Sang-il Oum - KAIST

31
Testing Branch-width Sang-il Oum School of Mathematics Georgia Institute of Technology January 23, 2006 Joint work with Paul Seymour Princeton University

Transcript of Sang-il Oum - KAIST

Page 1: Sang-il Oum - KAIST

Testing Branch-width

Sang-il Oum

School of MathematicsGeorgia Institute of Technology

January 23, 2006

Joint work withPaul Seymour

Princeton University

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A function f : 2V → Z is a connectivity function if(i) f (X) + f (Y ) ≥ f (X ∩ Y ) + f (X ∪ Y ), (submodular)(ii) f (X) = f (V \ X), (symmetric)(iii) f (∅) = 0.

v(X) =numberof verticesmeeting both Xand E \ X .

e(X) =numberof edgesmeeting both Xand V \ X .

M: matroid, λ(X) =r(X) + r(E(M)− X)− r(E(M)).

For a graph G, let A =adjacency matrix.ρG(X) = rank A[X , V \ X ].

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A function f : 2V → Z is a connectivity function if(i) f (X) + f (Y ) ≥ f (X ∩ Y ) + f (X ∪ Y ), (submodular)(ii) f (X) = f (V \ X), (symmetric)(iii) f (∅) = 0.

v(X) =numberof verticesmeeting both Xand E \ X .

e(X) =numberof edgesmeeting both Xand V \ X .

M: matroid, λ(X) =r(X) + r(E(M)− X)− r(E(M)).

For a graph G, let A =adjacency matrix.ρG(X) = rank A[X , V \ X ].

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A function f : 2V → Z is a connectivity function if(i) f (X) + f (Y ) ≥ f (X ∩ Y ) + f (X ∪ Y ), (submodular)(ii) f (X) = f (V \ X), (symmetric)(iii) f (∅) = 0.

v(X) =numberof verticesmeeting both Xand E \ X .

e(X) =numberof edgesmeeting both Xand V \ X .

M: matroid, λ(X) =r(X) + r(E(M)− X)− r(E(M)).

For a graph G, let A =adjacency matrix.ρG(X) = rank A[X , V \ X ].

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A function f : 2V → Z is a connectivity function if(i) f (X) + f (Y ) ≥ f (X ∩ Y ) + f (X ∪ Y ), (submodular)(ii) f (X) = f (V \ X), (symmetric)(iii) f (∅) = 0.

v(X) =numberof verticesmeeting both Xand E \ X .

e(X) =numberof edgesmeeting both Xand V \ X .

M: matroid, λ(X) =r(X) + r(E(M)− X)− r(E(M)).

For a graph G, let A =adjacency matrix.ρG(X) = rank A[X , V \ X ].

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A function f : 2V → Z is a connectivity function if(i) f (X) + f (Y ) ≥ f (X ∩ Y ) + f (X ∪ Y ), (submodular)(ii) f (X) = f (V \ X), (symmetric)(iii) f (∅) = 0.

v(X) =numberof verticesmeeting both Xand E \ X .

e(X) =numberof edgesmeeting both Xand V \ X .

M: matroid, λ(X) =r(X) + r(E(M)− X)− r(E(M)).

For a graph G, let A =adjacency matrix.ρG(X) = rank A[X , V \ X ].

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Branch-decomposition of f : a pair (T , L) ofa subcubic tree T and a bijection L : V → {leaves of T}.

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2 3

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67

8

Branch-widthCarving-width

M: matroid, λ(X) =r(X) + r(E(M)− X)− r(E(M)).Branch-width of matroids.

For a graph G, let A =adjacency matrix.ρG(X) = rank A[X , V \ X ].Rank-width of graphs

Testing Branch-width ≤ k for fixed kBranch-width of graphs: Linear (Bodlaender, Thilikos ’97)Carving-width of graphs: Linear (Thilikos, Serna, Bodlaendar’00)Branch-width of matroids represented over a fixed finite field:O(|E(M)|3) (Hlinený ’05)Rank-width of graphs: O(|V (G)|3) (Oum ’05)

Poly-time algorithm to test branch-width≤ k for any connectivityfunctions? assuming that f is given by an oracle.

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Branch-decomposition of f : a pair (T , L) ofa subcubic tree T and a bijection L : V → {leaves of T}.

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2 3

4

5

67

8 f ({1, 2, 3, 4})

Width of an edge e of T : f (Ae)(Ae, Be) is a partition of V givenby deleting e.

Branch-widthCarving-width

M: matroid, λ(X) =r(X) + r(E(M)− X)− r(E(M)).Branch-width of matroids.

For a graph G, let A =adjacency matrix.ρG(X) = rank A[X , V \ X ].Rank-width of graphs

Testing Branch-width ≤ k for fixed kBranch-width of graphs: Linear (Bodlaender, Thilikos ’97)Carving-width of graphs: Linear (Thilikos, Serna, Bodlaendar’00)Branch-width of matroids represented over a fixed finite field:O(|E(M)|3) (Hlinený ’05)Rank-width of graphs: O(|V (G)|3) (Oum ’05)

Poly-time algorithm to test branch-width≤ k for any connectivityfunctions? assuming that f is given by an oracle.

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Branch-decomposition of f : a pair (T , L) ofa subcubic tree T and a bijection L : V → {leaves of T}.

1

2 3

4

5

67

8

f ({1})f ({1

, 2})f ({3, 4})

f ({3}

)

f ({2}) f ({4})

f ({1, 2, 3, 4})

f ({5, 6})

f ({7, 8})Width of an edge e of T : f (Ae)(Ae, Be) is a partition of V givenby deleting e.

Width of (T , L): maxe width(e)

Branch-widthCarving-width

M: matroid, λ(X) =r(X) + r(E(M)− X)− r(E(M)).Branch-width of matroids.

For a graph G, let A =adjacency matrix.ρG(X) = rank A[X , V \ X ].Rank-width of graphs

Testing Branch-width ≤ k for fixed kBranch-width of graphs: Linear (Bodlaender, Thilikos ’97)Carving-width of graphs: Linear (Thilikos, Serna, Bodlaendar’00)Branch-width of matroids represented over a fixed finite field:O(|E(M)|3) (Hlinený ’05)Rank-width of graphs: O(|V (G)|3) (Oum ’05)

Poly-time algorithm to test branch-width≤ k for any connectivityfunctions? assuming that f is given by an oracle.

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Branch-decomposition of f : a pair (T , L) ofa subcubic tree T and a bijection L : V → {leaves of T}.

1

2 3

4

5

67

8

f ({1})f ({1

, 2})f ({3, 4})

f ({3}

)

f ({2}) f ({4})

f ({1, 2, 3, 4})

f ({5, 6})

f ({7, 8})Width of an edge e of T : f (Ae)(Ae, Be) is a partition of V givenby deleting e.

Width of (T , L): maxe width(e)

Branch-width: min(T ,L) width(T , L).(If |V | ≤ 1, then branch-width=0)

Branch-widthCarving-width

M: matroid, λ(X) =r(X) + r(E(M)− X)− r(E(M)).Branch-width of matroids.

For a graph G, let A =adjacency matrix.ρG(X) = rank A[X , V \ X ].Rank-width of graphs

Testing Branch-width ≤ k for fixed kBranch-width of graphs: Linear (Bodlaender, Thilikos ’97)Carving-width of graphs: Linear (Thilikos, Serna, Bodlaendar’00)Branch-width of matroids represented over a fixed finite field:O(|E(M)|3) (Hlinený ’05)Rank-width of graphs: O(|V (G)|3) (Oum ’05)

Poly-time algorithm to test branch-width≤ k for any connectivityfunctions? assuming that f is given by an oracle.

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Branch-decomposition of f : a pair (T , L) ofa subcubic tree T and a bijection L : V → {leaves of T}.

1

2 3

4

5

67

8

f ({1})f ({1

, 2})f ({3, 4})

f ({3}

)

f ({2}) f ({4})

f ({1, 2, 3, 4})

f ({5, 6})

f ({7, 8})Width of an edge e of T : f (Ae)(Ae, Be) is a partition of V givenby deleting e.

Width of (T , L): maxe width(e)

Branch-width: min(T ,L) width(T , L).(If |V | ≤ 1, then branch-width=0)

Branch-widthCarving-width

M: matroid, λ(X) =r(X) + r(E(M)− X)− r(E(M)).Branch-width of matroids.

For a graph G, let A =adjacency matrix.ρG(X) = rank A[X , V \ X ].Rank-width of graphs

Testing Branch-width ≤ k for fixed kBranch-width of graphs: Linear (Bodlaender, Thilikos ’97)Carving-width of graphs: Linear (Thilikos, Serna, Bodlaendar’00)Branch-width of matroids represented over a fixed finite field:O(|E(M)|3) (Hlinený ’05)Rank-width of graphs: O(|V (G)|3) (Oum ’05)

Poly-time algorithm to test branch-width≤ k for any connectivityfunctions? assuming that f is given by an oracle.

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Branch-widthCarving-width

M: matroid, λ(X) =r(X) + r(E(M)− X)− r(E(M)).Branch-width of matroids.

For a graph G, let A =adjacency matrix.ρG(X) = rank A[X , V \ X ].Rank-width of graphs

Testing Branch-width ≤ k for fixed kBranch-width of graphs: Linear (Bodlaender, Thilikos ’97)Carving-width of graphs: Linear (Thilikos, Serna, Bodlaendar’00)Branch-width of matroids represented over a fixed finite field:O(|E(M)|3) (Hlinený ’05)Rank-width of graphs: O(|V (G)|3) (Oum ’05)

Poly-time algorithm to test branch-width≤ k for any connectivityfunctions? assuming that f is given by an oracle.

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Branch-widthCarving-width

M: matroid, λ(X) =r(X) + r(E(M)− X)− r(E(M)).Branch-width of matroids.

For a graph G, let A =adjacency matrix.ρG(X) = rank A[X , V \ X ].Rank-width of graphs

Testing Branch-width ≤ k for fixed kBranch-width of graphs: Linear (Bodlaender, Thilikos ’97)Carving-width of graphs: Linear (Thilikos, Serna, Bodlaendar’00)Branch-width of matroids represented over a fixed finite field:O(|E(M)|3) (Hlinený ’05)Rank-width of graphs: O(|V (G)|3) (Oum ’05)

Poly-time algorithm to test branch-width≤ k for any connectivityfunctions? assuming that f is given by an oracle.

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Branch-widthCarving-width

M: matroid, λ(X) =r(X) + r(E(M)− X)− r(E(M)).Branch-width of matroids.

For a graph G, let A =adjacency matrix.ρG(X) = rank A[X , V \ X ].Rank-width of graphs

Testing Branch-width ≤ k for fixed kBranch-width of graphs: Linear (Bodlaender, Thilikos ’97)Carving-width of graphs: Linear (Thilikos, Serna, Bodlaendar’00)Branch-width of matroids represented over a fixed finite field:O(|E(M)|3) (Hlinený ’05)Rank-width of graphs: O(|V (G)|3) (Oum ’05)

Poly-time algorithm to test branch-width≤ k for any connectivityfunctions? assuming that f is given by an oracle.

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f -tangle of order k + 1 (Robertson and Seymour)A set T of subsets of V satisfying(T1) If f (X) ≤ k , then X ∈ T or V \ X ∈ T .(T2) If A, B, C ∈ T , then A ∪ B ∪C 6= V .(T3) V \ {v} /∈ T for all v ∈ V .

Robertson, Seymour (’91)Branch-width≤ k if and only if no f -tangle of order k + 1 exists.

Naive algorithm: Choose one from X or V \ X if f (X) ≤ k and seewhether (T2) and (T3) are satisfied.

loose f -tangle of order k + 1A set T of subsets of V satisfying(L1) V /∈ T .(L2) If A, B ∈ T , C ⊆ A ∪ B, and f (C) ≤ k , then C ∈ T .(L3) If |X | ≤ 1 and f (X) ≤ k , then X ∈ T .

THM: An f -tangle of order k + 1 exists if and only ifa loose f -tangle of order k + 1 exists.

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f -tangle of order k + 1 (Robertson and Seymour)A set T of subsets of V satisfying(T1) If f (X) ≤ k , then X ∈ T or V \ X ∈ T .(T2) If A, B, C ∈ T , then A ∪ B ∪C 6= V .(T3) V \ {v} /∈ T for all v ∈ V .

Robertson, Seymour (’91)Branch-width≤ k if and only if no f -tangle of order k + 1 exists.

Naive algorithm: Choose one from X or V \ X if f (X) ≤ k and seewhether (T2) and (T3) are satisfied.

loose f -tangle of order k + 1A set T of subsets of V satisfying(L1) V /∈ T .(L2) If A, B ∈ T , C ⊆ A ∪ B, and f (C) ≤ k , then C ∈ T .(L3) If |X | ≤ 1 and f (X) ≤ k , then X ∈ T .

THM: An f -tangle of order k + 1 exists if and only ifa loose f -tangle of order k + 1 exists.

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f -tangle of order k + 1 (Robertson and Seymour)A set T of subsets of V satisfying(T1) If f (X) ≤ k , then X ∈ T or V \ X ∈ T .(T2) If A, B, C ∈ T , then A ∪ B ∪C 6= V .(T3) V \ {v} /∈ T for all v ∈ V .

Robertson, Seymour (’91)Branch-width≤ k if and only if no f -tangle of order k + 1 exists.

Naive algorithm: Choose one from X or V \ X if f (X) ≤ k and seewhether (T2) and (T3) are satisfied.

loose f -tangle of order k + 1A set T of subsets of V satisfying(L1) V /∈ T .(L2) If A, B ∈ T , C ⊆ A ∪ B, and f (C) ≤ k , then C ∈ T .(L3) If |X | ≤ 1 and f (X) ≤ k , then X ∈ T .

THM: An f -tangle of order k + 1 exists if and only ifa loose f -tangle of order k + 1 exists.

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f -tangle of order k + 1 (Robertson and Seymour)A set T of subsets of V satisfying(T1) If f (X) ≤ k , then X ∈ T or V \ X ∈ T .(T2) If A, B, C ∈ T , then A ∪ B ∪C 6= V .(T3) V \ {v} /∈ T for all v ∈ V .

loose f -tangle of order k + 1A set T of subsets of V satisfying(L1) V /∈ T .(L2) If A, B ∈ T , C ⊆ A ∪ B, and f (C) ≤ k , then C ∈ T .(L3) If |X | ≤ 1 and f (X) ≤ k , then X ∈ T .

THM: An f -tangle of order k + 1 exists if and only ifa loose f -tangle of order k + 1 exists.

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loose f -tangle of order k + 1A set T of subsets of V satisfying(L1) V /∈ T .(L2) If A, B ∈ T , C ⊆ A ∪ B, and f (C) ≤ k , then C ∈ T .(L3) If |X | ≤ 1 and f (X) ≤ k , then X ∈ T .

Naive algorithm to find a loose f -tangle(1) Begin with T = {X : |X | ≤ 1, f (X) ≤ k}.(2) Test (L1).

If it fails, then no loose f -tangle of order k + 1.(3) Test (L2).

If it fails, then find C and add it to T . Go back to 2.(4) T is a loose f -tangle of order k + 1.

Problem: |T | can be exponentially large.

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loose f -tangle of order k + 1A set T of subsets of V satisfying(L1) V /∈ T .(L2) If A, B ∈ T , C ⊆ A ∪ B, and f (C) ≤ k , then C ∈ T .(L3) If |X | ≤ 1 and f (X) ≤ k , then X ∈ T .

Naive algorithm to find a loose f -tangle(1) Begin with T = {X : |X | ≤ 1, f (X) ≤ k}.(2) Test (L1).

If it fails, then no loose f -tangle of order k + 1.(3) Test (L2).

If it fails, then find C and add it to T . Go back to 2.(4) T is a loose f -tangle of order k + 1.

Problem: |T | can be exponentially large.

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loose f -tangle of order k + 1A set T of subsets of V satisfying(L1) V /∈ T .(L2) If A, B ∈ T , C ⊆ A ∪ B, and f (C) ≤ k , then C ∈ T .(L3) If |X | ≤ 1 and f (X) ≤ k , then X ∈ T .

Naive algorithm to find a loose f -tangle(1) Begin with T = {X : |X | ≤ 1, f (X) ≤ k}.(2) Test (L1).

If it fails, then no loose f -tangle of order k + 1.(3) Test (L2).

If it fails, then find C and add it to T . Go back to 2.(4) T is a loose f -tangle of order k + 1.

Problem: |T | can be exponentially large.

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loose f -tangle of order k + 1A set T of subsets of V satisfying(L1) V /∈ T .(L2) If A, B ∈ T , C ⊆ A ∪ B, and f (C) ≤ k , then C ∈ T .(L3) If |X | ≤ 1 and f (X) ≤ k , then X ∈ T .

Lemma

X Y

Let fmin(A, B) = min{f (X) : A ⊆ X ⊆ V \ B}.If fmin(X , Y ) = m, then ∃Z such that

(i) f (Z) = m

(ii) X ⊆ Z ⊆ V \ Y .Conversely, if f (Z) = m, then ∃X , Y such that

(i) |X |, |Y | ≤ m and X ⊆ Z ⊆ V \ Y ,(ii) fmin(X , Y ) = m.

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loose f -tangle of order k + 1A set T of subsets of V satisfying(L1) V /∈ T .(L2) If A, B ∈ T , C ⊆ A ∪ B, and f (C) ≤ k , then C ∈ T .(L3) If |X | ≤ 1 and f (X) ≤ k , then X ∈ T .

Lemma 2

Z2 Z1

X Y

Suppose fmin(X , Y ) = m, X ⊆ Z1, Z2 ⊆ V \ Y .If

f (Z1) = f (Z2) = m,

thenf (Z1 ∪ Z2) = m.

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loose f -tangle of order k + 1(L1) V /∈ T .(L2) If A, B ∈ T , C ⊆ A ∪ B, and f (C) ≤ k , then C ∈ T .(L3) If |X | ≤ 1 and f (X) ≤ k , then X ∈ T .

loose f -tangle kit of order k + 1A pair (P, µ) whereP = {(A, B) : A ∩ B = ∅, max(|A|, |B|) ≤ fmin(A, B) ≤ k .}and µ : P → 2V is a function satisfying the following.(K1) µ(∅, ∅) 6= V if (∅, ∅) ∈ P.(K2) If (A, B), (C, D), (E, F) ∈ P, E ⊆ X ⊆ µ(A, B) ∪ µ(C, D)− F , and

fmin(E, F) = f (X), then X ⊆ µ(E, F).(K3) If |X | ≤ 1, f (X) ≤ 1,

then there exists (A, B) ∈ P such that A ⊆ X ⊆ V \ B,f (X) = fmin(A, B), and X ⊆ µ(A, B).

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(K1) µ(∅, ∅) 6= V if (∅, ∅) ∈ P.(K2) If (A, B), (C, D), (E, F) ∈ P, E ⊆ X ⊆ µ(A, B) ∪ µ(C, D)− F , and

fmin(E, F) = f (X), then X ⊆ µ(E, F).(K3) If |X | ≤ 1, f (X) ≤ 1,

then there exists (A, B) ∈ P such that A ⊆ X ⊆ V \ B,f (X) = fmin(A, B), and X ⊆ µ(A, B).

Poly-time algorithm to find a loose f -tangle(A1) Let P = {(A, B) : A ∩ B = ∅, max(|A|, |B|) ≤ fmin(A, B) ≤ k}.(A2) For each v ∈ V , if 0 < f ({v}) ≤ k , then find B ⊆ V \ {v} such

that |B| ≤ fmin({v}, B) ≤ k . Let µ({v}, B) = {v}.Let µ(∅, ∅) = {v ∈ V : f ({v}) = 0} if (∅, ∅) ∈ P.For all other (A, B) ∈ P, let µ(A, B) = ∅.

(A3) Test (K1). If it fails, then no loose f -tangle kit of order k + 1.(A4) Test (K2).

If it fails, then find X and enlarge µ(E, F). Go back to (A3).(A5) (P, µ) is a loose f -tangle kit of order k + 1.

Time Complexity: O(n2knn6k+1nn5 log n)

Consequence to MatroidsPoly-time algorithm to test matroid branch-width ≤ k for fixed k ,when the input matroid is given by an independence oracle.

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(K1) µ(∅, ∅) 6= V if (∅, ∅) ∈ P.(K2) If (A, B), (C, D), (E, F) ∈ P, E ⊆ X ⊆ µ(A, B) ∪ µ(C, D)− F , and

fmin(E, F) = f (X), then X ⊆ µ(E, F).(K3) If |X | ≤ 1, f (X) ≤ 1,

then there exists (A, B) ∈ P such that A ⊆ X ⊆ V \ B,f (X) = fmin(A, B), and X ⊆ µ(A, B).

Poly-time algorithm to find a loose f -tangle(A1) Let P = {(A, B) : A ∩ B = ∅, max(|A|, |B|) ≤ fmin(A, B) ≤ k}.(A2) For each v ∈ V , if 0 < f ({v}) ≤ k , then find B ⊆ V \ {v} such

that |B| ≤ fmin({v}, B) ≤ k . Let µ({v}, B) = {v}.Let µ(∅, ∅) = {v ∈ V : f ({v}) = 0} if (∅, ∅) ∈ P.For all other (A, B) ∈ P, let µ(A, B) = ∅.

(A3) Test (K1). If it fails, then no loose f -tangle kit of order k + 1.(A4) Test (K2).

If it fails, then find X and enlarge µ(E, F). Go back to (A3).(A5) (P, µ) is a loose f -tangle kit of order k + 1.

Time Complexity: O(n2knn6k+1nn5 log n)

Consequence to MatroidsPoly-time algorithm to test matroid branch-width ≤ k for fixed k ,when the input matroid is given by an independence oracle.

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Poly-time algorithm to find a loose f -tangle(A1) Let P = {(A, B) : A ∩ B = ∅, max(|A|, |B|) ≤ fmin(A, B) ≤ k}.(A2) For each v ∈ V , if 0 < f ({v}) ≤ k , then find B ⊆ V \ {v} such

that |B| ≤ fmin({v}, B) ≤ k . Let µ({v}, B) = {v}.Let µ(∅, ∅) = {v ∈ V : f ({v}) = 0} if (∅, ∅) ∈ P.For all other (A, B) ∈ P, let µ(A, B) = ∅.

(A3) Test (K1). If it fails, then no loose f -tangle kit of order k + 1.(A4) Test (K2).

If it fails, then find X and enlarge µ(E, F). Go back to (A3).(A5) (P, µ) is a loose f -tangle kit of order k + 1.

Time Complexity: O(n2knn6k+1nn5 log n)

Consequence to MatroidsPoly-time algorithm to test matroid branch-width ≤ k for fixed k ,when the input matroid is given by an independence oracle.

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Constructing Branch-decomposition of width ≤ k

Is it possible to construct the branch-decomposition of width ≤ kif there exists one in polynomial time (in |V |)? Yes.

Jim Geelen (2005, private communication):Recursively find a pair a, b ∈ V such thatmerging them does not increase branch-width.We only need O(n3) calls to testing branch-width at most k .

Further topicsIs it fixed parameter tractable?In other words, is it possible to have a running time O(f (k)|V |c) forall k?

Thank you!

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Constructing Branch-decomposition of width ≤ k

Is it possible to construct the branch-decomposition of width ≤ kif there exists one in polynomial time (in |V |)? Yes.

Jim Geelen (2005, private communication):Recursively find a pair a, b ∈ V such thatmerging them does not increase branch-width.We only need O(n3) calls to testing branch-width at most k .

Further topicsIs it fixed parameter tractable?In other words, is it possible to have a running time O(f (k)|V |c) forall k?

Thank you!

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Constructing Branch-decomposition of width ≤ k

Is it possible to construct the branch-decomposition of width ≤ kif there exists one in polynomial time (in |V |)? Yes.

Jim Geelen (2005, private communication):Recursively find a pair a, b ∈ V such thatmerging them does not increase branch-width.We only need O(n3) calls to testing branch-width at most k .

Further topicsIs it fixed parameter tractable?In other words, is it possible to have a running time O(f (k)|V |c) forall k?

Thank you!

Page 31: Sang-il Oum - KAIST

Constructing Branch-decomposition of width ≤ k

Is it possible to construct the branch-decomposition of width ≤ kif there exists one in polynomial time (in |V |)? Yes.

Jim Geelen (2005, private communication):Recursively find a pair a, b ∈ V such thatmerging them does not increase branch-width.We only need O(n3) calls to testing branch-width at most k .

Further topicsIs it fixed parameter tractable?In other words, is it possible to have a running time O(f (k)|V |c) forall k?

Thank you!