a generalization of nemhauser and trotter's local ... · a generalization of nemhauser and...
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A Generalization of Nemhauser and Trotter’sLocal Optimization Theorem
Michael R. Fellows1, Jiong Guo2,Hannes Moser2, and Rolf Niedermeier2
1 University of Newcastle, Australia
2 Friedrich-Schiller-Universitat Jena, Germany
STACS 2009
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 1/16
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The Vertex Cover Problem
Vertex Cover
Input: An undirected graph G = (V ,E ) and aparameter k ≥ 0.
Question: Can we find a vertex set S ⊆ V , |S | ≤ k, such thateach edge has a least one endpoint in S .
Example
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 2/16
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The Vertex Cover Problem
Vertex Cover
Input: An undirected graph G = (V ,E ) and aparameter k ≥ 0.
Question: Can we find a vertex set S ⊆ V , |S | ≤ k, such thateach edge has a least one endpoint in S .
Example
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 2/16
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Nemhauser and Trotter’s Local Optimization Theorem
NT-Theorem [Nemhauser & Trotter, Math. Program. 1975]
For G = (V ,E ) one can compute in polynomial time a partitionof V into three subsets A, B, and C :
A
BC
1. There is a min.-cardinality vertex cover S of G with A ⊆ S
2. If S ′ is a vertex cover of G [C ], then A ∪ S ′ is a vertex coverof G
3. Every vertex cover of G [C ] has size at least |C |/2
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 3/16
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Consequences
A
BC
I A ∪ C is a factor-2 approximate vertex cover of G .
I G [C ] is a 2k-vertex problem kernel for Vertex Cover.
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 4/16
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Fixed-Parameter Tractability and Problem Kernel
Fixed-Parameter Tractability
A parameterized problem with input instance (I , k) isfixed-parameter tractable with respect to parameter k if it can besolved in f (k) · poly(|I |) time.
Problem Kernel
(I , k)data reduction rules
(I ′, k ′)poly(|I |) time
I (I , k) ∈ L if and only if (I ′, k ′) ∈ L,
I k ′ ≤ k, and
I |I ′| ≤ g(k) for some function g
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 5/16
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Generalizing Vertex Cover
d -Bounded-Degree Deletion
Input: An undirected graph G = (V ,E ) and aparameter k ≥ 0.
Question: Can we find a vertex set S ⊆ V , |S | ≤ k, such thateach vertex in G [V \ S ] has degree at most d?
Example for d = 2
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 6/16
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Generalizing Vertex Cover
d -Bounded-Degree Deletion
Input: An undirected graph G = (V ,E ) and aparameter k ≥ 0.
Question: Can we find a vertex set S ⊆ V , |S | ≤ k, such thateach vertex in G [V \ S ] has degree at most d?
Example for d = 2
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 6/16
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Generalizing Vertex Cover
d -Bounded-Degree Deletion
Input: An undirected graph G = (V ,E ) and aparameter k ≥ 0.
Question: Can we find a vertex set S ⊆ V , |S | ≤ k, such thateach vertex in G [V \ S ] has degree at most d?
Example for d = 2
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 6/16
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Motivation: Finding Dense Subgraphs
I Finding max.-cardinality cliques is an important task inBioinformatics
I Successful approach: Transform to the dual Vertex Coverproblem[Chesler et al., Nature Genetics, 2005]
[Baldwin et al., J. Biomed. Biotechnol., 2005]
[Abu-Khzam et al., Theory Comput. Syst., 2007]
I Drawback: cliques are overly restrictive
I Use s-plexes instead of cliques
s-plex
A graph is an s-plex if each vertex isadjacent to all but ≤ s − 1 vertices. 3-plex
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 7/16
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Motivation: Finding Dense Subgraphs
I Finding max.-cardinality cliques is an important task inBioinformatics
I Successful approach: Transform to the dual Vertex Coverproblem[Chesler et al., Nature Genetics, 2005]
[Baldwin et al., J. Biomed. Biotechnol., 2005]
[Abu-Khzam et al., Theory Comput. Syst., 2007]
I Drawback: cliques are overly restrictive
I Use s-plexes instead of cliques
s-plex
A graph is an s-plex if each vertex isadjacent to all but ≤ s − 1 vertices. 3-plex
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 7/16
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Motivation: Finding Dense Subgraphs
Maximum-cardinality 4-plex infission yeast protein-protein in-teraction network
Corresponding complement
(Data source: www.thebiogrid.org)
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 8/16
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Known Results for d -Bounded-Degree Deletion
I NP-complete for all d ≥ 0[Lewis and Yannakakis, J. Comput. System Sci., 1980]
I Can be solved in time O((d + k)k+1 · n)[Nishimura, Ragde, Thilikos, Discrete Appl. Math., 2005]
I Enumeration of all minimal solutions intime O((d + 2)k · (k + d)2 ·m)[Komusiewicz, Huffner, Moser, Niedermeier, Theor. Comput. Sci.]
I Problem kernel of size 15k for d = 1Problem kernel of size O(k2) for constant d ≥ 2[Prieto and Sloper, Theor. Comput. Sci., 2006]
I Experimental study for d = 0 (Vertex Cover)[Abu-Khzam et al., ALENEX 2004]
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 9/16
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“NT-Theorem” for d -Bounded-Degree Deletion
For G = (V ,E ) one can compute in polynomial time a partitionof V into three subsets A, B, and C :
A
BC
1. There is a min.-cardinality solution S for G with A ⊆ S
2. If S ′ is a solution for G [C ], then A ∪ S ′ is a solution for G
3. Every solution for G [C ] has size at least
|C |d3 + 4d2 + 6d + 4
⇒ G [C ] is a (d3 + 4d2 + 6d + 4) · k-vertex problem kernel.
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 10/16
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“NT-Theorem” for d -Bounded-Degree Deletion
For G = (V ,E ) one can compute in polynomial time a partitionof V into three subsets A, B, and C :
A
BC
1. There is a min.-cardinality solution S for G with A ⊆ S
2. If S ′ is a solution for G [C ], then A ∪ S ′ is a solution for G
3. Every solution for G [C ] has size at least
|C |d3 + 4d2 + 6d + 4
⇒ G [C ] is a (d3 + 4d2 + 6d + 4) · k-vertex problem kernel.
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 10/16
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O(k2)-Vertex Problem Kernel for d -Bounded-Degree Del.
High-Degree Reduction Rule
If there exists a vertex v ∈ Vwith deg(v) > d + k, thendelete v and set k := k − 1.
v
Low-Degree Reduction Rule
If there exists a vertex v ∈ V suchthat ∀w ∈ N[v ] : deg(w) ≤ d ,then delete v .
v N(v)
≤ d
≤ d + k
≤ k. . .
“low-degree vertices”
“high-degree vertices” . . .
. . .
A
B
C
⇒ O(k2)-vertex kernelfor constant d
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 11/16
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O(k2)-Vertex Problem Kernel for d -Bounded-Degree Del.
High-Degree Reduction Rule
If there exists a vertex v ∈ Vwith deg(v) > d + k, thendelete v and set k := k − 1.
v
Low-Degree Reduction Rule
If there exists a vertex v ∈ V suchthat ∀w ∈ N[v ] : deg(w) ≤ d ,then delete v .
v N(v)
≤ d
≤ d + k
≤ k. . .
“low-degree vertices”
“high-degree vertices” . . .
. . .
A
B
C
⇒ O(k2)-vertex kernelfor constant d
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 11/16
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A Linear-Vertex Kernel
Basic ObservationA star with d + 1 leaves is a forbiddensubgraph for graphs of maximum degree d .
d = 2
First Step of Kernelization
Find a maximal collection of vertex-disjoint copies of a star withd + 1 leaves.
X
N(X )
N2(X )
. . . I G [V \ X ] hasmaximum degree d
I There are ≤ k starsin the collection
I |X | = O(k)
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 12/16
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A Linear-Vertex Kernel
Basic ObservationA star with d + 1 leaves is a forbiddensubgraph for graphs of maximum degree d .
d = 2
First Step of Kernelization
Find a maximal collection of vertex-disjoint copies of a star withd + 1 leaves.
X
N(X )
N2(X )
. . .
I G [V \ X ] hasmaximum degree d
I There are ≤ k starsin the collection
I |X | = O(k)
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 12/16
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A Linear-Vertex Kernel
Basic ObservationA star with d + 1 leaves is a forbiddensubgraph for graphs of maximum degree d .
d = 2
First Step of Kernelization
Find a maximal collection of vertex-disjoint copies of a star withd + 1 leaves.
X
N(X )
N2(X )
. . . I G [V \ X ] hasmaximum degree d
I There are ≤ k starsin the collection
I |X | = O(k)
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 12/16
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Ideal Situation
X
N2(X )
N(X )
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 13/16
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Ideal Situation
X
N(X )
N2(X )
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 13/16
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Ideal Situation
X
N(X )
N2(X )
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 13/16
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Ideal Situation
X
N(X )
N2(X )
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 13/16
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Ideal Situation
X
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 13/16
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Main Idea for Linear-Vertex Kernel
X
N(X )
N2(X )
ObservationFor each gray vertex in X there are at most d · (d + 1) greenvertices in V \ X .
⇒ The remaining graph contains O(k) vertices for constant d .⇒ d-Bounded-Degree Deletion admits an O(k)-vertex problemkernel.
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 14/16
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Main Idea for Linear-Vertex Kernel
X
N(X )
N2(X )
ObservationFor each gray vertex in X there are at most d · (d + 1) greenvertices in V \ X .
⇒ The remaining graph contains O(k) vertices for constant d .⇒ d-Bounded-Degree Deletion admits an O(k)-vertex problemkernel.
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 14/16
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Main Idea for Linear-Vertex Kernel
X
N(X )
N2(X )
ObservationFor each gray vertex in X there are at most d · (d + 1) greenvertices in V \ X .
⇒ The remaining graph contains O(k) vertices for constant d .⇒ d-Bounded-Degree Deletion admits an O(k)-vertex problemkernel.
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 14/16
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Main Idea for Linear-Vertex Kernel
u v
X
N(X )
N2(X )N[{u, v}] \ X
ObservationFor each gray vertex in X there are at most d · (d + 1) greenvertices in V \ X .
⇒ The remaining graph contains O(k) vertices for constant d .⇒ d-Bounded-Degree Deletion admits an O(k)-vertex problemkernel.
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 14/16
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Main Idea for Linear-Vertex Kernel
u v
X
N(X )
N2(X )N[{u, v}] \ X
ObservationFor each gray vertex in X there are at most d · (d + 1) greenvertices in V \ X .
⇒ The remaining graph contains O(k) vertices for constant d .⇒ d-Bounded-Degree Deletion admits an O(k)-vertex problemkernel.
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 14/16
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Main Idea for Linear-Vertex Kernel
u v
X
N(X )
N2(X )N[{u, v}] \ XN[{x , y , z}] \ X
x y z
ObservationFor each gray vertex in X there are at most d · (d + 1) greenvertices in V \ X .
⇒ The remaining graph contains O(k) vertices for constant d .⇒ d-Bounded-Degree Deletion admits an O(k)-vertex problemkernel.
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 14/16
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Main Idea for Linear-Vertex Kernel
u v
X
N(X )
N2(X )N[{u, v}] \ XN[{x , y , z}] \ X
x y z
ObservationFor each gray vertex in X there are at most d · (d + 1) greenvertices in V \ X .
⇒ The remaining graph contains O(k) vertices for constant d .⇒ d-Bounded-Degree Deletion admits an O(k)-vertex problemkernel.
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 14/16
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Main Idea for Linear-Vertex Kernel
u v
X
N[{u, v}] \ XN[{x , y , z}] \ X
x y z
ObservationFor each gray vertex in X there are at most d · (d + 1) greenvertices in V \ X .
⇒ The remaining graph contains O(k) vertices for constant d .⇒ d-Bounded-Degree Deletion admits an O(k)-vertex problemkernel.
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 14/16
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Main Idea for Linear-Vertex Kernel
u v
X
N[{u, v}] \ XN[{x , y , z}] \ X
x y z
ObservationFor each gray vertex in X there are at most d · (d + 1) greenvertices in V \ X .
⇒ The remaining graph contains O(k) vertices for constant d .⇒ d-Bounded-Degree Deletion admits an O(k)-vertex problemkernel.
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 14/16
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Outlook
Further Results
I Bounded-Degree Deletion is W [2]-complete for unbounded d .
I Implementation and experiments[Moser, Niedermeier, Sorge, Manuscript, submitted]
Future Research
I Further improvement of the kernel size.
I For which other problems does this technique work?
Fellows, Guo, Moser, Niedermeier A Generalization of Nemhauser and Trotter’s Local Optimization Theorem 15/16
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Thank you!
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