Abstract 1 Introduction References

Submodular Hypergraph Partitioning: Metric Relaxations and Fast Algorithms via an Improved Cut-Matching Game

Antares Chen ORCID University of Chicago, IL, USA Lorenzo Orecchia ORCID University of Chicago, IL, USA Erasmo Tani ORCID University of Chicago, IL, USA
Sapienza University, Rome, Italy
Abstract

Despite there being significant work on developing spectral- [12, 36, 35], and metric-embedding-based [50] approximation algorithms for hypergraph conductance, little is known regarding the approximability of other hypergraph partitioning objectives.

This work proposes algorithms for a general model of hypergraph partitioning that unifies both undirected and directed versions of many well-studied partitioning objectives. The first contribution of this paper introduces polymatroidal cut functions, a large class of cut functions amenable to approximation algorithms via metric embeddings and routing multicommodity flows. We demonstrate a simple O(logn)-approximation, where n is the number of vertices in the hypergraph, for these problems by rounding relaxations to metrics of negative-type.

The second contribution of this paper generalizes the cut-matching game framework of Khandekar et al. [31] to tackle polymatroidal cut functions. This yields an almost-linear time O(logn)-approximation algorithm for standard versions of undirected and directed hypergraph partitioning [35]. A technical contribution of our construction is a novel cut-matching game, which greatly relaxes the set of allowed actions by the cut player and allows for the use of approximate s-t maximum flows by the matching player. We believe this to be of independent interest.

Keywords and phrases:
Hypergraph Partitioning, Cut Improvement, Cut-Matching Game
Category:
Track A: Algorithms, Complexity and Games
Funding:
Antares Chen: NSF DGE 2140001.
Lorenzo Orecchia: NSF CAREER 1943510.
Copyright and License:
[Uncaptioned image] © Antares Chen, Lorenzo Orecchia, and Erasmo Tani; licensed under Creative Commons License CC-BY 4.0
2012 ACM Subject Classification:
Theory of computation Facility location and clustering
Related Version:
Full Version: https://arxiv.org/abs/2301.08920 [2]
Acknowledgements:
The authors would like to thank anonymous reviewers for suggesting related work on polymatroidal networks, Thatchaphol Saranurak for suggesting related work on the use of cut-matching games beyond partitioning graphs, and Jeffrey Negrea for assisting on topics related to online learning. AC would like to dedicate this submission to Luca Trevisan. A portion of this work was done while AC visited Bocconi; among many other fond memories, AC recalls long discussions with Luca on this project and other topics of research. AC would also like to thank Max Ovsiankin and Marco Pirazzini for their steadfast support in finding the sparsest of cuts.
Editors:
Keren Censor-Hillel, Fabrizio Grandoni, Joël Ouaknine, and Gabriele Puppis

1 Introduction

Increasing complexity in real-world data has necessitated generalizing graph models to capture relations between multiple objects [1, 11]. Hypergraphs provide such a model, with tasks involving hypergraphs being featured prominently in recent practical [9, 67] and theoretical developments [12, 13, 49, 50].

Hypergraph partitioning is such a task. In machine learning, hypergraph partitioning enables detecting clusters in complex higher-order relations arising in social [67, 71] and biological [24, 33] networks. In combinatorial optimization, partitioning algorithms are a fundamental primitive, used in the development of divide-and-conquer methods for other hypergraph problems. Unlike in graphs, hypergraphs admit multiple manners of quantifying the cost of how a hyperedge is cut. This yields a wide spectrum partitioning objectives, and allows end-users to better design partitioning problems that suite their application domain. While there has been significant work on approximation algorithms for specific hypergraph partitioning objectives, such as undirected hypergraph conductance  [12, 36], vertex expansion [51, 50, 35] and conductance in directed graphs [36], little is known about the approximability of hypergraph partitioning objectives at broad, with even less known regarding fast algorithms for these objectives.

The first part of this work defines ratio-cut problems with polymatroidal cut functions, a class of partitioning objects inspired by classical work on polymatroidal networks [27, 42]. This is a subset of submodular cut functions considered in [45, 46], but with structure well-suited for approximation methods via metric embeddings. It captures undirected and directed versions of many partitioning objectives found in current literature, and for all objectives in this class, we construct polynomial-time O(logn)-approximation algorithms by rounding 22-metric relaxations. The second part of the paper constructs fast algorithms for polymatroidal cut function objectives. Our algorithms do not require solving super-quadratic-size semidefinite programs, and achieve O(logn)-approximations. Instead, we generalize the cut-matching framework of [31], and produce the first almost-linear time polylogarithmic approximation for undirected and directed hypergraph conductance.

To describe partitioning task in more detail, recall that a hypergraph is a tuple G=(V,E), where E2V is a set of hyperedges. The rank of a hyperedge hE is its cardinality |h|. If every hE satisfies |h|=k, the hypergraph G is called k-uniform. Hence, an undirected graph is a 2-uniform hypergraph. With each hyperedge hE, we associate a suitable cut function δh:2h[0,1]. This expresses the cost incurred when h is cut by (A,A¯) into Ah and A¯h. This work focuses on submodular cut functions, defined by [45, 46]:

Definition 1 (Submodular Cut Function).

A function δh:2h[0,1] is a submodular cut function if it is submodular and satisfies δh()=δh(h)=0.

The space of cut functions for an edge {i,j} is spanned by the undirected edge cut function δ{i,j}(S)=|𝟏iS𝟏jS| and the directed edge cut function δ(i,j)(S)=(𝟏iS𝟏jS)+, giving a complete classification of cut functions for rank-2 hyperedges. Higher-rank hyperedges support broader choices of cut functions. The most common example is the standard hypergraph cut function [12, 49]: δh𝖼𝗎𝗍(S)=defmin{1,|S|,|hS|}, which takes value 1 if h is cut by S and 0 otherwise. If the hypergraph G=(V,E) is associated with positive hyperedge weights 𝐰>0E, we can succinctly write the cut function for the entire graph as δG(S)=defhEwhδh(S).

With this, we can define our hypergraph partitioning objective of interest: submodular hypergraph ratio-cut.

Definition 2 (Submodular Hypergraph Ratio-Cut).

Given a hypergraph G=(V,E) with positive hyperedge weights 𝐰>0E, non-negative vertex weights 𝛍0V, and a collection of submodular cut functions {δh}hE, the ratio-cut objective on G is defined for all SV as:

ΨG(S)=defδG(S)min{μ(S),μ(S¯)}=hEwhδh(Sh)min{μ(S),μ(S¯)}.

The ratio-cut of G is ΨG=defminSVΨG(S).

When specialized to graphs111In the rest of the paper, for any undirected (resp. directed) graphs G, unless otherwise specified, we assume that δG and ΨG are formed using the undirected (resp. directed) cut functions. ratio-cut objectives include both graph expansion (𝝁=𝟏), graph conductance (iV,μi=hE:ihwh). It also captures their directed counterparts, by replacing the undirected cut function δ{i,j} with its directed analogue δ(i,j).

Previous Work on Polymatroidal Networks

Polymatroidal networks are a model of network flows first introduced independently by Hassin [27], and Lawler and Martel [42], with the latter drawing inspiration from scheduling problems [54, 55, 53]. This class of networks generalized the standard notion of s-t flows over graphs by replacing edge capacity constraints with submodular function constraints over subsets of edges [40] (in [7]). Among many others which studied submodular constrained flow networks [20, 23, 25, 41, 43], this body of work explored how to prove the existence of integral solutions to different flow and partitioning linear programs, and subsequently generalize combinatorial strong duality results such as the maxflow-mincut of Ford & Fulkerson, and Kőnig’s matching theorem [63, 64]. Along this theme, Chekuri et al.  [14] defined a notion of multicommodity flow for polymatroidal networks, and used metric embedding techniques to prove polylogarithmic upper bounds on multicommodity flow-cut gaps in this setting.

Previous Work on Submodular Hypergraph Partitioning

Hypergraph partitioning has been extensively studied from both a practical and theoretical perspective. Early work was driven by interest in optimal placement of circuit components [38, 52, 57], designing scheduling layouts [62, 22], and information management [39, 56]. Later interest was driven by applications in parallel numerical algorithms [11], and scientific computing [21].

In the realm of tractable algorithms with provable approximation guarantees, a number of metric embedding and spectral algorithms are known for approximating specific hypergraph partitioning objectives. Louis & Makarychev [50] gave the first randomized polynomial-time O(log|V|)-approximation algorithm for hypergraph expansion, matching the best known bound on graph expansion [6]. Louis [49] and Chan et al.  [12] show Cheeger-like guarantees for hypergraph conductance. Their algorithms require solving a semidefinite program (SDP) to approximate a notion of hypergraph spectral gap, returning a partition with conductance at most O(ΨGlogmaxhE|h|). Their results are known to be tight under the small-set expansion conjecture. Recently, Lau et al.  [36] introduced a different notion of hypergraph spectral gap, called reweighted eigenvalues, and use it to obtain Cheeger-like guarantees of the form O(ΨGlog(1/ΨG)) for undirected and directed hypergraph conductance.

Less is known for larger classes of ratio-cut objectives. Yoshida [72] showed a Cheeger-like bound on ratio-cut with submodular cut functions of O(ΨGlog|V|). Li & Milenkovic [46] improved this to O(ΨGmaxhE|h|) and conjectured that improving the dependence on rank is 𝖭𝖯-hard. In broader generality, Svitkina & Fleischer [66] showed that an ω(|V|/log|V|)-approximation for a submodular version of sparsest cut requires an exponential number of function-value queries. Thus, one must assume additional structure to construct non-trivial approximations to submodular hypergraph partitioning objectives.

Previous Work on Cut-Matching Games

Many of the spectral and metric embedding methods mentioned above solve SDP relaxations over |V|×|V| symmetric matrices, with at least |E| constraints. No faster algorithms are currently known beyond generic SDP solvers [30], which run in Ω(|V|2|E|) time. Even for the simplest form of hypergraph partitioning, the resulting SDP relaxation is a mixed packing-and-covering program for which no almost-linear time algorithm is currently known [29].

To circumvent this issue, one can apply primal-dual methods which solve the dual to the metric embedding SDPs. However, the dual is typically a multicommodity flow problem whose demand graph is dense, and solving this black-box may require quadratic time [5]. The cut-matching game [32] was designed to approximately reduce this multicommodity flow to a polylogarithmic number of exact single-commodity flows. Each single-commodity flow problem would route a perfect matching in the graph, until the union of matchings approximates the desired demand graph.

Using the cut-matching game, Khandekar et al.  [32] obtained a O(log2|V|)-approximation to undirected graph expansion by exactly solving O(log3|V|) maximum flows. Orecchia et al.  [61] reduced both of these measures by O(logn). Subsequent work has vastly generalized the applicability of the cut-matching game to directed graph expansion [48], to vertex and hypergraph expansion [47]. Nanongkai & Saranurak [58] demonstrate how to replace exact single-commodity flows with approximate ones at the cost of an additional O(logn)-factor in the approximation ratio. The cut-matching game has since become ubiquitous in designing fast deterministic algorithms for various static, and dynamic graph problems [3, 10, 17, 18, 19, 26].

Our Work

We provide the following technical contributions. Further detail is given in the parenthesized subsections.

  • (1.1.1) A metric approach to submodular hypergraph ratio-cut – We identify a subset of submodular cut functions, called polymatroidal cut functions, that are amenable to metric and flow techniques. For this class, we develop a notion of hypergraph flow, and show how to certify lower bounds to the ratio-cut objective by flow-embedding graphs into hypergraphs. We give polynomial-time O(log|V|)-approximation algorithms for minimum ratio-cut for submodular hypergraphs with polymatroidal cut functions by providing an 22-metric relaxation, generalizing the result of Arora, Rao & Vazirani [5] to submodular hypergraph partitioning.

  • (1.1.2) Local Formulations of the Submodular Hypergraph Ratio-Cut Problem – Generalizing the cut-matching game requires an algorithm that can locally probe the hypergraph cut structure by finding a sparse hypergraph cut near a given input partition. The cut-improvement algorithm of Andersen & Lang [4] does this for graphs. We generalize this to submodular hypergraphs. Our cut improvement problem can be solved using decomposable submodular minimization. We provide a simple interpretation of this problem by characterizing it as a surrogate estimate for the submodular hypergraph ratio-cut objective that is both local and convex.

  • (1.1.3) An Improved Cut-Matching Game – We extend the cut-matching game framework to approximate minimum ratio-cuts on submodular hypergraphs with polymatroidal cut functions. Our approach relies on an improved formulation of the cut-matching games that allows for a larger set of cut and matching strategies. As a result, we produce an O(log|V|)-approximation algorithm for minimum ratio-cut that uses only a single O(1)-approximate decomposable submodular minimization solve per round. For the special case of graph partitioning, our algorithm improves on existing applications of the cut-matching game by enabling the use of approximate maxflow solutions at no cost to the approximation ratio. This is to be compared with previous work that require either a single on(1)-approximate, or a polylogarithmic number of O(1)-approximate maxflow computations per round [10, 47, 58], which results in a polylogarithmic loss in approximation ratio.

Concurrent and Subsequent Work

Subsequent to an initial version of this paper [2], Veldt [68] gave a O(log|V|)-approximation for ratio-cut with cardinality-based symmetric submodular cut functions, a special case of submodular hypergraph partitioning where δh(S)=δh(hS) and δh(S)=δh(T) if |S|=|T|. This assumption on δh allows a certain combinatorial gadget, hypergraph cut preservers, to certify lower bounds to the ratio-cut objective [70, 69]. Their work gives an algorithm that generalizes the cut-matching framework to allow for a matching player to construct such a gadget using exact single commodity flows. [68] additionally provides compelling empirical evaluations.

Independently, Lau et al. [37] gave the first almost-linear time O(logn)-approximation for undirected and directed hypergraph expansion. Their algorithm does not use the cut-matching game, and instead solves the dual to the relaxation of the minimum reweighted eigenvalue problem in [36] equipped with additional 22-metric constraints. This produces a multicommodity flow problem which they show to be approximable by a sequence of single-commodity flows via a generalization of Sherman’s algorithmic chaining [65].

Finally in very recent work, Chekuri & Louis [15] extend our algorithm and analysis to produce a O(log|V|)-approximation to the larger class of ratio-cut problems in polymatroidal networks (see section 1.2 of [15] for a comparison). However, their work explicitly does not address the design of fast algorithms.

1.1 Technical Summary of Contributions

In this section, we provide a technical overview of our work, including the key definitions and the statement of the main theorems and lemmata. Full proofs, together with more detailed discussions and examples, are available in the full version of the paper [16].

1.1.1 A Metric Approach to Submodular Hypergraph Ratio-Cut

The best polynomial-time approximation algorithms for graph ratio-cut problems solve for a metric relaxation, then round using a metric embedding result. Relaxing to general metrics results in an O(logn)-approximation [44], while relaxing to 22-metrics achieve O(logn) [6]. Our goal is to understand the applicability of this approach to submodular hypergraph ratio-cut problems. First, one should not hope to obtain polylogarithmic approximations for general submodular cut functions without further assumptions, as Svitkina & Fleischer (Section 3 in [66]) exhibit a submodular cut function for which any o(n)-approximation requires exponentially many queries to a value oracle.

What kind of restrictions allow for metric relaxation methods to work? The cut function must exhibit some kind of monotonicity, so that its value is preserved under low-distortion metric embeddings. However, a cut function cannot be monotone without being equal to 0, so the monotone structure must appear in some other way. To solve this issue, we introduce the class of polymatroidal cut functions.

Polymatroidal Cut Functions and Metric Embeddings

Our first contribution is to define polymatroidal cut functions, a subclass of submodular cut functions given by the infimal symmetrization of non-decreasing submodular functions.

Definition 3 (Polymatroidal Cut Function).

A cut function δh:2h0 is polymatroidal if, for all Sh, it can be expressed as

δh(S) =min{Fh(S),Fh+(hS)},

where the associated functions Fh,Fh+:2h are non-decreasing submodular functions such that Fh()=Fh+()=0. When the associated functions Fh and Fh+ are identical, we refer to the cut function δh as symmetric polymatroidal.

Note, δh penalizes a cut (A,A¯) intersecting h in a directed manner: F penalizes Ah and F+ penalizes A¯h. For a symmetric polymatroidal cut function δh, such penalty is the same, so that δh(Ah)=δh(A¯h), generalizing the cut function of undirected graphs and hypergraphs. In the full version [16], we show that Definition 3 captures most of the cut functions proposed in previous work, including all directed and undirected standard hypergraph cut functions. Moreover, our setup also captures possible combinations of all these types of cut functions into a single framework. We highlight two examples relevant in applications:

  • Fh is a cardinality-based non-decreasing submodular function [69], a non-decreasing concave function of the cardinality. This is useful when we wish to modify the star-expansion cut function by diminishing the penalty on balanced partitions of h, e.g., by taking δh(S)=min{|S|p,|hS|p} for p(0,1).

  • Fh is the entropy of a subset of random variables associated with vertices of h. When the hypergraph G is the factor graph of an undirected graphical model, this yields a variant of the minimum information partition [28, 59].

We investigate the polynomial-time approximability of ratio-cut problems over submodular hypergraphs with polymatroidal cut functions. In particular, we prove the following result.

Theorem 4.

There exists a randomized polynomial-time O(logn)-approximation algorithm for solving the minimum ratio-cut problem on weighted submodular hypergraphs equipped with polymatroidal cut functions.

This result extends the O(logn)-approximation of Louis and Makarychev [50] for standard hypergraph sparsest cut to all ratio-cut problems for the more general class of polymatroidal functions. Our algorithm for Theorem 4 solves a new 22-metric relaxation of the ratio-cut problem for weighted submodular hypergraphs This relaxation crucially exploits the form of the Lovász extension of polymatroidal cut functions. To the best of our knowledge, our results constitutes the broadest known generalization of the seminal result of Arora, Rao and Vazirani for sparsest cut, capturing a wealth of partitioning objectives, including directed and undirected graph ratio-cut, vertex-based ratio cut and hypergraph ratio-cut within a single simple analysis. It also improves on the best known approximation ratio for many other objectives in this class, such as cardinality-based cut functions [69]. For details, see the full version of this paper [16].

Polymatroidal Cut Functions and Hypergraph Flows

In the second part of the paper, we explore the efficient solution of the minimum submodular hypergraph ratio cut problem via flow methods based on the cut-matching game. We start by developing the notions of flow-cut duality and flow embeddings over hypergraphs. For this purpose, we define a novel notion of hypergraph flows for weighted submodular hypergraphs equipped with polymatroidal cut functions. Specifically, for a weighted submodular hypergraph G=(V,E,𝐰,𝝁), we show that the subgradients of polymatroidal cut functions behave like network flows over the factor graph, a graph analogue of the input submodular hypergraph that includes an auxiliary vertex for each hyperedge hE and an edge {i,h} for each ih. For each hyperedge hE, the monotone functions Fh and Fh+ define capacity constraints on the flows entering and leaving the corresponding auxiliary node. The main algorithmic result in this part of the paper is a flow decomposition result for hypergraph flows, which allows us to construct flow embeddings of directed and undirected graphs into hypergraphs while lower bounding the size of the hypergraph cut with the size of the corresponding graph cut. This will become a fundamental building block for our cut-matching game construction.

Connection to Polymatroidal Networks and Flows

A polymatroidal network [42] is characterized by a directed (2-uniform) graph H representing a flow network where the edges are not individually capacitated, but rather, for every vertex vV(H), there exist monotone submodular functions ρ and ρ+ on the constraining the amount of flow entering and exiting a vertex respectively. When viewed as network flows over the factor graph, our hypergraph flows are exactly an instantiation of polymatroidal flows, where each auxiliary node hE has associated submodular capacity functions Fh and Fh+. In the same way, it is also possible to interpret the submodular hypergraph partitioning problem with polymatroidal cut functions, as a special case of the ratio-cut problem over polymatroidal networks [14]. This is achieved by casting the factor graph as a polymatroidal network in which the original vertices vV have no restrictions on their capacities and the auxiliary vertices hH do not contribute to the denominator in the ratio-cut problem. See the recent paper of Chekuri & Louis [15] for more details of this reduction.

1.1.2 Local Formulations of the Submodular Hypergraph Ratio-Cut Problem

To generalize the cut-matching game framework to the setting of submodular hypergraphs with polymatroidal cut functions, we introduce an optimization approach based on a continuous non-convex formulation (RC-NonCvx) of the submodular hypergraph ratio-cut problem. Given a weighted submodular hypergraph G=(V,E,𝐰,𝝁), and cut functions {δh}hE with Lovász extensions {δ¯h}hE, we define (RC-NonCvx) as the following non-convex optimization problem over V:

Ψ¯G(𝐱) =defhEwhδ¯h(𝐱)minu𝐱u𝟏𝝁,1
Ψ¯G =defmin𝐱VΨ¯G(𝐱) (RC-NonCvx)

The fact that solving (RC-NonCvx) is equivalent to solving the minimum ratio-cut problem (expressed in the following lemma) is a direct consequence of the submodularity of the cut functions.

Lemma 5.

Given a weighted hypergraph G=(V,E,𝛍,𝐰), we have ΨG=Ψ¯G. Furthermore, there exists an algorithm that, given any 𝐱V, recovers a cut SV satisfying

ΨG(S)Ψ¯G(𝐱)

in time O(|V|log|V|+𝗌𝗉(G)), where the sparsity of G is defined as 𝗌𝗉(G)=defhE|h|.

The main idea in our approach is to address the non-convexity of the Ψ¯G objective by replacing the non-concave denominator with a linear lower bound. In particular, we can exploit the dual characterization of the 1-norm in the lower bound to write:

minu𝐱u𝟏𝝁,1=minumax𝐲1𝐲,𝐱u𝟏𝝁=max𝐲1𝐲,𝟏𝝁=0𝐲,𝐱𝝁 (2)

This calculation directly leads us to define a novel localized, convex formulation of the (RC-NonCvx) problem for each vector 𝐬V with 𝐬1 and 𝐬,𝟏𝝁=0, which we call a seed:

Ψ¯G,𝐬(𝐱) =defhEwhδ¯h(𝐱)max{0,𝐬,𝐱𝝁}
Ψ¯G,𝐬 =defmin𝐱VΨ¯G,𝐬(𝐱) (Local-RC)

Intuitively, the program (Local-RC) seeks a distribution over cuts 𝐱 with small expected cut size and high correlation with the seed 𝐬. In the full version [16], we investigate the properties of this formulation, including its equivalence with an integral cut problem, the ratio-cut improvement problem, which generalizes the cut improvement problem of Andersen and Lang [4] to submodular hypergraphs. For polymatroidal cut functions, we show that the natural notion of a dual solution for the convex problem Local-RC is exactly a hypergraph flow with demand vector diag(𝝁)𝐬. By applying our flow decomposition result, we can turn such a hypergraph flow into a dual graph certificate, a graph D such that ΨG,𝐬δD(S)δG(S) for all SV. To construct these objects, we prove the following algorithmic result:

Theorem 6 (Informal. See full version).

An approximate primal-dual solution to (Local-RC) can be computed by solving O(log|V|)-many decomposable submodular minimization problems for general polymatroidal cut functions or O(log|V|)-many 1/2-approximate maximum flow problems over a graph of size O(𝗌𝗉(G)).

Finally, and crucially for our purposes, for all 𝐱V, we have Ψ¯G(𝐱)Ψ¯G,𝐬(𝐱) by Equation 2. Hence, any (not necessarily optimal) solution 𝐱 to (Local-RC) for a seed 𝐬 yields a solution to (RC-NonCvx) of value at most Ψ¯G,𝐬(𝐱) and, via Lemma 5, a cut S with ΨG(S)Ψ¯G,𝐬(𝐱). In other words, if we can somehow find a seed 𝐬 such that Ψ¯G,𝐬αΨG, then solving the (Local-RC) will yield an α-approximation algorithm for ΨG.

This reduction may not seem very useful, as finding the required seed may be as hard as solving the original problem. However, we can now exploit the dual feedback obtained when solving (Local-RC) for a sequence of seeds 𝐒=(𝐬1,𝐬2,,𝐬T) to effectively guide our search for a good seed and boost our approximation ratio. In particular, suppose we can adaptively construct a sequence of T seeds 𝐒 such that the sum H=t=1TDt of the corresponding dual demand graphs Dt has large ratio-cut, i.e., ΨHβ. Then, it is easy to show that we must have mint=1TΨG,𝐬tT/βΨG. Hence, solving (Local-RC) for one of the seeds in the sequence 𝐒 must yield a T/β-approximation to the minimum ratio-cut problem. It turns out that the problem of constructing such sequence 𝐒 is precisely the task addressed by the cut-matching framework of Khandekar et al.  [32], which we generalize and strengthen in our next contribution.

1.1.3 An Improved Cut-Matching Game

We now describe our construction of the cut-matching game. Later, we use this to design an approximation algorithm for hypergraph partitioning. We first define what constitutes admissible actions played by the cut and matching players.

Definition 7.

Let V be a set of vertices with vertex weights 𝛍0V. A cut action is a pair (A,B) of non-empty disjoint subsets A,BV such that 𝛍(A)𝛍(B).

The cut player is not restricted to returning a partition of the vertex set, let alone a bisection. The set of valid actions for a matching player is similarly relaxed, as the edges of the response do not need to form a perfect matching. It is only required that enough of the available degree is routed across the cut action.

Definition 8.

An approximate matching response to a cut action (A,B) is a weighted bipartite graph D=(AB,ED,𝐰D,𝛍) satisfying the following conditions.

  1. 1.

    Bounded degree: for all iV

    degD(i){μiiA𝝁(A)𝝁(B)μiiB.
  2. 2.

    Largeness: 𝐰D(E(A,B))𝝁(A)2.

We may emphasize D is a directed approximate matching response if edges of D are directed from A to B.

This definition is motivated by how an approximate matching response naturally arises from the dual graph certificate obtained by approximately solving the (Local-RC) program above. The matching response will be undirected when we wish to approximate the minimum ratio-cut of a hypergraph with symmetric cut functions and directed when generically considering asymmetric cut functions. We now define the cut-matching game.

Definition 9.

Let n>0, and 𝛍0V be a weighting over a set of vertices V where |V|=n. A (n,𝛍)-generalized cut-matching game is a multi-round, two-player game between a cut player 𝒞, and a matching player that proceeds as follows. In the t-th round of the game, following definitions 7 and 8:

  1. 1.

    𝒞 plays a cut action (At,Bt),

  2. 2.

    responds with an approximate matching response Dt to (At,Bt).

The state graph after t rounds is Ht=s=1tDs the edge-wise sum of the matching response seen thus far.

For comparison, the original cut-matching game of [32] is captured by the above definition when one specifically considers graph expansion 𝝁=𝟏, restricts cut actions to be exact bisections (A,A¯) where |A|=|A¯|, and requires exact matching responses: 𝐰D(E(A,A¯))=𝝁(A).

To approximate minimum ratio-cut using our cut-matching games, we seek a cut player 𝒞 that outputs a sequence of cuts which, in as few cuts as possible, forces the matching player to place edges that make the minimum ratio-cut objective for the state graph HT large. We call a cut player good if it can achieve this.

Definition 10.

A cut strategy is a randomized algorithm 𝒜𝖼𝗎𝗍 that takes as input the current state graph H, and outputs a cut action (A,B). A cut strategy 𝒜𝖼𝗎𝗍 is (f(n),g(n))-good if a cut player 𝒞 using 𝒜𝖼𝗎𝗍 at every iteration, achieves:

ΨHg(n)f(n)

with constant probability, for any (potentially adaptive) sequence of approximate matching responses D1,,Dg(n).

Note that, when dealing with symmetric cut functions, H will be undirected, so that ΨH refers to the undirected ratio-cut objective. If the cut functions are asymmetric, then ΨH is naturally replaced with the directed ratio-cut objective.

These definitions reveal the mechanism by which the cut-matching framework adaptively constructs a sequence of seeds to (Local-RC): the seed we choose at every iteration of cut-matching game is just a 𝝁-weighted indicator vector for the cut action of that round. A good strategy guarantees that, no matter what dual graph certificates we receive for each seed, at the end of the game at T=g(n) iterations, we can guarantee HT has ratio-cut objective at least β=f(n). This shows that a good strategy yields an approximation ratio of O(g(n)/f(n)) for minimum ratio cut. Our main technical theorem regarding the new cut-matching game shows the existence of good strategies for the cut-player for both the cases of symmetric and general polymatroidal cut functions.

Theorem 11.

Let H=(V,EH,𝐰H,𝛍) be the state graph for an (n,𝛍)-generalized cut-matching game. There exists a cut strategy 𝒜𝖼𝗎𝗍 satisfying the following:

  1. 1.

    If H is undirected, then 𝒜𝖼𝗎𝗍 is (Ω(logn),O(log2n))-good with probability O(1).

  2. 2.

    If H is directed, then 𝒜𝖼𝗎𝗍 is (Ω(log2n),O(log3n))-good with probability O(1).

Furthermore, 𝒜𝖼𝗎𝗍 outputs a cut action in time O~(𝗌𝗉(H)).

Together with Theorem 6, our results on good cut-player strategies directly imply the following results for the solution of the minimum ratio-cut problem.

Corollary 12.

There exists an O(logn)-approximation algorithm for the minimum ratio-cut problem over submodular hypergraphs with polymatroidal cut functions whose running time is dominated by the solution of a polylogarthmic number of decomposable submodular minimization problems over the hypergraph cut function.

Corollary 13.

There exists an O(logn)-approximation algorithm for the minimum ratio-cut problem over submodular hypergraphs equipped with the directed or standard hypergraph cut function whose running time is almost linear in the hypergraph sparsity.

In many cases of interest, the decomposable submodular minimization problem can be solved in almost linear time [8], so that Corollary 12 yields the first O(logV|)-approximation algorithms for many submodular hypergraph ratio-cut problems with polymatroidal cut functions. Corollary 13 shows that the specialization of this algorithm to the directed and standard hypergraph cut functions also yields O(log|V|)-approximation almost linear time algorithms. Concurrently to our work, this latter result was improved to an O(log|V|-approximation in almost linear time by Lau et al.  [37]. However, our algorithm is still the fastest one to achieve an O(log|V|)-approximation ratio for general polymatroidal cut functions.

Approximate Matching Responses

The concept of an approximate matching response in Definition 8 is carefully tailored to enable its implementation by a O(1)-approximate maximum flow algorithm (for directed and standard hypergraph problems) or a O(1)-approximate decomposable submodular minimization problem (for general polymatroidal cut functions). This contrasts with previous applications of the cut-matching game, which either require a single on(1)-approximate solve per round [32], at the cost of higher running time, or a logarithmic number of O(1)-approximate solves per round [58], at the cost of a logarithmic loss in approximation ratio. As a result, our novel cut-matching game can be plugged in to improve the analyses of down-stream applications of the cut-matching game [10, 17, 18, 19, 47, 58]. The low-precision required by our algorithm may also impact the deployment of cut-matching games in practice as high precision solves for maximum flow or submodular minimization are a running-time bottleneck of algorithms based on cut-matching games [60, 68].

Note that the additional freedom in the matching responses, which can now fail to match up to a constant fraction of vertices, makes it harder to establish the existence of good cut strategies. Showing Theorem 11 thus requires a new geometric result regarding well-separated sets of embedding vectors, discussed below.

Separated Sets

In our new cut-matching game, the cut player must pay considerable more care in choosing a cut action (A,B) than in the original version, as the approximate nature of the matching response means that the matching player can easily keep some small subset of vertices disconnected from the rest of the graph. Suppose for instance that the cut player restricted itself to play μ-bisections. Then, the matching player could select a small cut T, with μ(T)1/10μ(V) and never place any edge across the boundary of T. The only way the cut player can force progress on T is to depart from playing bisections and play a cut action (A,B) where A is close or equal to T.

How is this intuition formalized in our design of cut-player strategies? As in the analysis of Orecchia et al.  [61], our cut strategy maintains at each iteration t a spectral embedding for the current state graph Ht. We make progress if we can force the matching player to connect vertices that are far away in this embedding. We show that we can achieve this, even against an approximate matching player, by choosing a cut action coming from the following notion of separated sets:

Definition 14 (σ-separated sets).

Given an embedding {𝐯i}i[n] and a measure 𝛍0n we say two sets S,T[n] are σ-separated if:

𝝁(S)𝝁(T)miniSjT𝐯i𝐯j2σi,j(V2)μiμj𝐯i𝐯j2.

This definition generalizes the idea of well-separated sets [6], which was applied only to large balanced sets, to possibly unbalanced sets, as long as S and T contribute a σ-fraction to the total variance of the embedding. In the full version [16], we show how to efficiently find separated sets and prove the following result.

Theorem 15.

Given an embedding {𝐯i}i[n]d and a measure 𝛍0n, with μ(V)=𝗉𝗈𝗅𝗒(n), there exists a randomized algorithm running in time O~(nd) that outputs σ-separated sets S and T for σ=Ω(1/logn) with high probability.

1.1.4 Open Problems

Chekuri and Louis [15] have recently shown that our metric analysis, originally devised for submodular hypergraphs, can be applied more generally to partitioning problems over polymatroidal networks. This naturally leads to the question of whether our second contribution, the design of fast algorithms based on the cut-matching game, can also be extended to the general setting of polymatroidal networks to yield O(log|V|) approximation algorithms.

An even more ambitious goal is to adapt the recent work of Kolmogorov [34] which provides a simpler construction of Sherman’s O(logn)-approximation for sparsest cut, to the minimum-ratio cut problem over general polymatroidal networks. The ultimate goal would be to obtain O(log|V|/ϵ)-approximation algorithms that only requires the solution of O(|V|ϵ) decomposable sumbodular minimization problems, for ϵΘ(1/log|V|,Θ(1)).

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