The document discusses kernelization algorithms for graph modification problems. It begins by introducing graph modification problems, which take as input a graph and property and output the minimum number of modifications to the graph to satisfy the property. It then discusses using parameterized complexity to more efficiently solve NP-hard graph modification problems. In particular, it covers the concept of kernels, which are polynomial-time algorithms that reduce an instance to an equivalent instance of size bounded by a function of the parameter. The document provides an overview of generic reduction rules and the concept of branches that can be applied to graph modification problems. It also introduces the specific problem of proper interval completion and known results about its parameterized complexity.
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Kernelization algorithms for graph and other structure modification problems
1. Kernelization algorithms for graph and other
structure modification problems
Anthony P EREZ
´
Supervisors: Stephane B ESSY and Christophe PAUL
(AlGCo Team)
November 14
2. I NTRODUCTION
(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 to
satisfy G.
modification: adding edges, deleting edges, deleting vertices, ...
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3. I NTRODUCTION
(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 to
satisfy G.
modification: adding edges, deleting edges, deleting vertices, ...
2 / 42
4. I NTRODUCTION
(Graph) Modification problems
C LUSTER E DITING
Input: A graph G = (V , E).
Output: A set F ⊆ (V × V ) of minimum size such that the graph
H = (V , E F ) is a disjoint union of cliques.
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5. I NTRODUCTION
(Graph) Modification problems
C LUSTER E DITING
Input: A graph G = (V , E).
Output: A set F ⊆ (V × V ) of minimum size such that the graph
H = (V , E F ) is a disjoint union of cliques.
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6. I NTRODUCTION
(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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7. I NTRODUCTION
(Graph) Modification problems
Find applications in various domains:
bioinformatics
machine learning
relational databases
image processing
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8. I NTRODUCTION
Different approaches
Most modification problems are NP-hard.
How to solve them efficiently?
Approximation algorithms
Exact exponential algorithms
Preprocessing steps (heuristics)
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9. I NTRODUCTION
Different approaches
Most modification problems are NP-hard.
How to solve them efficiently?
Approximation algorithms
Exact exponential algorithms
Preprocessing steps (heuristics)
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10. I NTRODUCTION
Different approaches
Most modification problems are NP-hard.
How to solve them efficiently?
Approximation algorithms
Exact exponential algorithms
Preprocessing steps (heuristics)
How to measure the efficiency of heuristics?
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11. I NTRODUCTION
Different approaches
Most modification problems are NP-hard.
How to solve them efficiently?
Approximation algorithms
Exact exponential algorithms
Preprocessing steps (heuristics)
Exploit the fact that the number of modifications needed should be
small compared to the instance size n.
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12. Outline of the talk
1 Parameterized complexity
Part I. Graph Modification Problems
2 Branches and generic reduction rules
3 P ROPER I NTERVAL C OMPLETION
Part II. Different modification problems
4 Considered problems
5 F EEDBACK A RC S ET IN TOURNAMENTS
13. PARAMETERIZED COMPLEXITY
Parameterized problem
G-M ODIFICATION
Input: A graph G = (V , E), k ∈ N.
Parameter: k .
Output: A set F ⊆ (V × V ) of size at most k such that the graph
H = (V , E F ) belongs to G.
Idea. Measure the complexity of a problem with respect to
some parameter k.
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14. PARAMETERIZED COMPLEXITY
Parameterized problem
G-M ODIFICATION
Input: A graph G = (V , E), k ∈ N.
Parameter: k .
Output: A set F ⊆ (V × V ) of size at most k such that the graph
H = (V , E F ) belongs to G.
Parameterized algorithm
A problem parameterized by some k ∈ N admits a parameterized
algorithm if it can be solved in time f (k ) · nO(1) .
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15. 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 | h(k ) and k k
(I , k ) (I , k )
Poly (|I | + k )
|I | h(k )
k k
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16. 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 | h(k ) and k k
Theorem (Folklore)
Parameterized algorithm ⇔ Kernelization algorithm
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17. 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 | h(k ) and k k
Size: super-polynomial
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18. 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 | h(k ) and k k
Size: super-polynomial
Do all parameterized problems admit polynomial kernels?
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19. PARAMETERIZED COMPLEXITY
Lower bounds for kernels
There exist some parameterized problems that do not admit polynomial
kernels. (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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20. Graph modification problems
2 Branches and generic reduction rules
3 P ROPER I NTERVAL C OMPLETION
G-M ODIFICATION
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 F ) belongs to G.
21. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Generic reduction rules
Connected component.
If G is hereditary and closed under disjoint union, remove any
connected component C that belongs to G.
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22. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
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 obstructions
pairwise intersecting exactly in e, transform G into (V , E {e}).
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23. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
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 obstructions
pairwise intersecting exactly in e, transform G into (V , E {e}).
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24. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Generic reduction rules
Critical 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 .
u
v
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25. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Generic reduction rules
Critical 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 .
u
v
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26. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Generic reduction rules
Critical 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 .
k =1
Lemma [Bessy, Paul and P., 2010]
There always exists an optimal edition
that preserves the critical cliques.
k =1
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27. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Branches: a natural idea
Reduce set of vertices that induce a graph belonging to G.
The Connected Component rule is a Branch reduction rule.
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28. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Branches: a natural idea
Reduce set of vertices that induce a graph belonging to G.
The Connected Component rule is a Branch reduction rule.
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29. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
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-called
adjacency decomposition.
Branch: set of vertices B ⊆ V
such that:
(i) G[B] ∈ G and,
(ii) B is connected properly
to the rest of the graph.
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30. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
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-called
adjacency decomposition.
G [B ] ∈ G
Branch: set of vertices B ⊆ V B
such that:
(i) G[B] ∈ G and,
(ii) B is connected properly
to the rest of the graph.
GB
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31. Outline
2 Branches and generic reduction rules
Generic reduction rules
Branches
3 P ROPER I NTERVAL C OMPLETION
Definition and known results
Branches
Reducing the branches
32. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Definition
P ROPER I NTERVAL C OMPLETION
Input: A graph G = (V , E), k ∈ N.
Parameter: k .
Output: A set F ⊆ (V × V ) E of size at most k such that
H = (V , E ∪ F ) is a proper interval graph.
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33. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Definition
P ROPER I NTERVAL C OMPLETION
Input: A graph G = (V , E), k ∈ N.
Parameter: k .
Output: A set F ⊆ (V × V ) E of size at most k such that
H = (V , E ∪ F ) is a proper interval graph.
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34. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Definition
P ROPER I NTERVAL C OMPLETION
Input: A graph G = (V , E), k ∈ N.
Parameter: k .
Output: A set F ⊆ (V × V ) E of size at most k such that
H = (V , E ∪ F ) is a proper interval graph.
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35. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Definition
P ROPER I NTERVAL C OMPLETION
Input: A graph G = (V , E), k ∈ N.
Parameter: k .
Output: A set F ⊆ (V × V ) E of size at most k such that
H = (V , E ∪ F ) is a proper interval graph.
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36. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Definition
P ROPER I NTERVAL C OMPLETION
Input: A graph G = (V , E), k ∈ N.
Parameter: k .
Output: A set F ⊆ (V × V ) E of size at most k such that
H = (V , E ∪ F ) is a proper interval graph.
NP-Complete [Golumbic et al., 1994]
FPT : O(24k m) (motivated by applications in genomic research)
[Kaplan, Shamir and Tarjan, 1994]
Polynomial kernel?
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37. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Definition
P ROPER I NTERVAL C OMPLETION
Input: A graph G = (V , E), k ∈ N.
Parameter: k .
Output: A set F ⊆ (V × V ) E of size at most k such that
H = (V , E ∪ F ) is a proper interval graph.
Theorem [Bessy and P., 2011]
The P ROPER I NTERVAL C OMPLETION problem admits a kernel with
O(k 4 ) vertices.
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38. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
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 induced
subgraph.
claw 3-sun net p-cycle (p ≥ 4)
[Wegner, 1967]
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39. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Some useful results
A graph is a proper interval graph if and only if:
its vertices admit an ordering v1 . . . vn such that:
vi vj ∈ E i < j ⇒ vi vl , vl vj ∈ E, i < l < j
[Looges and Olartu, 1993]
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40. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
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.
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41. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
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.
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42. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
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.
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43. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
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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44. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Adjacency decomposition
1 3
6 8
(a) 2
4 5 7 9
4
6 7
(b) 3
8
2 1 2 3 4 5 6 7 8 9
5 9
1
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45. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Adjacency decomposition
1 3
6 8
(a) 2
4 5 7 9
4
6 7
(b) 3
8
2 1 2 3 4 5 6 7 8 9
5 9
1
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46. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Adjacency decomposition
1 3
6 8
(a) 2
4 5 7 9
4
6 7
(b) 3
8
2 1 2 3 4 5 6 7 8 9
5 9
1
Branches can be used on P ROPER I NTERVAL C OMPLETION.
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47. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
How to define a branch?
Consider the structure of a solution.
Look at unaffected vertices.
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48. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Branches
B
B1 BR B2
b1 bl
L R C
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 C
every vertex in L (resp. R) has a neighbor in B
NB (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 ) for
every l ≤ i < |B|
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49. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Branches
B
B1 BR B2
b1 b|B|
L R C
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 C
every vertex in L (resp. R) has a neighbor in B
NB (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 ) for
every l ≤ i < |B|
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50. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Branches
B
B1 BR B2
b1 b|B|
L R C
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 C
every vertex in L (resp. R) has a neighbor in B
NB (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 ) for
every l ≤ i < |B|
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51. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Branches
B
B1 BR B2
b1 bl bl b|B|
L R C
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 C
every vertex in L (resp. R) has a neighbor in B
NB (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 ) for
every l ≤ i < |B|
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52. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Branches
B
B1 BR B2
b1 bl bl b|B|
L R C
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 C
every vertex in L (resp. R) has a neighbor in B
NB (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 ) for
every l ≤ i < |B|
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53. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Branches
B
B1 BR B2
b1 bl bl b|B|
L R C
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 C
every vertex in L (resp. R) has a neighbor in B
NB (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 ) for
every l ≤ i < |B|
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54. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Branches
B
B1 BR B2
b1 bl bl b|B|
L R C
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 C
every vertex in L (resp. R) has a neighbor in B
NB (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 ) for
every l ≤ i < |B|
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55. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Branches
B
B1 BR B2
b1 bl
L R C
If L = ∅ (or R = ∅), B is a 1-branch, otherwise B is a 2-branch
If B is a clique, we call B a K-join
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56. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Branches
B
B1 BR B2
b1 bl
L R C
If L = ∅ (or R = ∅), B is a 1-branch, otherwise B is a 2-branch
If B is a clique, we call B a K-join
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57. Outline
2 Branches and generic reduction rules
Generic reduction rules
Branches
3 P ROPER I NTERVAL C OMPLETION
Definition and known results
Branches
Reducing the branches
58. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Reducing the K -joins
Cannot be done directly.
x y z t
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59. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Reducing the K -joins
Cannot be done directly.
A clean K -join does not intersect any claw or C4 .
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60. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
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 remove
O(k 3 ) vertices from any K -join to obtain a clean K -join.
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61. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Reducing the clean K -joins
Let B be a clean K -join of size at least 2k + 2. Let Bf be the k + 1 first
vertices of B, Bl be its k + 1 last vertices and M = B (Bf ∪ Bl ).
Remove the set of vertices M from G.
Bf (k + 1 vertices) M Bl (k + 1 vertices)
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62. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Reducing the clean K -joins
Let B be a clean K -join of size at least 2k + 2. Let Bf be the k + 1 first
vertices 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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63. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Reducing the branches
In polynomial time, the 1- and 2-branches can be reduced to O(k 3 )
vertices.
Remove 2k + 1 vertices
BR B1
R G (B ∪ R )
B
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64. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Reducing the branches
In polynomial time, the 1- and 2-branches can be reduced to O(k 3 )
vertices.
Remove 2k + 1 vertices
BR B1
R G (B ∪ R )
B
2k + 1 vertices Remove 2k + 1 vertices
B1 B1 BR B2 B2
L R
B
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65. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Main result
Theorem [Bessy and P., 2011]
The P ROPER I NTERVAL C OMPLETION problem admits a kernel with
O(k 4 ) vertices.
1-branch K -join K -join 2-branch K -join 1-branch
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66. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Main result
Theorem [Bessy and P., 2011]
The P ROPER I NTERVAL C OMPLETION problem admits a kernel with
O(k 4 ) vertices.
1-branch K -join K -join 2-branch K -join 1-branch
O (k 3 ) O (k 3 ) O (k 3 ) O (k 3 ) O (k 3 ) O (k 3 )
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67. B RANCHES AND GENERIC REDUCTION RULES P ROPER I NTERVAL C OMPLETION
Main result
Theorem [Bessy and P., 2011]
The P ROPER I NTERVAL C OMPLETION problem admits a kernel with
O(k 4 ) vertices.
Related result [Bessy, Paul and P., 2010]
The C LOSEST 3-L EAF P OWER problem admits a kernel with O(k 3 )
vertices.
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68. Different modification problems
4 Considered problems
5 F EEDBACK A RC S ET IN TOURNAMENTS
Π-E DITION
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.
69. C ONSIDERED PROBLEMS F EEDBACK A RC S ET IN TOURNAMENTS
F EEDBACK A RC S ET 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 acyclic
tournament.
1 4
3 1 4 2
2 3
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70. C ONSIDERED PROBLEMS F EEDBACK A RC S ET IN TOURNAMENTS
F EEDBACK A RC S ET 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 acyclic
tournament.
NP-Complete [Charbit et al., 2007]
Admits constant-factor approximation algorithms [Kenyon-Mathieu and
Schudy, 2007]
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71. C ONSIDERED PROBLEMS F EEDBACK A RC S ET IN TOURNAMENTS
D ENSE R OOTED T RIPLET I NCONSISTENCY (RTI)
Input: A set of leaves V and a dense collection R of rooted binary
trees over three leaves of V .
Parameter: k .
Output: A set of at most k triplets whose modification leads to a
collection admitting a consistent rooted binary tree defined over V .
t1 t2 t3 t4
a b c c d b a b d a c d
R := {t1 , t2 , t3 , t4 }
R := {ab|c, cd|b, ab|d, ac|d}
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72. C ONSIDERED PROBLEMS F EEDBACK A RC S ET IN TOURNAMENTS
D ENSE R OOTED T RIPLET I NCONSISTENCY (RTI)
Input: A set of leaves V and a dense collection R of rooted binary
trees over three leaves of V .
Parameter: k .
Output: A set of at most k triplets whose modification leads to a
collection admitting a consistent rooted binary tree defined over V .
t1 t2 t3 t4
a b c c d b a b d a c d
R := {t1 , t2 , t3 , t4 }
R := {ab|c, cd|b, ab|d, ac|d}
T is not consistent with R
a b c d
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73. C ONSIDERED PROBLEMS F EEDBACK A RC S ET IN TOURNAMENTS
D ENSE R OOTED T RIPLET I NCONSISTENCY (RTI)
Input: A set of leaves V and a dense collection R of rooted binary
trees over three leaves of V .
Parameter: k .
Output: A set of at most k triplets whose modification leads to a
collection admitting a consistent rooted binary tree defined over V .
t1 t2 t3 t4
a b c c d b a b d c d a
R := {t1 , t2 , t3 , t4 }
R := {ab|c, cd|b, ab|d, cd|a}
T is consistent with R
a b c d
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74. C ONSIDERED PROBLEMS F EEDBACK A RC S ET IN TOURNAMENTS
D ENSE R OOTED T RIPLET I NCONSISTENCY (RTI)
Input: A set of leaves V and a dense collection R of rooted binary
trees over three leaves of V .
Parameter: k .
Output: A set of at most k triplets whose modification leads to a
collection 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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75. Outline
4 Considered problems
F EEDBACK A RC S ET IN TOURNAMENTS
D ENSE R OOTED T RIPLET I NCONSISTENCY
Conflict Packing
5 F EEDBACK A RC S ET IN TOURNAMENTS
Reduction rules
Conflict Packing
76. C ONSIDERED PROBLEMS F EEDBACK A RC S ET 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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77. C ONSIDERED PROBLEMS F EEDBACK A RC S ET 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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78. C ONSIDERED PROBLEMS F EEDBACK A RC S ET IN TOURNAMENTS
Parameterized complexity
√
FAST RTI
1/3
FPT O ∗ (2 k log k ) a FPT O ∗ (2k log k ) b
Kernel with O(k 2 ) verticesa Kernel with O(k 2 ) 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]
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79. C ONSIDERED PROBLEMS F EEDBACK A RC S ET IN TOURNAMENTS
Parameterized complexity
√
FAST RTI
1/3
FPT O ∗ (2 k log k ) a FPT O ∗ (2k log k ) b
Kernel with O(k 2 ) verticesa Kernel with O(k 2 ) 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]
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80. C ONSIDERED PROBLEMS F EEDBACK A RC S ET IN TOURNAMENTS
Parameterized complexity
√
FAST RTI
1/3
FPT O ∗ (2 k log k ) a FPT O ∗ (2k log k ) b
Kernel with O(k 2 ) verticesa Kernel with O(k 2 ) 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]
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81. C ONSIDERED PROBLEMS F EEDBACK A RC S ET IN TOURNAMENTS
Parameterized complexity
√
FAST RTI
1/3
FPT O ∗ (2 k log k ) a FPT O ∗ (2k log k ) b
Kernel with O(k 2 ) verticesa Kernel with O(k 2 ) 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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82. C ONSIDERED PROBLEMS F EEDBACK A RC S ET IN TOURNAMENTS
Parameterized complexity
√
FAST RTI
1/3
FPT O ∗ (2 k log k ) a FPT O ∗ (2k log k ) b
Kernel with O(k 2 ) verticesa Kernel with O(k 2 ) 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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83. C ONSIDERED PROBLEMS F EEDBACK A RC S ET 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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84. C ONSIDERED PROBLEMS F EEDBACK A RC S ET IN TOURNAMENTS
Reduction rules
Remove any vertex that is not part of any directed triangle. a .
a
can be carried out in polynomial time.
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85. C ONSIDERED PROBLEMS F EEDBACK A RC S ET IN TOURNAMENTS
Reduction rules
Safe partition
Assume V (T ) is ordered under some ordering σ, and let P be a
partition of σ into intervals.
V1 V2 Vl
AI := {uv ∈ A | ∃ i u , v ∈ Vi }
AO := A AI
B is the set of backward arcs of AO (arcs vi vj with i > j).
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86. C ONSIDERED PROBLEMS F EEDBACK A RC S ET IN TOURNAMENTS
Reduction rules
Safe partition
Assume V (T ) is ordered under some ordering σ, and let P be a
partition of σ into intervals.
V1 V2 Vl
AI := {uv ∈ A | ∃ i u , v ∈ Vi }
AO := A AI
B is the set of backward arcs of AO (arcs vi vj with i > j).
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87. C ONSIDERED PROBLEMS F EEDBACK A RC S ET IN TOURNAMENTS
Reduction rules
Safe partition
P is safe if there exist |B| arc-disjoint conflicts using arcs of AO
only.
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88. C ONSIDERED PROBLEMS F EEDBACK A RC S ET 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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89. C ONSIDERED PROBLEMS F EEDBACK A RC S ET 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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90. C ONSIDERED PROBLEMS F EEDBACK A RC S ET 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 question
How to compute a safe partition in polynomial time?
Use constant-factor approximation algorithm.
Use Conflict Packing.
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91. C ONSIDERED PROBLEMS F EEDBACK A RC S ET 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 question
How to compute a safe partition in polynomial time?
Use constant-factor approximation algorithm.
Use Conflict Packing.
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92. C ONSIDERED PROBLEMS F EEDBACK A RC S ET 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 question
How to compute a safe partition in polynomial time?
Use constant-factor approximation algorithm.
Use Conflict Packing.
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93. C ONSIDERED PROBLEMS F EEDBACK A RC S ET IN TOURNAMENTS
Conflict Packing
A conflict packing of a tournament is a maximal collection of
arc-disjoint directed triangles.
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94. C ONSIDERED PROBLEMS F EEDBACK A RC S ET IN TOURNAMENTS
Conflict Packing
A conflict packing of a tournament is a maximal collection of
arc-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)| 3k.
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95. C ONSIDERED PROBLEMS F EEDBACK A RC S ET IN TOURNAMENTS
Conflict Packing
A conflict packing of a tournament is a maximal collection of
arc-disjoint directed triangles.
Conflict Packing Lemma [Paul, P. and Thomasse, 2011]
´
Let T = (V , A) be a tournament. There exists an ordering of T whose
backward arcs uv are such that u, v ∈ V (C).
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96. C ONSIDERED PROBLEMS F EEDBACK A RC S ET IN TOURNAMENTS
Conflict Packing
A conflict packing of a tournament is a maximal collection of
arc-disjoint directed triangles.
Lemma [Paul, P. and Thomasse, 2011]
´
Let T = (V , A) be a tournament such that |V | > 4k. There exists a safe
partition that can be computed in polynomial time.
proof
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97. C ONSIDERED PROBLEMS F EEDBACK A RC S ET IN TOURNAMENTS
Conflict Packing
A conflict packing of a tournament is a maximal collection of
arc-disjoint directed triangles.
Corollary [Paul, P. and Thomasse, 2011]
´
F EEDBACK A RC S ET IN TOURNAMENTS admits a kernel with at most 4k
vertices.
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98. C ONSIDERED PROBLEMS F EEDBACK A RC S ET IN TOURNAMENTS
Application to the RTI problem
Remove vertices that do not belong to any conflict
Safe Partition reduction rule
Conflict Packing allows to find a Safe Partition
Theorem [Paul, P. and Thomasse, 2011]
´
D ENSE R OOTED T RIPLET I NCONSISTENCY admits a kernel with at most
5k vertices.
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100. O UR RESULTS O PEN PROBLEMS
Main results
Polynomial kernels
First polynomial kernels:
(i) C LOSEST 3-L EAF P OWER
(ii) P ROPER I NTERVAL C OMPLETION
(iii) C OGRAPH E DGE -E DITION
Improved polynomial kernels:
(i) F EEDBACK A RC S ET IN TOURNAMENTS
(ii) D ENSE R OOTED T RIPLET I NCONSISTENCY
(iii) D ENSE B ETWEENNESS and D ENSE C IRCULAR O RDERING
joint works with: S. Bessy, F. Fomin, S. Gaspers, S. Guillemot, F. Havet, C. Paul,
S. Saurabh and S. Thomasse. ´
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101. O UR RESULTS O PEN PROBLEMS
Main results
Lower bounds on kernelization:
(i) For any l 7, the Pl -F REE E DGE -D ELETION problem
does not admit a polynomial kernel.
(ii) For any l 4, the Cl -F REE E DGE -D ELETION problem
does not admit a polynomial kernel.
joint work with: S. Guillemot, F. Havet and C. Paul.
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102. O UR RESULTS O PEN PROBLEMS
Open problems
Do the F EEDBACK V ERTEX S ET IN TOURNAMENTS and C LUSTER
V ERTEX D ELETION problems admit linear vertex-kernels?
Characterize lower bounds for modification problems. details
Can we use branches on other problems?
(e.g. C HORDAL D ELETION)
Can we use Conflict Packing on other problems?
(e.g. (weakly)-fragile constraint modification problems)
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103. O UR RESULTS O PEN PROBLEMS
Open problems
Do the F EEDBACK V ERTEX S ET IN TOURNAMENTS and C LUSTER
V ERTEX D ELETION problems admit linear vertex-kernels?
Characterize lower bounds for modification problems. details
Can we use branches on other problems?
(e.g. C HORDAL D ELETION)
Can we use Conflict Packing on other problems?
(e.g. (weakly)-fragile constraint modification problems)
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104. O UR RESULTS O PEN PROBLEMS
Open problems
Do the F EEDBACK V ERTEX S ET IN TOURNAMENTS and C LUSTER
V ERTEX D ELETION problems admit linear vertex-kernels?
Characterize lower bounds for modification problems. details
Can we use branches on other problems?
(e.g. C HORDAL D ELETION)
Can we use Conflict Packing on other problems?
(e.g. (weakly)-fragile constraint modification problems)
41 / 42