A structural approach to kernels for ILPs:Treewidth and Total Unimodularity

# A structural approach to kernels for ILPs: Treewidth and Total Unimodularity

Bart M. P. Jansen
Eindhoven University of Technology
The Netherlands
b.m.p.jansen@tue.nl
Supported by ERC Starting Grant 306992 while working for the University of Bergen.
Stefan Kratsch
University of Bonn
Germany
kratsch@cs.uni-bonn.de
Supported by the German Research Foundation (DFG), KR 4286/1.
###### Abstract

Kernelization is a theoretical formalization of efficient preprocessing for -hard problems. Empirically, preprocessing is highly successful in practice, for example in state-of-the-art ILP-solvers like CPLEX. Motivated by this, previous work studied the existence of kernelizations for ILP related problems, e.g., for testing feasibility of . In contrast to the observed success of CPLEX, however, the results were largely negative. Intuitively, practical instances have far more useful structure than the worst-case instances used to prove these lower bounds.

In the present paper, we study the effect that subsystems that have (a Gaifman graph of) bounded treewidth or that are totally unimodular have on the kernelizability of the ILP feasibility problem. We show that, on the positive side, if these subsystems have a small number of variables on which they interact with the remaining instance, then we can efficiently replace them by smaller subsystems of size polynomial in the domain without changing feasibility. Thus, if large parts of an instance consist of such subsystems, then this yields a substantial size reduction. Complementing this we prove that relaxations to the considered structures, e.g., larger boundaries of the subsystems, allow worst-case lower bounds against kernelization. Thus, these relaxed structures give rise to instance families that cannot be efficiently reduced, by any approach.

## 1 Introduction

The notion of kernelization from parameterized complexity is a theoretical formalization of preprocessing (i.e., data reduction) for -hard combinatorial problems. Within this framework it is possible to prove worst-case upper and lower bounds for preprocessing; see, e.g., recent surveys on kernelization [17, 18]. Arguably one of the most successful examples of preprocessing in practice are the simplification routines within modern integer linear program (ILP) solvers like CPLEX (see also [1, 12, 19]). Since ILPs have high expressive power, already the problem of testing feasibility of an ILP is -hard; there are immediate reductions from a variety of well-known -hard problems. Thus, the problem also inherits many lower bounds, in particular, lower bounds against kernelization.

integer linear program feasibilityilpf
Input: A matrix and a vector .
Question: Is there an integer vector  with ?

Despite this negative outlook, a formal theory of preprocessing, such as kernelization aims to be, needs to provide a more detailed view on one of the most successful practical examples of preprocessing, even if worst-case bounds will rarely match empirical results. With this premise we take a structural approach to studying kernelization for ilpf. We pursue two main structural aspects of ILPs. The first one is the treewidth of the so-called Gaifman graph underlying the constraint matrix . As a second aspect we consider ILPs whose constraint matrix has large parts that are totally unimodular. Both bounded treewidth and total unimodularity of the whole system imply that feasibility (and optimization) are tractable.111Small caveat: For bounded treewidth this also requires bounded domain. We study the effect of having subsystems that have bounded treewidth or that are totally unimodular. We determine when such subsystems allow for a substantial reduction in instance size. Our approach differs from previous work [15, 16] in that we study structural parameters related to treewidth and total unimodularity rather than considering parameters such as the dimensions and of the constrain matrix or the sparsity thereof.

#### Treewidth and ILPs.

The Gaifman graph  of a matrix  is a graph with one vertex per column of , i.e., one vertex per variable, such that variables that occur in a common constraint form a clique in  (see Section 3.1). This perspective allows us to consider the structure of an ILP by graph-theoretical means. In the context of graph problems, a frequently employed preprocessing strategy is to replace a simple (i.e., constant-treewidth) part of the graph that attaches to the remainder through a constant-size boundary, by a smaller gadget that enforces the same restrictions on potential solutions. There are several meta-kernelization theorems (cf. [13]) stating that large classes of graph problems can be effectively preprocessed by repeatedly replacing such protrusions by smaller structures. It is therefore natural to consider whether large protrusions in the Gaifman graph , corresponding to subsystems of the ILP, can safely be replaced by smaller subsystems.

We give an explicit dynamic programming algorithm to determine which assignments to the boundary variables (see Section 3.3) of the protrusions can be extended to feasible assignments to the remaining variables in the protrusion. Then we show that, given a list of feasible assignments to the boundary of the protrusion, the corresponding subsystem of the ILP can be replaced by new constraints. If there are  variables in the boundary and their domain is bounded by , we find a replacement system with  variables and constraints that can be described in  bits. By an information-theoretic argument we prove that equivalent replacement systems require  bits to encode. Moreover, we prove that large-domain structures are indeed an obstruction for effective kernelization by proving that a family of instances with a single variable of large domain (all others have ), and with given Gaifman decompositions into protrusions and a small shared part of encoding size , admit no kernelization or compression to size polynomial in .

On the positive side, we apply the replacement algorithm to protrusion decompositions of the Gaifman graph to shrink ilpf instances. When an ilpf instance can be decomposed into a small number of protrusions with small boundary domains, replacing each protrusion by a small equivalent gadget yields an equivalent instance whose overall size is bounded. The recent work of Kim et al. [13] on meta-kernelization has identified a structural graph parameter such that graphs from an appropriately chosen family with parameter value  can be decomposed into  protrusions. If the Gaifman graph of an ilpf instance satisfies these requirements, the ilpf problem has kernels of size polynomial in . Concretely, one can show that bounded-domain ilpf has polynomial kernels when the Gaifman graph excludes a fixed graph  as a topological minor and the parameter  is the size of a modulator of the graph to constant treewidth. We do not pursue this application further in the paper, as it follows from our reduction algorithms in a straight-forward manner.

#### Total unimodularity.

Recall that a matrix is totally unimodular (TU) if every square submatrix has determinant , , or . If is TU then feasibility of , for any integral vector , can be tested in polynomial time. (Similarly, one can efficiently optimize any function subject to .) We say that a matrix is totally unimodular plus columns if it can be obtained from a TU matrix by changing entries in at most columns. Clearly, changing a single entry may break total unimodularity, but changing only few entries should still give a system of constraints that is much simpler than the worst-case. Indeed, if, e.g., all variables are binary (domain ) then one may check feasibility by simply trying all assignments to variables with modified column in . The system on the remaining variables will be TU and can be tested efficiently.

From the perspective of kernelization it is interesting whether a small value of allows a reduction in size for or, in other words, whether one can efficiently find an equivalent system of size polynomial in . We prove that this depends on the structure of the system on variables with unmodified columns. If this remaining system decomposes into separate subsystems, each of which depends only on a bounded number of variables in non-TU columns, then by a similar reduction rule as for the treewidth case we get a reduced instance of size polynomial in and the domain size . Complementing this we prove that in general, i.e., without this bounded dependence, there is no kernelization to size polynomial in ; this also holds even if counts the number of entry changes to obtain from a TU matrix, rather than the (usually smaller) number of modified columns.

#### Related work.

Several lower bounds for kernelization for ilpf and other ILP-related problems follow already from lower bounds for other (less general) problems. For example, unless and the polynomial hierarchy collapses222 is a standard assumption in computational complexity. It is stronger than and , and it is known that implies a collapse of the polynomial hierarchy., there is no efficient algorithm that reduces every instance  of ilpf to an equivalent instance of size polynomial in  (here  refers to the number of columns in ); this follows from lower bounds for hitting set [9] or for satisfiability [8] and, thus, holds already for binary variables (-ilpf). The direct study of kernelization properties of ILPs was initiated in [15, 16] and focused on the influence of row- and column-sparsity of  on having kernelization results in terms of the dimensions  and  of . At high level, the outcome is that unbounded domain variables rule out essentially all nontrivial attempts at polynomial kernelizations. In particular, ilpf admits no kernelization to size polynomial in  when variable domains are unbounded, unless ; this remains true under strict bounds on sparsity [15]. For bounded domain variables the situation is a bit more positive: there are generalizations of positive results for -hitting set and -satisfiability (when sets/clauses have size at most ). One can reduce to size polynomial in  in general [16], and to size polynomial in  when seeking a feasible  with  for a sparse covering ILP [15].

#### Organization.

Section 2 contains preliminaries about parameterized complexity, graphs, and treewidth. In Section 3 we analyze the effect of treewidth on preprocessing ILPs, while we consider the effect of large totally unimodular submatrices in Section 4. In Section 5 we discuss some differences between totally unimodular and bounded-treewidth subsystems. We conclude in Section 6.

## 2 Preliminaries

#### Parameterized complexity and kernelization.

A parameterized problem is a set  where  is any finite alphabet and  denotes the non-negative integers. In an instance  the second component is called the parameter. A parameterized problem  is fixed-parameter tractable () if there is an algorithm that, given any instance , takes time  and correctly determines whether ; here  is any computable function. A kernelization for  is an algorithm  that, given , takes time polynomial in  and returns an instance  such that  if and only if  (i.e., the two instances are equivalent) and ; here  is any computable function, and we also call it the size of the kernel. If  is polynomially bounded in , then  is a polynomial kernelization. We also define (polynomial) compression; the only difference with kernelization is that the output is any instance  with respect to any fixed language , i.e., we demand that  if and only if  and that . A polynomial-parameter transformation from a parameterized problem  to a parameterized problem  is a polynomial-time mapping that transforms each instance  of  into an equivalent instance  of , with the guarantee that  if and only if  and  for some polynomial .

#### Lower bounds for kernelization.

For one of our lower bound proofs we use the notion of a cross-composition from [7], which builds on the framework for lower bounds for kernelization by Bodlaender et al. [5] and Fortnow and Santhanam [11].

###### Definition 1.

An equivalence relation  on is called a polynomial equivalence relation if the following two conditions hold:

1. There is an algorithm that given two strings  decides whether  and  belong to the same equivalence class in  time.

2. For any finite set  the equivalence relation  partitions the elements of  into at most  classes.

###### Definition 2.

Let  be a set and let  be a parameterized problem. We say that  cross-composes into  if there is a polynomial equivalence relation  and an algorithm that, given  strings  belonging to the same equivalence class of , computes an instance  in time polynomial in  such that:

1. for some ,

2. is bounded by a polynomial in .

###### Theorem 1 ([7]).

If the set  is NP-hard under Karp reductions and  cross-composes into the parameterized problem , then there is no polynomial kernel or compression for  unless .

#### Graphs.

All graphs in this work are simple, undirected, and finite. For a finite set  and positive integer , we denote by  the family of size- subsets of . The set  is abbreviated as . An undirected graph  consists of a vertex set  and edge set . For a set  we use  to denote the subgraph of  induced by . We use  as a shorthand for . For  we use  to denote the open neighborhood of . For  we define . The boundary of  in , denoted , is the set of vertices in  that have a neighbor in .

#### Treewidth and protrusion decompositions.

A tree decomposition of a graph  is a pair , where  is a tree and  is a family of subsets of  called bags, such that (i) , (ii) for each edge  there is a node  with , and (iii) for each  the nodes  induce a connected subtree of . The width of the tree decomposition is . The treewidth of a graph , denoted , is the minimum width over all tree decompositions of . An optimal tree decomposition of an -vertex graph  can be computed in time  using Bodlaender’s algorithm [3]. A -approximation to treewidth can be computed in time  using the recent algorithm of Bodlaender et al. [6]. A vertex set  such that  is called a treewidth- modulator.

For a positive integer , an -protrusion in a graph  is a vertex set  such that  and . An -protrusion decomposition of a graph  is a partition  of  such that (1) for every  we have , (2) , and (3) for every  the set  is an -protrusion in . We sometimes refer to  as the shared part.

## 3 ILPs of bounded treewidth

We analyze the influence of treewidth for preprocessing ilpf. In Section 3.1 we give formal definitions to capture the treewidth of an ILP, and introduce a special type of tree decompositions to solve ILPs efficiently. In Section 3.2 we study the parameterized complexity of ilpf parameterized by treewidth. Tractability turns out to depend on the domain of the variables. An instance  of ilpf has domain size  if, for every variable , there are constraints  and  for some  and . (All positive results work also under more relaxed definitions of domain size , e.g., any choice of  integers for each variable, at the cost of technical complication.) The feasibility of bounded-treewidth, bounded-domain ILPs is used in Section 3.3 to formulate a protrusion replacement rule. It allows the number of variables in an ILP of domain size  that is decomposed by a -protrusion decomposition to be reduced to . In Section 3.4 we discuss limitations of the protrusion-replacement approach.

### 3.1 Tree decompositions of linear programs

Given a constraint matrix  we define the corresponding Gaifman graph  as follows [10, Chapter 11]. We let , i.e., the variables in  for . We let  if and only if there is an  with  and . Intuitively, two vertices are adjacent if the corresponding variables occur together in some constraint.

###### Observation 1.

For every row  of , the variables  with nonzero coefficients in row  form a clique in . Consequently (cf. [4]), any tree decomposition  of  has a node  with .

To simplify the description of our dynamic programming procedure, we will restrict the form of the tree decompositions that the algorithm is applied to. This is common practice when dealing with graphs of bounded treewidth: one works with nice tree decompositions consisting of leaf, join, forget, and introduce nodes. When using dynamic programming to solve ILPs it will be convenient to have another type of node, the constraint node, to connect the structure of the Gaifman graph to the constraints in the ILP. To this end, we define the notion of a nice Gaifman decomposition including constraint nodes.

###### Definition 3.

Let . A nice Gaifman decomposition of  of width  is a triple , where  is a rooted tree and  is a width  tree decomposition of the Gaifman graph  with:
(1) The tree  has at most  nodes.
(2) Every row of  is assigned to exactly one node of . If row  is mapped to node  then  is a list of pointers to the nonzero coefficients in row .
(3) Every node  of  has one of the following types:

leaf:

has no children and ,

join:

has exactly two children  and ,

introduce:

has exactly one child  and  with ,

forget:

has exactly one child  and  with ,

constraint:

has exactly one child , and  stores a constraint of  involving variables that are all contained in .

The following proposition shows how to construct the Gaifman graph  for a given matrix . It will be used in later proofs.

###### Proposition 1.

Given a matrix  in which each row contains at most  nonzero entries, the  adjacency matrix of  can be constructed in  time.

###### Proof.

Initialize an all-zero  adjacency matrix  in  time. Scan through  to collect the indices of the non-zero entries in each row in  time. For each row , for each of the  pairs  of distinct nonzero entries in the row, set the corresponding entries  of the adjacency matrix to one. ∎

We show how to obtain a nice Gaifman decomposition for a matrix  of width  from any tree decomposition of its Gaifman graph  of width .

###### Proposition 2.

There is an algorithm that, given  and a width- tree decomposition  of the Gaifman graph of , computes a nice Gaifman decomposition  of  having width  in  time.

###### Proof.

Building a nice tree decomposition. From the tree decomposition  of  we can derive a chordal supergraph of  with maximum clique size bounded by , by completing the vertices of each bag into a clique [14, Lemma 2.1.1]. This can be done in  time by scanning through the contents of the bags of . A perfect elimination order of the chordal supergraph can be used to obtain a nice tree decomposition  of  having width  on at most  nodes [14, Lemma 13.1.3]. The nice tree decomposition consists of leaf, join, introduce, and forget nodes.

Incorporating constraint nodes. We augment the nice tree decomposition with constraint nodes to obtain a nice Gaifman decomposition of , as follows. We scan through matrix  and store, for each row, a list of pointers to the nonzero entries in that row. This takes  time. Since a graph of treewidth  does not have cliques of size more than , by Observation 1 each row of  has at most  nonzero entries. We maintain a list of the rows in  that have not yet been associated to a constraint bag in the Gaifman decomposition. We traverse the rooted tree  in post-order. For each node , we inspect the corresponding bag  and test, for each constraint that is not yet represented by the decomposition, whether all variables involved in the constraint are contained in the bag. This can be determined in  time per constraint as follows. For each variable in  we test whether the corresponding row in  has a nonzero coefficient for that variable; if so, we increase a counter. If the final value of the counter matches the precomputed number of nonzero coefficients in the row then the bag contains all variables involved in the constraint. In that case we update the tree  as follows: we make a new node , assign , and let  be a copy of the precomputed list of pointers to the nonzero coefficients in the constraint. We make  the parent of . If  is not the root, then it originally had a parent ; we make  the parent of  instead. This operation effectively splices a node of degree two into the tree. Since the newly introduced node has the same bag as , the relation between the bags of parents and children for the existing nodes of the tree remains unaltered (e.g., a forget node in the old tree will be a forget node in the new tree). The newly introduced node  satisfies the requirements of a constraint node. We then continue processing the remainder of the tree to obtain the final nice Gaifman decomposition . As the original tree contains  nodes, while we spend  time per node to incorporate the constraint bags, this phase of the algorithm takes  time. By Observation 1, for each constraint of  the involved variables occur together in some bag. Hence we will detect such a bag in the procedure, which results in a constraint node for the row. Since the nice tree decomposition that we started from contained at most  nodes, while we introduce one node for each constraint in , the resulting tree has at most  nodes. This shows that  satisfies all properties of a nice Gaifman decomposition and concludes the proof. ∎

### 3.2 Feasibility on Gaifman graphs of bounded treewidth

We discuss the influence of treewidth on the complexity of ilpf. It turns out that for unbounded domain variables the problem remains weakly -hard on instances with Gaifman graphs of treewidth at most two (Theorem 2). On the other hand, the problem can be solved by a simple dynamic programming algorithm with runtime , where  is the domain size and  denotes the width of a given tree decomposition of the Gaifman graph (Theorem 3). In other words, the problem is fixed-parameter tractable in terms of , and efficiently solvable for bounded treewidth and  polynomially bounded in the input size.

Both results are not hard to prove and fixed-parameter tractability of ilpf() can also be derived from Courcelle’s theorem (cf. [10, Corollary 11.43]). Nevertheless, for the sake of self-containment and concrete runtime bounds we provide direct proofs. Theorem 3 is a subroutine of our protrusion reduction algorithm.

###### Theorem 2.

ilp feasibility remains weakly -hard when restricted to instances whose Gaifman graph  has treewidth two.

###### Proof.

We give a straightforward reduction from subset sum to this restricted variant of ilpf. Recall that an instance of subset sum consists of a set  of integers and a target value ; the task is to determine whether some subset of the  integers sums to exactly . Given such an instance  we create  variables  that encode the selection of a subset and  variables  that effectively store partial sums; the  variables are constrained to domain . Concretely, we aim to compute

 yj=j∑i=1aixi

for all . Clearly, this is correctly enforced by the following constraints.

 y1 =a1x1 yj =ajxj+yj−1 j∈{2,…,n}

Finally, we enforce . Clearly, a subset of the  integers with sum  translates canonically to a feasible assignment of the variables, and vice versa.

It remains to check the treewidth of the corresponding Gaifman graph. We note that for this purpose it is not necessary to split equalities into inequalities or performing similar normalizations since it does not affect whether sets of variables occur in at least one shared constraint. Thus, we can make a tree decomposition (in fact, a path decomposition) consisting of a path on nodes  with bags  where  and  for . Clearly, for each constraint there is a bag containing all its variables, correctly handling all edges of the Gaifman graph, and the bags containing any variable form a connected subtree. It follows that the Gaifman graph of the constructed instance has treewidth at most two. ∎

###### Theorem 3.

Instances  of ilpf of domain size  with a given nice Gaifman decomposition of width  can be solved in time .

###### Proof.

Let  denote an instance of ilpf of domain size  and let  denote a given nice Gaifman decomposition of width  for . We describe a simple dynamic programming algorithm for testing whether there exists an integer vector  such that . For ease of presentation we assume that each domain is ; it is straightforward, but technical, to use arbitrary (possibly different) domains of at most  values for each variable.

With a node , apart from its bag , we associate the set  of all variables appearing in  or in the bag  of any descendant  of . By  we denote all constraints  (rows of ) that the Gaifman decomposition maps to a descendant of  (including  itself).

Our goal is to compute for each node  the set of all feasible assignments to the variables in  when taking into account only constraints in . The set of feasible assignments for  will be recorded in a table  indexed by tuples  where . The entry at  corresponds to assigning  where  and . Its value will be  if we determined that there is a feasible assignment for  with respect to constraints  that extends ; otherwise the value is . We will now describe how to compute the tables  in a bottom-up manner, by outlining the behavior on each node type.

• Leaf node  with bag . Since  as  has no children, we simply have  for all  which is computed in  time.

• Forget node  with child  and bag . It suffices to project  down to only contain information about the feasible assignments for . To make this concrete, let  with  and . Thus, . We let

 Fi(a1,…,as−1,as+1,…,aℓ)=maxa∈{0,…,d−1}Fj(a1,…,as−1,a,as+1,…,aℓ),

for all , i.e., we set  to  if some choice of  extends the assignment to one that is feasible for  with respect to all constraints on ; else it takes value . Each table entry takes time  and in total we spend time ; note that  implies .

• Introduce node  with child  and bag . Let  with  and , implying that . As  is an introduce node we have , and therefore  does not constrain the value of  in any way. We set  as follows.

 Fi(a1,…,aℓ)=Fj(a1,…,as−1,as+1,…,aℓ),

for all . We use time .

• Join node  with children  and bag . At a join node , from child  we get all assignments that are feasible for  regarding constraints , and from child  we get the feasible assignments for constraints . We have , and therefore an assignment is feasible for  if and only if it is feasible for both  and . It suffices to merge the information of the two child nodes. Letting  with , we set

 Fi(a1,…,aℓ)=min{Fj(a1,…,aℓ),Fj′(a1,…,aℓ)},

for all . This takes time .

• Constraint node  with child  mapped to row . Let  with . We know that  and therefore an assignment of values to  is feasible with respect to the constraints  if and only if it is feasible for the constraints  and also satisfies constraint . We therefore initialize by setting  as follows.

 Fi(a1,…,aℓ)=Fj(a1,…,aℓ),

for all . Now, we need to discard those assignments  that are not feasible for the additional constraint . For each  with  we process the row . Using the pointers to the nonzero coefficients in row  that are stored in , along with the fact that all variables constrained by  are contained in bag  by Definition 3, we can evaluate the constraint in  time. If the sum of values times coefficients exceeds  then the constraint is not satisfied and we set  to . This takes time  per assignment. In total we use time .

At the end of the computation we have the table  where  denotes the root of the tree decomposition . By definition, it encodes all assignments to  that can be extended to assignments that are feasible for all constraints in . By Definition 3 the set  for the root contains all constraints of  and thus any entry  implies that  has an integer solution. Conversely, any integer solution must lead to a nonzero entry in . By Definition 3 the number of nodes in  is . The total time needed for the dynamic programming is therefore bounded by . ∎

If a nice Gaifman decomposition is not given, one can be computed by combining an algorithm for computing tree decompositions [3, 6] with Proposition 2.

### 3.3 Protrusion reductions

To formulate the protrusion replacement rule, which is the main algorithmic asset used in this section, we need some terminology. For a non-negative integer , an -boundaried ILP is an instance  of ilpf in which  distinct boundary variables  are distinguished among the total variable set . If  is a sequence of variables of , we will also use  to denote the corresponding -boundaried ILP. The feasible boundary assignments of a boundaried ILP are those assignments to the boundary variables that can be completed into an assignment that is feasible for the entire system.

###### Definition 4.

Two -boundaried ILPs  and  are equivalent if they have the same feasible boundary assignments.

The following lemma shows how to compute equivalent boundaried ILPs for any boundaried input ILP. The replacement system is built by adding, for each infeasible boundary assignment, a set of constraints on auxiliary variables that explicitly blocks that assignment.

###### Lemma 1.

There is an algorithm with the following specifications: (1) It gets as input an -boundaried ILP  with domain size , with , and a width- nice Gaifman decomposition  of . (2) Given such an input it takes time . (3) Its output is an equivalent -boundaried ILP  of domain size  containing  variables and constraints, and all entries of  in .

###### Proof.

The lemma is a combination of two ingredients. Using Theorem 3 we can efficiently test whether a given assignment to the boundary variables can be extended to a feasible solution for . Then, knowing the set of all assignments that can be feasibly extended, we block each infeasible partial assignment by introducing a small number of variables and constraints, allowing us to fully discard the original constraints and non-boundary variables. The latter step uses a construction from an earlier paper by Kratsch [16, Theorem 5], which we repeat here for completeness.

Finding feasible partial assignments. Consider a partial assignment  to the boundary variables with . To determine whether this assignment can be extended to a feasible assignment for , we enforce these equalities in the ILP. Concretely, for each  we replace the domain-bounding constraints for  in the system  by  and . We obtain a new system  with  constraints. Since the modified constraints involve only a single variable each, the modifications do not affect the Gaifman graph: . Moreover, the modifications do not affect which entries in the constraint matrix have nonzero values, implying that  also serves as a nice Gaifman decomposition of . The partial assignment to the boundary variables can be feasibly extended if and only if  is feasible. We may invoke the algorithm of Theorem 3 with  to decide feasibility in  time. By iterating over all  possible partial assignments to the boundary variables with values in , we determine which partial assignments can be feasibly extended. Let  be a list of the partial assignments that can not be extended to a feasible solution for .

Blocking infeasible partial assignments. Using  we construct an equivalent -boundaried ILP  as follows. Based on the length of  we can determine the number of variables that will be used in , which helps to write down the constraint matrix efficiently. The number of variables in the new system will be , the number of constraints will be . The system is built as follows. For each boundary variable  of  we introduce a corresponding variable  in  and constrain its domain to  using two inequalities; this yields  variables and  constraints.

For each infeasible partial assignment  in the list , we add new variables  and  for all , together with the following constraints:

 ∀i∈[r]: x′ti =aji+uji−d⋅vji uji∈{0,…,d−1},vji∈{0,1} (1) r∑i=1uji ≥1 (2)

We claim that an assignment to the boundary variables can be extended to the newly introduced variables to satisfy the constraints for  if and only if the partial assignment is not . In the first direction, assume that . Then , implying that  (taking into account the domains of  and ) for all . Therefore constraint (2) is violated which shows that the partial assignment can not be feasibly extended. In the other direction, if , then there is a position  with . It follows that  (due to the domain of ) which in turn implies that  since the contribution of  to the equality (1) is a multiple of . Therefore constraint (2) is fulfilled.

The only coefficients used in the constraints that block a partial assignment are ; as the equalities of (1) are represented using two inequalities, we get coefficients  and . The values of , which appear in the right-hand side vector , are in  since they arise from the coordinates of an attempted partial assignment (with negative values from representing equalities by inequalities). As the coefficients of the domain-enforcing constraints are all plus or minus one, with right-hand side in , the structure of the constructed system  matches that described in the lemma statement. For each infeasible partial assignment we introduce  variables with  domain-enforcing constraints each, along with  inequalities to express the  equalities of (1), and a single constraint for (2). The total system therefore has  variables and  constraints. Since there are only  partial assignments that we check, we have  and therefore the system has  variables and constraints. Consequently, the constraint matrix  has  entries and it is not hard to verify that it can be written down in linear time. It remains to prove that the -boundaried ILP  is equivalent to the input structure.

Consider an assignment to the boundary variables. If the assignment can be extended to a feasible assignment for , then the boundary variables take values in  (since  has domain size ) and therefore satisfy the domain restrictions of . Since the partial assignment has a feasible extension, it is not on the list . For each set of constraints that was added to block an infeasible partial assignment , the claim above therefore shows that the related variables  and  can be set to satisfy their constraints. Hence the partial assignment can be extended to a feasible assignment for . In the reverse direction, suppose that a partial assignment can be feasibly extended for . By the claim above, the partial assignment differs from each of the blocked points on . Since  contains all infeasible partial assignments with values in , and feasible partial assignments for  belong to  since we restricted the domain of , there is an extension feasible for . This shows that the two -boundaried ILPs are indeed equivalent, and concludes the proof. ∎

Intuitively, we can simplify an ilpf instance with a given protrusion decomposition by replacing all protrusions with equivalent boundaried ILPs of small size via Lemma 1. We get a new instance containing all replacement constraints plus all original constraints that are fully contained in the shared part.

###### Theorem 4.

For each constant  there is an algorithm that, given an instance of ilpf with domain size , along with a -protrusion decomposition  of the given Gaifman graph , outputs an equivalent instance  of ilpf with domain size  on  variables in time . Each constraint of  is either a constraint in  involving only variables from , or one of  new constraints with coefficients and right-hand side among .

###### Proof.

The main idea of the proof is to apply Lemma 1 to replace each protrusion in the Gaifman graph by a small subsystem that is equivalent with respect to the boundary variables. For the sake of efficiency, we start by scanning through  once to compute for each row of  a list of pointers to the nonzero coefficients in that row. This takes  time. We handle the protrusions  for  (this implies ) consecutively, iteratively replacing each protrusion by a small equivalent boundaried ILP to obtain an equivalent instance.

Replacing protrusions. Consider some  with ; we show how to replace the variables  by a small subsystem that has the same effect on the existence of global solutions. The definition of protrusion decomposition ensures that  and that  has treewidth at most . From the system  we extract the constraints involving at least one variable in . We collect these constraints, together with domain-enforcing constraints for , into a subsystem . Let  and  be the number of variables and constraints in , respectively. Since  by the definition of a protrusion decomposition, we have . Since the nonzero variables involved in a constraint induce a clique in the Gaifman graph, while  has treewidth at most  and therefore does not have cliques of size more than , it follows that a constraint involving a variable from  acts on at most  variables. We can therefore identify the constraints involving a variable in  by only inspecting the rows of  containing at most  nonzero entries, which implies that the system  can be extracted in  time.

Let  be the neighbors of  in , i.e., the variables of  that appear in a common constraint with a variable in . As  is a constant, we can compute a tree decomposition  of  of width  with  bags in  time [3, 6]. Using Proposition 2 this yields a nice Gaifman decomposition  of  in  time. Interpreting  as an -boundaried ILP, we invoke Lemma 1 to compute an equivalent -boundaried ILP  in  time for constant . By Lemma 1, the numbers in the system  are restricted to the set  and  has  variables and constraints. We modify the instance  as follows, while preserving the fact that it has domain size . We remove all variables from  and all constraints involving them from the system . For each non-boundary variable in  we add a corresponding new variable to . For each constraint in , containing some boundary variables and some non-boundary variables, we add a new constraint with the same coefficients and right-hand side to . All occurrences of boundary variables  of  are replaced by the corresponding existing variables  of ; occurrences of non-boundary variables are replaced by occurrences of the corresponding newly introduced variables.

Observe that these replacements preserve the variable set , and that the newly introduced constraints only involve  and newly introduced variables. We can therefore perform this replacement step independently for each protrusion  with . Since each variable set  for  is removed and replaced by a new set of  variables, the final system  resulting from these replacements has  variables, which is  since the definition of a -protrusion decomposition ensures that . When building , the procedure above removes from  all constraints that involve at least one variable in  with . Hence the only constraints in  are (1) the constraints of  that only involve variables in , and (2) the