# Set splitting problem

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In computational complexity theory, the Set Splitting problem is the following decision problem: given a family F of subsets of a finite set S, decide whether there exists a partition of S into two subsets S1, S2 such that all elements of F are split by this partition, i.e., none of the elements of F is completely in S1 or S2. Set Splitting is one of Garey&Johnson's classical NP-complete problems.[1]

## Variants

The optimization version of this problem is called Max Set Splitting and requires finding the partition which maximizes the number of split elements of F. It is an APX-complete[2] problem and hence in NPO.

The Set k-Splitting problem is stated as follows: given S, F, and an integer k, does there exist a partition of S which splits at least k subsets of F? The original formulation is the restricted case with k equal to the cardinality of F. The Set k-Splitting is fixed-parameter tractable, i.e., if k taken to be a fixed parameter, rather than a part of the input, then a polynomial algorithm exists for any fixed k.Dehne, Fellows and Rosamond presented an algorithm that solves it in time ${\displaystyle O(f(k)n^{c})}$ for some function f and constant c.[3]

When each element of F is restricted to be of cardinality exactly k, the decision variant is called Ek-Set Splitting and the optimization version Max Ek-Set Splitting. For k > 2 the former remains NP complete, and for k ≥ 2 the latter remains APX complete.[4] For k ≥ 4, Ek-Set Splitting is approximation resistant. That is, unless P=NP, there is no polynomial-time (factor) approximation algorithm which does essentially better than a random partition.[5][6]

The Weighted Set Splitting is a variant in which the subsets in F have weights and the objective is to maximize the total weight of the split subsets.

## Connection to other problems

Set Splitting is special case of the Not-All-Equal Satisfiability problem without negated variables. Additionally, Ek-Set Splitting equals non-monochromatic graph coloring of k-uniform hypergraphs. For k=2, the optimization variant reduces to the well-known Maximum cut.[6]

## References

1. ^ Garey, Michael R.; Johnson, David S. (1979). Computers and Intractability: A Guide to the Theory of NP-Completeness. New York: W.H. Freeman. ISBN 0-7167-1045-5.
2. ^ Petrank, Erez (1994). "The Hardness of Approximation: Gap Location". Computational Complexity. Springer.
3. ^ Dehne, Frank; Fellows, Michael; Rosamond, Frances (2003). An FPT Algorithm for Set Splitting (PDF). Graph Theoretic Concepts in Computer Science (WG2003), Lecture Notes in Computer Science. 2880. Springer. pp. 180–191.
4. ^ Lovász, László (1973). Coverings and Colorings of Hypergraphs. 4th Southeastern Conference on Combinatorics, Graph Theory, and Computing.
5. ^ Håstad, Johan (2001). "Some Optimal Inapproximability Results". Journal of the ACM. Association for Computing Machinery. 48: 798–859. doi:10.1145/502090.502098.
6. ^ a b Guruswami, Venkatesan (2003). "Inapproximability Results for Set Splitting and Satisfiability Problems with no Mixed Clauses". Algorithmica. Springer. 38: 451–469. doi:10.1007/s00453-003-1072-z.