kernelization algorithms for graph and other structure modification problems
DESCRIPTION
Thesis defense on November 14th, 2011, in Montpellier.Jury:Stéphane Bessy, Bruno Durand, Frédéric Havet, Rolf Niedermeier, Christophe Paul & Ioan Todinca.TRANSCRIPT
Kernelization algorithms for graph and otherstructure modification problems
Anthony PEREZ
Supervisors: Stephane BESSY and Christophe PAUL
(AlGCo Team)
November 14
INTRODUCTION
(Graph) Modification problems
Input: A graph (or another structure) and a (graph) property G.Output: A minimum number of modification of the graph in order tosatisfy G.
modification: adding edges, deleting edges, deleting vertices, ...
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INTRODUCTION
(Graph) Modification problems
Input: A graph (or another structure) and a (graph) property G.Output: A minimum number of modification of the graph in order tosatisfy G.
modification: adding edges, deleting edges, deleting vertices, ...
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INTRODUCTION
(Graph) Modification problems
CLUSTER EDITING
Input: A graph G = (V ,E).Output: A set F ⊆ (V × V ) of minimum size such that the graphH = (V ,E M F ) is a disjoint union of cliques.
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INTRODUCTION
(Graph) Modification problems
CLUSTER EDITING
Input: A graph G = (V ,E).Output: A set F ⊆ (V × V ) of minimum size such that the graphH = (V ,E M F ) is a disjoint union of cliques.
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INTRODUCTION
(Graph) Modification problems
Cover a broad range of NP-Hard problems:
VERTEX COVER
FEEDBACK VERTEX SET
More general: F -MINOR DELETION
EDGE-MULTICUT
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INTRODUCTION
(Graph) Modification problems
Find applications in various domains:
bioinformaticsmachine learningrelational databasesimage processing
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INTRODUCTION
Different approaches
Most modification problems are NP-hard.How to solve them efficiently?
Approximation algorithmsExact exponential algorithmsPreprocessing steps (heuristics)
5 / 42
INTRODUCTION
Different approaches
Most modification problems are NP-hard.How to solve them efficiently?
Approximation algorithmsExact exponential algorithmsPreprocessing steps (heuristics)
5 / 42
INTRODUCTION
Different approaches
Most modification problems are NP-hard.How to solve them efficiently?
Approximation algorithmsExact exponential algorithmsPreprocessing steps (heuristics)
How to measure the efficiency of heuristics?
5 / 42
INTRODUCTION
Different approaches
Most modification problems are NP-hard.How to solve them efficiently?
Approximation algorithmsExact exponential algorithmsPreprocessing steps (heuristics)
Exploit the fact that the number of modifications needed should besmall compared to the instance size n.
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Outline of the talk
1 Parameterized complexity
Part I. Graph Modification Problems
2 Branches and generic reduction rules
3 PROPER INTERVAL COMPLETION
Part II. Different modification problems
4 Considered problems
5 FEEDBACK ARC SET IN TOURNAMENTS
PARAMETERIZED COMPLEXITY
Parameterized problem
G-MODIFICATION
Input: A graph G = (V ,E), k ∈ N.Parameter: k .Output: A set F ⊆ (V × V ) of size at most k such that the graphH = (V ,E M F ) belongs to G.
Idea. Measure the complexity of a problem with respect tosome parameter k .
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PARAMETERIZED COMPLEXITY
Parameterized problem
G-MODIFICATION
Input: A graph G = (V ,E), k ∈ N.Parameter: k .Output: A set F ⊆ (V × V ) of size at most k such that the graphH = (V ,E M F ) belongs to G.
Parameterized algorithmA problem parameterized by some k ∈ N admits a parameterizedalgorithm if it can be solved in time f (k) · nO(1).
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PARAMETERIZED COMPLEXITY
Kernels
Given an instance (I, k) of a parameterized problem L,a kernelization algorithm:
runs in time Poly(|I|+ k)
and outputs an instance (I′, k ′) such that:(i) (I, k) ∈ YES ⇔ (I′, k ′) ∈ YES(ii) |I′| 6 h(k) and k ′ 6 k
(I, k) (I ′, k ′)
|I ′| 6 h(k)k ′ 6 k
Poly(|I|+ k)
Do all parameterized problems admit polynomial kernels?
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PARAMETERIZED COMPLEXITY
Kernels
Given an instance (I, k) of a parameterized problem L,a kernelization algorithm:
runs in time Poly(|I|+ k)
and outputs an instance (I′, k ′) such that:(i) (I, k) ∈ YES ⇔ (I′, k ′) ∈ YES(ii) |I′| 6 h(k) and k ′ 6 k
Theorem (Folklore)Parameterized algorithm⇔ Kernelization algorithm
Do all parameterized problems admit polynomial kernels?
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PARAMETERIZED COMPLEXITY
Kernels
Given an instance (I, k) of a parameterized problem L,a kernelization algorithm:
runs in time Poly(|I|+ k)
and outputs an instance (I′, k ′) such that:(i) (I, k) ∈ YES ⇔ (I′, k ′) ∈ YES(ii) |I′| 6 h(k) and k ′ 6 k
Size: super-polynomial
Do all parameterized problems admit polynomial kernels?
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PARAMETERIZED COMPLEXITY
Kernels
Given an instance (I, k) of a parameterized problem L,a kernelization algorithm:
runs in time Poly(|I|+ k)
and outputs an instance (I′, k ′) such that:(i) (I, k) ∈ YES ⇔ (I′, k ′) ∈ YES(ii) |I′| 6 h(k) and k ′ 6 k
Size: super-polynomial
Do all parameterized problems admit polynomial kernels?
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PARAMETERIZED COMPLEXITY
Lower bounds for kernels
There exist some parameterized problems that do not admit polynomialkernels. (under a complexity-theoretic assumption)
(i) Or-composition [Bodlaender et al., 2008 - Fortnow and Santhanam, 2008]
(ii) Polynomial time and parameter transformations[Bodlaender et al., 2009]
(iii) Cross-composition [Bodlaender et al., 2011]
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Graph modification problems
2 Branches and generic reduction rules
3 PROPER INTERVAL COMPLETION
G-MODIFICATION
Input: A graph G = (V ,E), k ∈ N.
Parameter: k .
Output: A set F ⊆ (V × V ) of size at most k s.t. the graph H = (V ,E M F ) belongs to G.
BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Generic reduction rules
Connected component.
If G is hereditary and closed under disjoint union, remove anyconnected component C that belongs to G.
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Generic reduction rules
Sunflower.
Consider a finite forbidden induced subgraph of G (obstruction).For any pair e ⊆ (V ×V ) that belongs to a set of k + 1 obstructionspairwise intersecting exactly in e, transform G into (V ,E M {e}).
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Generic reduction rules
Sunflower.
Consider a finite forbidden induced subgraph of G (obstruction).For any pair e ⊆ (V ×V ) that belongs to a set of k + 1 obstructionspairwise intersecting exactly in e, transform G into (V ,E M {e}).
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Generic reduction rulesCritical clique.
Assume G is hereditary and closed under true twin addition.For any critical clique T with |T | > k + 1, remove |T | − (k + 1)arbitrary vertices from T .
v
u
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Generic reduction rulesCritical clique.
Assume G is hereditary and closed under true twin addition.For any critical clique T with |T | > k + 1, remove |T | − (k + 1)arbitrary vertices from T .
v
u
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Generic reduction rulesCritical clique.
Assume G is hereditary and closed under true twin addition.For any critical clique T with |T | > k + 1, remove |T | − (k + 1)arbitrary vertices from T .
Lemma [Bessy, Paul and P., 2010]
There always exists an optimal editionthat preserves the critical cliques.
k = 1
k = 1
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Branches: a natural idea
Reduce set of vertices that induce a graph belonging to G.The Connected Component rule is a Branch reduction rule.
Context: can be used on problems where G admits a so-calledadjacency decomposition.
Branch: set of vertices B ⊆ Vsuch that:
(i) G[B] ∈ G and,(ii) B is connected properly
to the rest of the graph.
B
G[B] ∈ G
G \ B
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Branches: a natural idea
Reduce set of vertices that induce a graph belonging to G.The Connected Component rule is a Branch reduction rule.
Context: can be used on problems where G admits a so-calledadjacency decomposition.
Branch: set of vertices B ⊆ Vsuch that:
(i) G[B] ∈ G and,(ii) B is connected properly
to the rest of the graph.
B
G[B] ∈ G
G \ B
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Branches: a natural idea
Reduce set of vertices that induce a graph belonging to G.The Connected Component rule is a Branch reduction rule.
Context: can be used on problems where G admits a so-calledadjacency decomposition.
Branch: set of vertices B ⊆ Vsuch that:
(i) G[B] ∈ G and,(ii) B is connected properly
to the rest of the graph.
B
G[B] ∈ G
G \ B
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Branches: a natural idea
Reduce set of vertices that induce a graph belonging to G.The Connected Component rule is a Branch reduction rule.
Context: can be used on problems where G admits a so-calledadjacency decomposition.
Branch: set of vertices B ⊆ Vsuch that:
(i) G[B] ∈ G and,(ii) B is connected properly
to the rest of the graph.
B
G[B] ∈ G
G \ B
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Outline
2 Branches and generic reduction rulesGeneric reduction rulesBranches
3 PROPER INTERVAL COMPLETION
Definition and known resultsBranchesReducing the branches
BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
DefinitionPROPER INTERVAL COMPLETION
Input: A graph G = (V ,E), k ∈ N.Parameter: k .Output: A set F ⊆ (V × V ) \ E of size at most k such thatH = (V ,E ∪ F ) is a proper interval graph.
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
DefinitionPROPER INTERVAL COMPLETION
Input: A graph G = (V ,E), k ∈ N.Parameter: k .Output: A set F ⊆ (V × V ) \ E of size at most k such thatH = (V ,E ∪ F ) is a proper interval graph.
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
DefinitionPROPER INTERVAL COMPLETION
Input: A graph G = (V ,E), k ∈ N.Parameter: k .Output: A set F ⊆ (V × V ) \ E of size at most k such thatH = (V ,E ∪ F ) is a proper interval graph.
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
DefinitionPROPER INTERVAL COMPLETION
Input: A graph G = (V ,E), k ∈ N.Parameter: k .Output: A set F ⊆ (V × V ) \ E of size at most k such thatH = (V ,E ∪ F ) is a proper interval graph.
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
DefinitionPROPER INTERVAL COMPLETION
Input: A graph G = (V ,E), k ∈ N.Parameter: k .Output: A set F ⊆ (V × V ) \ E of size at most k such thatH = (V ,E ∪ F ) is a proper interval graph.
NP-Complete [Golumbic et al., 1994]
FPT : O(24km) (motivated by applications in genomic research)[Kaplan, Shamir and Tarjan, 1994]
Polynomial kernel?
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
DefinitionPROPER INTERVAL COMPLETION
Input: A graph G = (V ,E), k ∈ N.Parameter: k .Output: A set F ⊆ (V × V ) \ E of size at most k such thatH = (V ,E ∪ F ) is a proper interval graph.
Theorem [Bessy and P., 2011]
The PROPER INTERVAL COMPLETION problem admits a kernel withO(k4) vertices.
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Some useful results
A graph is a proper interval graph if and only if:
it does not contain any of the following graphs as an inducedsubgraph.
claw p-cycle (p ≥ 4)3-sun net
[Wegner, 1967]
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Some useful results
A graph is a proper interval graph if and only if:
its vertices admit an ordering v1 . . . vn such that:
vivj ∈ E i < j ⇒ vivl , vlvj ∈ E , i < l < j
[Looges and Olartu, 1993]
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Generic reduction rules
Remarks. Proper interval graphs are hereditary and:
(i) closed under disjoint union:the Connected Component rule can be applied.
(ii) do not admit any claw or C4 as an induced subgraph:the Sunflower rule can be applied.
(iii) closed under true twin addition:the Critical Clique rule can be applied.
What about branches?
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Generic reduction rules
Remarks. Proper interval graphs are hereditary and:
(i) closed under disjoint union:the Connected Component rule can be applied.
(ii) do not admit any claw or C4 as an induced subgraph:the Sunflower rule can be applied.
(iii) closed under true twin addition:the Critical Clique rule can be applied.
What about branches?
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Generic reduction rules
Remarks. Proper interval graphs are hereditary and:
(i) closed under disjoint union:the Connected Component rule can be applied.
(ii) do not admit any claw or C4 as an induced subgraph:the Sunflower rule can be applied.
(iii) closed under true twin addition:the Critical Clique rule can be applied.
What about branches?
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Generic reduction rules
Remarks. Proper interval graphs are hereditary and:
(i) closed under disjoint union:the Connected Component rule can be applied.
(ii) do not admit any claw or C4 as an induced subgraph:the Sunflower rule can be applied.
(iii) closed under true twin addition:the Critical Clique rule can be applied.
What about branches?
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Adjacency decomposition
2 3 4 5 6 7 8 9(b)
(a)
9
2
1
4
3
5
6
7
8
7895
4321
6
1
Branches can be used on PROPER INTERVAL COMPLETION.
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Adjacency decomposition
2 3 4 5 6 7 8 9(b)
(a)
9
2
1
4
3
5
6
7
8
7895
4321
6
1
Branches can be used on PROPER INTERVAL COMPLETION.
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Adjacency decomposition
2 3 4 5 6 7 8 9(b)
(a)
9
2
1
4
3
5
6
7
8
7895
4321
6
1
Branches can be used on PROPER INTERVAL COMPLETION.
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
How to define a branch?
Consider the structure of a solution.Look at unaffected vertices.
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Branches
B1 BR B2
L b1 R C
B
bl
A subset B of V is a branch if:
(i) G[B] is a connected PIG with umbrella ordering σB = b1, . . . ,b|B|,(ii) The vertex set V \ B can be partitioned into sets L,R and C with:
no edges between B and Cevery vertex in L (resp. R) has a neighbor in BNB(L) ⊂ NB[b1] = {b1, . . . ,bl′}NB(R) ⊂ NB[b|B|] = {bl , . . . ,b|B|}NL(bi+1) ⊆ NL(bi) for every 1 ≤ i < l ′ and NR(bi) ⊆ NR(bi+1) forevery l ≤ i < |B|
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Branches
B1 BR B2
L b1 b|B| R C
B
A subset B of V is a branch if:
(i) G[B] is a connected PIG with umbrella ordering σB = b1, . . . ,b|B|,(ii) The vertex set V \ B can be partitioned into sets L,R and C with:
no edges between B and Cevery vertex in L (resp. R) has a neighbor in BNB(L) ⊂ NB[b1] = {b1, . . . ,bl′}NB(R) ⊂ NB[b|B|] = {bl , . . . ,b|B|}NL(bi+1) ⊆ NL(bi) for every 1 ≤ i < l ′ and NR(bi) ⊆ NR(bi+1) forevery l ≤ i < |B|
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Branches
B1 BR B2
L b1 b|B| R C
B
A subset B of V is a branch if:
(i) G[B] is a connected PIG with umbrella ordering σB = b1, . . . ,b|B|,(ii) The vertex set V \ B can be partitioned into sets L,R and C with:
no edges between B and Cevery vertex in L (resp. R) has a neighbor in BNB(L) ⊂ NB[b1] = {b1, . . . ,bl′}NB(R) ⊂ NB[b|B|] = {bl , . . . ,b|B|}NL(bi+1) ⊆ NL(bi) for every 1 ≤ i < l ′ and NR(bi) ⊆ NR(bi+1) forevery l ≤ i < |B|
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Branches
B1 BR B2
L b1 b|B| R C
B
blbl ′
A subset B of V is a branch if:
(i) G[B] is a connected PIG with umbrella ordering σB = b1, . . . ,b|B|,(ii) The vertex set V \ B can be partitioned into sets L,R and C with:
no edges between B and Cevery vertex in L (resp. R) has a neighbor in BNB(L) ⊂ NB[b1] = {b1, . . . ,bl′}NB(R) ⊂ NB[b|B|] = {bl , . . . ,b|B|}NL(bi+1) ⊆ NL(bi) for every 1 ≤ i < l ′ and NR(bi) ⊆ NR(bi+1) forevery l ≤ i < |B|
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Branches
B1 BR B2
L b1 b|B| R C
B
bl ′ bl
A subset B of V is a branch if:
(i) G[B] is a connected PIG with umbrella ordering σB = b1, . . . ,b|B|,(ii) The vertex set V \ B can be partitioned into sets L,R and C with:
no edges between B and Cevery vertex in L (resp. R) has a neighbor in BNB(L) ⊂ NB[b1] = {b1, . . . ,bl′}NB(R) ⊂ NB[b|B|] = {bl , . . . ,b|B|}NL(bi+1) ⊆ NL(bi) for every 1 ≤ i < l ′ and NR(bi) ⊆ NR(bi+1) forevery l ≤ i < |B|
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Branches
B1 BR B2
L b|B| R C
B
b1 bl ′ bl
A subset B of V is a branch if:
(i) G[B] is a connected PIG with umbrella ordering σB = b1, . . . ,b|B|,(ii) The vertex set V \ B can be partitioned into sets L,R and C with:
no edges between B and Cevery vertex in L (resp. R) has a neighbor in BNB(L) ⊂ NB[b1] = {b1, . . . ,bl′}NB(R) ⊂ NB[b|B|] = {bl , . . . ,b|B|}NL(bi+1) ⊆ NL(bi) for every 1 ≤ i < l ′ and NR(bi) ⊆ NR(bi+1) forevery l ≤ i < |B|
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Branches
B1 BR B2
L b|B| R C
B
b1 bl ′ bl
A subset B of V is a branch if:
(i) G[B] is a connected PIG with umbrella ordering σB = b1, . . . ,b|B|,(ii) The vertex set V \ B can be partitioned into sets L,R and C with:
no edges between B and Cevery vertex in L (resp. R) has a neighbor in BNB(L) ⊂ NB[b1] = {b1, . . . ,bl′}NB(R) ⊂ NB[b|B|] = {bl , . . . ,b|B|}NL(bi+1) ⊆ NL(bi) for every 1 ≤ i < l ′ and NR(bi) ⊆ NR(bi+1) forevery l ≤ i < |B|
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Branches
B1 BR B2
L b1 R C
B
bl
If L = ∅ (or R = ∅), B is a 1-branch, otherwise B is a 2-branchIf B is a clique, we call B a K-join
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Branches
B1 BR B2
L b1 R C
B
bl
If L = ∅ (or R = ∅), B is a 1-branch, otherwise B is a 2-branchIf B is a clique, we call B a K-join
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Outline
2 Branches and generic reduction rulesGeneric reduction rulesBranches
3 PROPER INTERVAL COMPLETION
Definition and known resultsBranchesReducing the branches
BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Reducing the K -joins
Cannot be done directly.
x y z t
Assuming the graph is reduced by the generic rules, we can removeO(k3) vertices from any K -join to obtain a clean K -join.
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Reducing the K -joins
Cannot be done directly.
A clean K -join does not intersect any claw or C4.
Assuming the graph is reduced by the generic rules, we can removeO(k3) vertices from any K -join to obtain a clean K -join.
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Reducing the K -joins
Cannot be done directly.
A clean K -join does not intersect any claw or C4.
Assuming the graph is reduced by the generic rules, we can removeO(k3) vertices from any K -join to obtain a clean K -join.
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Reducing the clean K -joins
Let B be a clean K -join of size at least 2k + 2. Let Bf be the k + 1 firstvertices of B, Bl be its k + 1 last vertices and M = B \ (Bf ∪ Bl).Remove the set of vertices M from G.
Bl (k + 1 vertices)MBf (k + 1 vertices)
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Reducing the clean K -joins
Let B be a clean K -join of size at least 2k + 2. Let Bf be the k + 1 firstvertices of B, Bl be its k + 1 last vertices and M = B \ (Bf ∪ Bl).Remove the set of vertices M from G.
Can be carried out in polynomial time!
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Reducing the branches
In polynomial time, the 1- and 2-branches can be reduced to O(k3)vertices.
B1
Remove
B
BR
2k + 1 vertices
R G \ (B ∪ R)
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Reducing the branches
In polynomial time, the 1- and 2-branches can be reduced to O(k3)vertices.
B1
Remove
B
BR
2k + 1 vertices
R G \ (B ∪ R)
Remove
B
2k + 1 vertices
BR
2k + 1 vertices
B1
LB2
RB′2B′1
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Main result
Theorem [Bessy and P., 2011]
The PROPER INTERVAL COMPLETION problem admits a kernel withO(k4) vertices.
1-branch 1-branch2-branchK -join K -join K -join
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Main result
Theorem [Bessy and P., 2011]
The PROPER INTERVAL COMPLETION problem admits a kernel withO(k4) vertices.
O(k3) O(k3) O(k3) O(k3) O(k3)O(k3)
1-branch 1-branch2-branchK -join K -join K -join
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BRANCHES AND GENERIC REDUCTION RULES PROPER INTERVAL COMPLETION
Main result
Theorem [Bessy and P., 2011]
The PROPER INTERVAL COMPLETION problem admits a kernel withO(k4) vertices.
Related result [Bessy, Paul and P., 2010]
The CLOSEST 3-LEAF POWER problem admits a kernel with O(k3)vertices.
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Different modification problems
4 Considered problems
5 FEEDBACK ARC SET IN TOURNAMENTS
Π-EDITION
Input: A dense set R of p-sized relations defined over an universe V , an integer k ∈ N.
Parameter: k .
Output: A set F ⊆ R of size at most k whose modification satisfies Π.
CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
FEEDBACK ARC SET IN TOURNAMENTS (FAST)
Input: A tournament T = (V ,A) and an integer k ∈ N.Parameter: k .Output: A set at most k arcs whose reversal results in an acyclictournament.
23 4 21
3
1 4
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CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
FEEDBACK ARC SET IN TOURNAMENTS (FAST)
Input: A tournament T = (V ,A) and an integer k ∈ N.Parameter: k .Output: A set at most k arcs whose reversal results in an acyclictournament.
NP-Complete [Charbit et al., 2007]
Admits constant-factor approximation algorithms [Kenyon-Mathieu andSchudy, 2007]
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CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
DENSE ROOTED TRIPLET INCONSISTENCY (RTI)
Input: A set of leaves V and a dense collection R of rooted binarytrees over three leaves of V .Parameter: k .Output: A set of at most k triplets whose modification leads to acollection admitting a consistent rooted binary tree defined over V .
ab bd dc cca b da
t1 t2 t3 t4
R := {ab|c, cd |b, ab|d , ac|d}R := {t1, t2, t3, t4}
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CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
DENSE ROOTED TRIPLET INCONSISTENCY (RTI)
Input: A set of leaves V and a dense collection R of rooted binarytrees over three leaves of V .Parameter: k .Output: A set of at most k triplets whose modification leads to acollection admitting a consistent rooted binary tree defined over V .
a ab bd d dc cca b
a b c d
t1 t2 t3 t4
R := {ab|c, cd |b, ab|d , ac|d}
T is not consistent withR
R := {t1, t2, t3, t4}
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CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
DENSE ROOTED TRIPLET INCONSISTENCY (RTI)
Input: A set of leaves V and a dense collection R of rooted binarytrees over three leaves of V .Parameter: k .Output: A set of at most k triplets whose modification leads to acollection admitting a consistent rooted binary tree defined over V .
a cb bd d ac dca b
a b c d
t1 t2 t3 t4
R := {ab|c, cd |b, ab|d , cd |a}
T is consistent withR
R := {t1, t2, t3, t4}
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CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
DENSE ROOTED TRIPLET INCONSISTENCY (RTI)
Input: A set of leaves V and a dense collection R of rooted binarytrees over three leaves of V .Parameter: k .Output: A set of at most k triplets whose modification leads to acollection admitting a consistent rooted binary tree defined over V .
NP-Complete [Barky et al., 2010]
Does not admit a constant-factor approximation algorithm yet
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Outline
4 Considered problemsFEEDBACK ARC SET IN TOURNAMENTS
DENSE ROOTED TRIPLET INCONSISTENCY
Conflict Packing
5 FEEDBACK ARC SET IN TOURNAMENTS
Reduction rulesConflict Packing
CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
Consistency
FAST (folklore)The following properties are equivalent:
(i) T is acyclic(ii) T does not contain any directed triangle
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CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
Consistency
RTI [Guillemot and Mnich, 2010]
The following properties are equivalent:(i) R is consistent(ii) R does not contain any conflict on four leaves
Conflict. Set of vertices C ⊆ V that does not admit a consistent rooted binary tree.
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CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
Parameterized complexity
FAST RTIFPT O∗(2
√k log k ) a FPT O∗(2k1/3 log k ) b
Kernel with O(k2) verticesa Kernel with O(k2) verticesb
Linear vertex-kernel c No such result known before.
a[Alon et al., 2009]b[Guillemot and Mnich, 2010]c[Bessy et al., 2009]
The linear vertex-kernel for FAST described by [Bessy et al., 2009]uses a constant-factor approximation algorithm.Their reduction rules can be adapted to RTI.But no constant-factor approximation!
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CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
Parameterized complexity
FAST RTIFPT O∗(2
√k log k ) a FPT O∗(2k1/3 log k ) b
Kernel with O(k2) verticesa Kernel with O(k2) verticesb
Linear vertex-kernel c No such result known before.
a[Alon et al., 2009]b[Guillemot and Mnich, 2010]c[Bessy et al., 2009]
The linear vertex-kernel for FAST described by [Bessy et al., 2009]uses a constant-factor approximation algorithm.Their reduction rules can be adapted to RTI.But no constant-factor approximation!
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CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
Parameterized complexity
FAST RTIFPT O∗(2
√k log k ) a FPT O∗(2k1/3 log k ) b
Kernel with O(k2) verticesa Kernel with O(k2) verticesb
Linear vertex-kernel c No such result known before.
a[Alon et al., 2009]b[Guillemot and Mnich, 2010]c[Bessy et al., 2009]
The linear vertex-kernel for FAST described by [Bessy et al., 2009]uses a constant-factor approximation algorithm.Their reduction rules can be adapted to RTI.But no constant-factor approximation!
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CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
Parameterized complexity
FAST RTIFPT O∗(2
√k log k ) a FPT O∗(2k1/3 log k ) b
Kernel with O(k2) verticesa Kernel with O(k2) verticesb
Linear vertex-kernel c No such result known before.
a[Alon et al., 2009]b[Guillemot and Mnich, 2010]c[Bessy et al., 2009]
The linear vertex-kernel for FAST described by [Bessy et al., 2009]uses a constant-factor approximation algorithm.Their reduction rules can be adapted to RTI.But no constant-factor approximation!
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CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
Parameterized complexity
FAST RTIFPT O∗(2
√k log k ) a FPT O∗(2k1/3 log k ) b
Kernel with O(k2) verticesa Kernel with O(k2) verticesb
Linear vertex-kernel c No such result known before.
a[Alon et al., 2009]b[Guillemot and Mnich, 2010]c[Bessy et al., 2009]
The linear vertex-kernel for FAST described by [Bessy et al., 2009]uses a constant-factor approximation algorithm.Their reduction rules can be adapted to RTI.But no constant-factor approximation!
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CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
Conflict Packing
[Paul, P. and Thomasse, 2011]
works on problems characterized by some finite conflicts.maximal collection of p-uplets disjoint conflits C.provides a lower bound on the number of modification required.implies that the instance induced by V \ V (C) is consistent.
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CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
Reduction rules
Remove any vertex that is not part of any directed triangle. a.acan be carried out in polynomial time.
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CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
Reduction rules
Safe partition
Assume V (T ) is ordered under some ordering σ, and let P be apartition of σ into intervals.
Vl
AI := {uv ∈ A | ∃ i u, v ∈ Vi}
V1 V2
AO := A \ AI
B is the set of backward arcs of AO (arcs vivj with i > j).
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CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
Reduction rules
Safe partition
Assume V (T ) is ordered under some ordering σ, and let P be apartition of σ into intervals.
Vl
AI := {uv ∈ A | ∃ i u, v ∈ Vi}
V1 V2
AO := A \ AI
B is the set of backward arcs of AO (arcs vivj with i > j).
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CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
Reduction rules
Safe partition
P is safe if there exist |B| arc-disjoint conflicts using arcs of AOonly.
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CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
Safe Partition Reduction Rule
[Bessy et al., 2009]
Let P be a safe partition of an ordered tournament T = (V ,A, σ).Reverse every arc of B and decrease k accordingly.
Use constant-factor approximation algorithm.Use Conflict Packing.
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CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
Safe Partition Reduction Rule
[Bessy et al., 2009]
Let P be a safe partition of an ordered tournament T = (V ,A, σ).Reverse every arc of B and decrease k accordingly.
Use constant-factor approximation algorithm.Use Conflict Packing.
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CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
Safe Partition Reduction Rule
[Bessy et al., 2009]
Let P be a safe partition of an ordered tournament T = (V ,A, σ).Reverse every arc of B and decrease k accordingly.
Main questionHow to compute a safe partition in polynomial time?
Use constant-factor approximation algorithm.Use Conflict Packing.
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CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
Safe Partition Reduction Rule
[Bessy et al., 2009]
Let P be a safe partition of an ordered tournament T = (V ,A, σ).Reverse every arc of B and decrease k accordingly.
Main questionHow to compute a safe partition in polynomial time?
Use constant-factor approximation algorithm.Use Conflict Packing.
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CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
Safe Partition Reduction Rule
[Bessy et al., 2009]
Let P be a safe partition of an ordered tournament T = (V ,A, σ).Reverse every arc of B and decrease k accordingly.
Main questionHow to compute a safe partition in polynomial time?
Use constant-factor approximation algorithm.Use Conflict Packing.
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CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
Conflict Packing
A conflict packing of a tournament is a maximal collection ofarc-disjoint directed triangles.
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CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
Conflict Packing
A conflict packing of a tournament is a maximal collection ofarc-disjoint directed triangles.
Can be computed greedily (i.e. in polynomial time).Let C be a conflict packing. If T = (V ,A) is a positive instance then|V (C)| 6 3k .
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CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
Conflict Packing
A conflict packing of a tournament is a maximal collection ofarc-disjoint directed triangles.
Conflict Packing Lemma [Paul, P. and Thomasse, 2011]
Let T = (V ,A) be a tournament. There exists an ordering of T whosebackward arcs uv are such that u, v ∈ V (C).
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CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
Conflict Packing
A conflict packing of a tournament is a maximal collection ofarc-disjoint directed triangles.
Lemma [Paul, P. and Thomasse, 2011]
Let T = (V ,A) be a tournament such that |V | > 4k . There exists a safepartition that can be computed in polynomial time.
proof
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CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
Conflict Packing
A conflict packing of a tournament is a maximal collection ofarc-disjoint directed triangles.
Corollary [Paul, P. and Thomasse, 2011]
FEEDBACK ARC SET IN TOURNAMENTS admits a kernel with at most 4kvertices.
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CONSIDERED PROBLEMS FEEDBACK ARC SET IN TOURNAMENTS
Application to the RTI problem
Remove vertices that do not belong to any conflictSafe Partition reduction ruleConflict Packing allows to find a Safe Partition
Theorem [Paul, P. and Thomasse, 2011]
DENSE ROOTED TRIPLET INCONSISTENCY admits a kernel with at most5k vertices.
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Conclusion
6 Our results
7 Open problems
OUR RESULTS OPEN PROBLEMS
Main results
Polynomial kernelsFirst polynomial kernels:
(i) CLOSEST 3-LEAF POWER(ii) PROPER INTERVAL COMPLETION(iii) COGRAPH EDGE-EDITION
Improved polynomial kernels:
(i) FEEDBACK ARC SET IN TOURNAMENTS(ii) DENSE ROOTED TRIPLET INCONSISTENCY(iii) DENSE BETWEENNESS and DENSE CIRCULAR ORDERING
joint works with: S. Bessy, F. Fomin, S. Gaspers, S. Guillemot, F. Havet, C. Paul,S. Saurabh and S. Thomasse.
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OUR RESULTS OPEN PROBLEMS
Main results
Lower bounds on kernelization:(i) For any l > 7, the Pl -FREE EDGE-DELETION problem
does not admit a polynomial kernel.(ii) For any l > 4, the Cl -FREE EDGE-DELETION problem
does not admit a polynomial kernel.
joint work with: S. Guillemot, F. Havet and C. Paul.
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OUR RESULTS OPEN PROBLEMS
Open problems
Do the FEEDBACK VERTEX SET IN TOURNAMENTS and CLUSTER
VERTEX DELETION problems admit linear vertex-kernels?
Characterize lower bounds for modification problems. details
Can we use branches on other problems?(e.g. CHORDAL DELETION)Can we use Conflict Packing on other problems?(e.g. (weakly)-fragile constraint modification problems)
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OUR RESULTS OPEN PROBLEMS
Open problems
Do the FEEDBACK VERTEX SET IN TOURNAMENTS and CLUSTER
VERTEX DELETION problems admit linear vertex-kernels?
Characterize lower bounds for modification problems. details
Can we use branches on other problems?(e.g. CHORDAL DELETION)Can we use Conflict Packing on other problems?(e.g. (weakly)-fragile constraint modification problems)
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OUR RESULTS OPEN PROBLEMS
Open problems
Do the FEEDBACK VERTEX SET IN TOURNAMENTS and CLUSTER
VERTEX DELETION problems admit linear vertex-kernels?
Characterize lower bounds for modification problems. details
Can we use branches on other problems?(e.g. CHORDAL DELETION)Can we use Conflict Packing on other problems?(e.g. (weakly)-fragile constraint modification problems)
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Merci de votre attention !