conflict-free covering.[4] In this problem:

  • There is a set O of m objects, and a conflict-graph GO on O.
  • A subset Q of O is called conflict-free if it is an independent set in GO, that is, no two objects in Q are connected by an edge in GO.
  • A rainbow set is a conflict-free set in the special case in which GO is made of disjoint cliques, where each clique represents a color.

Conflict-free set cover is the problem of finding a conflict-free subset of O that is a covering of P. Banik, Panolan, Raman, Sahlot and Saurabh[5] prove the following for the special case in which the conflict-graph has bounded arboricity:

  • If the geometric cover problem is fixed-parameter tractable (FPT), then the conflict-free geometric cover problem is FPT.
  • If the geometric cover problem admits an r-approximation algorithm, then the conflict-free geometric cover problem admits a similar approximation algorithm in FPT time.

References

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  1. ^ Vazirani, Vijay V. (2001). Approximation Algorithms. Springer-Verlag. ISBN 3-540-65367-8.: 112 
  2. ^ Douek-Pinkovich, Y., Ben-Gal, I., & Raviv, T. (2022). "The Stochastic Test Collection Problem: Models, Exact and Heuristic Solution Approaches" (PDF). European Journal of Operational Research, 299 (2022), 945–959}.{{cite web}}: CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link)
  3. ^ Arkin, Esther M.; Banik, Aritra; Carmi, Paz; Citovsky, Gui; Katz, Matthew J.; Mitchell, Joseph S. B.; Simakov, Marina (2018-12-11). "Selecting and covering colored points". Discrete Applied Mathematics. 250: 75–86. doi:10.1016/j.dam.2018.05.011. ISSN 0166-218X.
  4. ^ Banik, Aritra; Sahlot, Vibha; Saurabh, Saket (2020-08-01). "Approximation algorithms for geometric conflict free covering problems". Computational Geometry. 89: 101591. doi:10.1016/j.comgeo.2019.101591. ISSN 0925-7721. S2CID 209959954.
  5. ^ Banik, Aritra; Panolan, Fahad; Raman, Venkatesh; Sahlot, Vibha; Saurabh, Saket (2020-01-01). "Parameterized Complexity of Geometric Covering Problems Having Conflicts". Algorithmica. 82 (1): 1–19. doi:10.1007/s00453-019-00600-w. ISSN 1432-0541. S2CID 254027914.