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«A Tractable hypergraph properties for constraint satisfaction and conjunctive queries1 D´niel Marx, Computer and Automation Research Institute, ...»

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A different version of CSP was investigated in [Marx 2011], where each variable has a different domain, and each constraint relation is represented by a full truth table (see the exact definition in [Marx 2011]). Let us denote by CSPtt (H) this variant of the problem. It is easy to see that CSPtt (H) can be reduced to CSP(H) in polynomial time, but a reduction in the other direction can possibly increase the representation of a constraint by an exponential factor. Nevertheless, the hardness results of this section apply to the “easier” problem CSPtt (H) as well. What we have to verify is that the proof of Lemma 7.5 works even if I2 is an instance of CSPtt, i.e., the constraint relations have to be represented by truth tables. Inspection of the proof shows that it indeed works: the product in Inequality (4) is exactly the size of the truth table describing the constraint corresponding to edge e, thus the |D1 |q upper bound remains valid even if constraints are represented by truth tables. Therefore, the hardness results of [Marx 2011] are subsumed by the following

corollary:

Corollary 7.8. If H is a recursively enumerable class of hypergraphs with unbounded submodular width, then CSPtt (H) is not fixed-parameter tractable, unless the Exponential Time Hypothesis fails.

8. CONCLUSIONS The main result of the paper is introducing submodular width and proving that bounded submodular width is the property that determines the fixed-parameter tractability of CSP(H). The hardness result is proved assuming the Exponential Time Hypothesis. This conjecture was formulated relatively recently [Impagliazzo et al. 2001], but it turned out to be very useful in proving lower bounds in a variety of settings [Marx 2010b; Andoni et al.

2006; Marx 2007; Pˇtra¸cu and Williams 2010].

as For the hardness proof, we had to understand what large submodular width means and we had to explore the connection between submodular width and other combinatorial properties. We have obtained several equivalent characterizations of bounded submodular width, in particular, we have showed that bounded submodular width is equivalent to bounded

adaptive width:

Corollary 8.1. The following are equivalent for every class H of hypergraphs:

(1 ) There is a constant c1 such that µ-width(H) ≤ c1 for every H ∈ H and fractional independent set µ.

(2 ) There is a constant c2 such that b-width(H) ≤ c2 for every H ∈ H and edgedominated monotone submodular function b on V (H) with b(∅) = 0.

(3 ) There is a constant c3 such that b∗ -width(H) ≤ c3 for every H ∈ H and edgedominated monotone submodular function b on V (H) with b(∅) = 0.

Journal of the ACM, Vol. V, No. N, Article A, Publication date: January YYYY.

Tractable hypergraph properties for constraint satisfaction and conjunctive queries A:45 (4 ) There is a constant c4 such that conλ (H) ≤ c4 for every H ∈ H, where λ 0 is a universal constant.

(5 ) There is a constant c5 such that emb(H) ≤ c5 for every H ∈ H.

Implications (2)⇒(1) and (3)⇒(2) are trivial; (4)⇒(3) follows from Lemma 5.10; (5)⇒(4) follows from Corollary 6.2; (1)⇒(5) follows from Lemma 6.9.

Let us briefly review the main ideas that were necessary for proving the main result of

the paper:

— Recognizing that submodular width is the right property characterizing the complexity of the problem.

— A CSP instance can be partitioned into a bounded number of uniform instances (Section 4.2).

— The number of solutions in a uniform CSP instance can be described by a submodular function (Section 4.3).

— There is a connection between fractional separation and finding a separator minimizing an edge-dominated submodular cost function (Section 5.2).

— The transformation that turns b into b∗, and the properties of b∗ that are more suitable than b for recursively constructing a tree decomposition (Section 5.1).

— Our results on fractional separation and the standard framework of finding tree decompositions show that large submodular width implies that there is a highly connected set (Section 5.3).

— A highly connected set can be turned into a highly connected set that is partitioned into cliques in an appropriate way (Section 6.1).

— A highly connected set with appropriate cliques implies that there is a uniform concurrent flow of large value between the cliques (Section 6.2).

— Similarly to [Marx 2010b], we use the observation that a concurrent flow is analogous to a line graph of a clique, hence it has good embedding properties (Section 6.2).

— Similarly to [Marx 2010b], an embedding in a hypergraph gives a way of simulating 3SAT with CSP(H) (Section 7).

It is possible that the main result can be proved in a simpler way by bypassing some of the ideas above. In particular, a surprising consequence of our results is that bounded submodular width and bounded adaptive width are the same, i.e., if a class H has unbounded submodular width, then for every k there is a Hk ∈ H and a fractional independent set µk such that µk -width(Hk ) ≥ k, or in other words, large submodular width can be certified by the modular function µk. To prove this, we need all the results of Sections 5 and 6. Having a better understanding and an independent proof of this fact could simplify the proofs considerably. Another possible target for simplification is Section 6.1, where a lot of effort is spent on proving that if there is a large highly connected set, then there is a large highly connected set that is partitioned into cliques in an appropriate way. It might be possible to strengthen the results of Section 5 (perhaps by better understanding the role of cliques in separators) so that they give such a highly connected set directly.





An obvious question for further research is whether it is possible to prove a similar dichotomy result with respect to polynomial-time solvability. At this point, it is hard to see what the answer could be if we investigate the same question using the more restricted notion of polynomial time solvability. We know that bounded fractional hypertree width implies polynomial-time solvability [Marx 2010a] and Theorem 7.1 shows that unbounded submodular width implies that the problem is not polynomial-time solvable (as it is not even fixed-parameter tractable). So only those classes of hypergraphs are in the “gray zone” that have bounded submodular width but unbounded fractional hypertree width.

What could be the truth in this gray zone? A first possibility is that CSP(H) is polynomial-time solvable for every such class, i.e., Theorem 4.1 can be improved from Journal of the ACM, Vol. V, No. N, Article A, Publication date: January YYYY.

A:46 D. Marx fixed-parameter tractability to polynomial-time solvability. However, Theorem 4.1 uses the power of fixed-parameter tractability in an essential way (splitting into a double-exponential number of uniform instances), so it is not clear how such improvement is possible. A second possibility is that unbounded fractional hypertree width implies that CSP(H) is not polynomial-time solvable. Substantially new techniques would be required for such a hardness proof. The hardness proofs of this paper and of [Grohe 2007; Marx 2010b] are based on showing that a large problem space can be efficiently embedded into an instance with a particular hypergraph. However, the fixed-parameter tractability results show that no such embedding is possible in case of classes with bounded submodular width. Therefore, a possible hardness proof should embed a problem space that is comparable (in some sense) with the size of the hypergraph and should create instances where the domain size is bounded by a function of the size of the hypergraph. A third possibility is that the boundary of polynomial-time solvability is somewhere between bounded fractional hypertree width and bounded submodular width. Currently, there is no natural candidate for a property that could correspond to this boundary and, again, the hardness part of the characterization should be substantially different than what was done before. Finally, there is a fourth possibility: the boundary of the polynomial-time cases cannot be elegantly characterized by a simple combinatorial property. In general, if we consider the restriction of a problem to all possible classes of (hyper)graphs, then there is no a priori reason why an elegant characterization should exist that describes the easy and hard classes. For example, it is highly unlikely that there is an elegant characterization of those classes of graphs where solving the Maximum Independent Set problem is polynomial-time solvable. As discussed earlier, the fixed-parameter tractability of CSP(H) is a more robust question than its polynomialtime solvability, hence it is very well possible that only the former question has an elegant answer.

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