graph3 - sjtubasics.sjtu.edu.cn/~chen/teaching/gr07/graph3.pdf · 2007-09-29 · graph theory(iii)...
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![Page 1: graph3 - SJTUbasics.sjtu.edu.cn/~chen/teaching/GR07/graph3.pdf · 2007-09-29 · GRAPH THEORY(III) Page 3 Two edges e = f are adjacentif they have an end in common. Pairwise non-adjacentvertices](https://reader034.vdocuments.net/reader034/viewer/2022050112/5f49c0fc9b30a9510b120338/html5/thumbnails/1.jpg)
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GRAPH THEORY
Yijia ChenShanghai Jiaotong University
2007/2008
Shanghai
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GRAPH THEORY (III) Page 2
Chapter 2. Matching Covering and Packing
Shanghai
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GRAPH THEORY (III) Page 3
Two edges e �= f are adjacent if they have an end in common.
Shanghai
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GRAPH THEORY (III) Page 3
Two edges e �= f are adjacent if they have an end in common.
Pairwise non-adjacent vertices or edges are called independent.
Shanghai
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GRAPH THEORY (III) Page 3
Two edges e �= f are adjacent if they have an end in common.
Pairwise non-adjacent vertices or edges are called independent. A set of edges or of edges
is independent (or stable) if no two of its elements are adjacent.
Shanghai
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GRAPH THEORY (III) Page 3
Two edges e �= f are adjacent if they have an end in common.
Pairwise non-adjacent vertices or edges are called independent. A set of edges or of edges
is independent (or stable) if no two of its elements are adjacent.
A set M of independent edges in a graph G = (V, E) is called a matching.
Shanghai
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GRAPH THEORY (III) Page 3
Two edges e �= f are adjacent if they have an end in common.
Pairwise non-adjacent vertices or edges are called independent. A set of edges or of edges
is independent (or stable) if no two of its elements are adjacent.
A set M of independent edges in a graph G = (V, E) is called a matching. M is a
matching of U ⊆ V if every vertex in U is incident with an edge in M .
Shanghai
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GRAPH THEORY (III) Page 3
Two edges e �= f are adjacent if they have an end in common.
Pairwise non-adjacent vertices or edges are called independent. A set of edges or of edges
is independent (or stable) if no two of its elements are adjacent.
A set M of independent edges in a graph G = (V, E) is called a matching. M is a
matching of U ⊆ V if every vertex in U is incident with an edge in M . The vertices in U
are then called matched (by M ); vertices not incident with any edge of M are unmatched.
Shanghai
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GRAPH THEORY (III) Page 3
Two edges e �= f are adjacent if they have an end in common.
Pairwise non-adjacent vertices or edges are called independent. A set of edges or of edges
is independent (or stable) if no two of its elements are adjacent.
A set M of independent edges in a graph G = (V, E) is called a matching. M is a
matching of U ⊆ V if every vertex in U is incident with an edge in M . The vertices in U
are then called matched (by M ); vertices not incident with any edge of M are unmatched.
A k-regular spanning subgraph is called a k-factor.
Shanghai
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GRAPH THEORY (III) Page 3
Two edges e �= f are adjacent if they have an end in common.
Pairwise non-adjacent vertices or edges are called independent. A set of edges or of edges
is independent (or stable) if no two of its elements are adjacent.
A set M of independent edges in a graph G = (V, E) is called a matching. M is a
matching of U ⊆ V if every vertex in U is incident with an edge in M . The vertices in U
are then called matched (by M ); vertices not incident with any edge of M are unmatched.
A k-regular spanning subgraph is called a k-factor.
Thus, a subgraph H ⊆ G is a 1-factor of G if and only if E(H) is a matching of V .
Shanghai
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GRAPH THEORY (III) Page 4
2.1 Matching in bipartite graphs
Shanghai
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GRAPH THEORY (III) Page 4
2.1 Matching in bipartite graphs
We fix a bipartite graph G = (V, E) with bipartition {A, B}.
Shanghai
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GRAPH THEORY (III) Page 4
2.1 Matching in bipartite graphs
We fix a bipartite graph G = (V, E) with bipartition {A, B}.
Let M be an arbitrary matching in G. A path in G which starts in A at an unmatched vertex
and then contains, alternately, edges from E \ M and from M , is an alternating path with
respect to M .
Shanghai
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GRAPH THEORY (III) Page 4
2.1 Matching in bipartite graphs
We fix a bipartite graph G = (V, E) with bipartition {A, B}.
Let M be an arbitrary matching in G. A path in G which starts in A at an unmatched vertex
and then contains, alternately, edges from E \ M and from M , is an alternating path with
respect to M .
An alternating path that ends in an unmatched vertex of B is called an augmenting path.
Shanghai
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GRAPH THEORY (III) Page 4
2.1 Matching in bipartite graphs
We fix a bipartite graph G = (V, E) with bipartition {A, B}.
Let M be an arbitrary matching in G. A path in G which starts in A at an unmatched vertex
and then contains, alternately, edges from E \ M and from M , is an alternating path with
respect to M .
An alternating path that ends in an unmatched vertex of B is called an augmenting path.
An augmenting path leads to a larger matching.
Shanghai
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GRAPH THEORY (III) Page 5
A set U ⊆ V a vertex cover of E if every edge of G is incident with a vertex in U .
Shanghai
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GRAPH THEORY (III) Page 5
A set U ⊆ V a vertex cover of E if every edge of G is incident with a vertex in U .
Theorem.[Konig 1931] The maximum cardinality of a matching in G is equal to the
minimum cardinality of a vertex cover of its edges.
Shanghai
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GRAPH THEORY (III) Page 5
A set U ⊆ V a vertex cover of E if every edge of G is incident with a vertex in U .
Theorem.[Konig 1931] The maximum cardinality of a matching in G is equal to the
minimum cardinality of a vertex cover of its edges.
Proof.
Shanghai
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GRAPH THEORY (III) Page 5
A set U ⊆ V a vertex cover of E if every edge of G is incident with a vertex in U .
Theorem.[Konig 1931] The maximum cardinality of a matching in G is equal to the
minimum cardinality of a vertex cover of its edges.
Proof. Let M be a matching of maximum cardinality.
Shanghai
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GRAPH THEORY (III) Page 5
A set U ⊆ V a vertex cover of E if every edge of G is incident with a vertex in U .
Theorem.[Konig 1931] The maximum cardinality of a matching in G is equal to the
minimum cardinality of a vertex cover of its edges.
Proof. Let M be a matching of maximum cardinality.
Define a set U in the following way:
Shanghai
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GRAPH THEORY (III) Page 5
A set U ⊆ V a vertex cover of E if every edge of G is incident with a vertex in U .
Theorem.[Konig 1931] The maximum cardinality of a matching in G is equal to the
minimum cardinality of a vertex cover of its edges.
Proof. Let M be a matching of maximum cardinality.
Define a set U in the following way: For every ab ∈ M , if there is an alternating path ending
in b, then b ∈ U ; otherwise a ∈ U .
�
Shanghai
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GRAPH THEORY (III) Page 6
Theorem.[Hall 1935] G contains a matching of A if and only if |N(S)| ≥ |S| for all
S ⊆ A.
Shanghai
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GRAPH THEORY (III) Page 7
First Proof.
Shanghai
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GRAPH THEORY (III) Page 7
First Proof.
Let M be a matching that leaves a vertex in A unmatched.
Shanghai
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GRAPH THEORY (III) Page 7
First Proof.
Let M be a matching that leaves a vertex in A unmatched. We will construct a augmenting
path with respect to M .
Shanghai
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GRAPH THEORY (III) Page 7
First Proof.
Let M be a matching that leaves a vertex in A unmatched. We will construct a augmenting
path with respect to M .
Let a0, b1, a1, b2, a2, . . . be a maximal sequence of distinct vertices ai ∈ A and bi ∈ B
satisfying the following conditions for all i ≥ 1:
Shanghai
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GRAPH THEORY (III) Page 7
First Proof.
Let M be a matching that leaves a vertex in A unmatched. We will construct a augmenting
path with respect to M .
Let a0, b1, a1, b2, a2, . . . be a maximal sequence of distinct vertices ai ∈ A and bi ∈ B
satisfying the following conditions for all i ≥ 1:
(i) a0 is unmatched;
(ii) bi is adjacent to some vertex af(i) ∈ {a0, . . . , ai−1};
(iii) aibi ∈ M .
The sequence will end in some vertex bk ∈ B.
Shanghai
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GRAPH THEORY (III) Page 7
First Proof.
Let M be a matching that leaves a vertex in A unmatched. We will construct a augmenting
path with respect to M .
Let a0, b1, a1, b2, a2, . . . be a maximal sequence of distinct vertices ai ∈ A and bi ∈ B
satisfying the following conditions for all i ≥ 1:
(i) a0 is unmatched;
(ii) bi is adjacent to some vertex af(i) ∈ {a0, . . . , ai−1};
(iii) aibi ∈ M .
The sequence will end in some vertex bk ∈ B.
Consider
P := bkaf(k)bf(k)af2(k)bf2(k)af3(k) . . . , afr(k)
with fr(k) = 0 is an alternating path.
Shanghai
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GRAPH THEORY (III) Page 7
First Proof.
Let M be a matching that leaves a vertex in A unmatched. We will construct a augmenting
path with respect to M .
Let a0, b1, a1, b2, a2, . . . be a maximal sequence of distinct vertices ai ∈ A and bi ∈ B
satisfying the following conditions for all i ≥ 1:
(i) a0 is unmatched;
(ii) bi is adjacent to some vertex af(i) ∈ {a0, . . . , ai−1};
(iii) aibi ∈ M .
The sequence will end in some vertex bk ∈ B.
Consider
P := bkaf(k)bf(k)af2(k)bf2(k)af3(k) . . . , afr(k)
with fr(k) = 0 is an alternating path.
P is an augmenting path. �
Shanghai
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GRAPH THEORY (III) Page 8
Second Proof.
Shanghai
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GRAPH THEORY (III) Page 8
Second Proof.
Induction on |A|.
Shanghai
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GRAPH THEORY (III) Page 8
Second Proof.
Induction on |A|.Trivial for |A| = 1.
Shanghai
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GRAPH THEORY (III) Page 8
Second Proof.
Induction on |A|.Trivial for |A| = 1.
If |N(S)| ≥ |S| + 1 for every non-empty set S � A, we pick an edge ab ∈ G and apply
induction hypothesis on the graph G′ := G − {a, b}.
Shanghai
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GRAPH THEORY (III) Page 8
Second Proof.
Induction on |A|.Trivial for |A| = 1.
If |N(S)| ≥ |S| + 1 for every non-empty set S � A, we pick an edge ab ∈ G and apply
induction hypothesis on the graph G′ := G − {a, b}.
If A has a non-empty proper subset A′ with |B′| = |A′| for B′ := N(A).
Shanghai
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GRAPH THEORY (III) Page 8
Second Proof.
Induction on |A|.Trivial for |A| = 1.
If |N(S)| ≥ |S| + 1 for every non-empty set S � A, we pick an edge ab ∈ G and apply
induction hypothesis on the graph G′ := G − {a, b}.
If A has a non-empty proper subset A′ with |B′| = |A′| for B′ := N(A).
Apply induction hypothesis on G[A′ ∪ B′] and G − (A′ ∪ B′). �
Shanghai
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GRAPH THEORY (III) Page 9
Third Proof.
Shanghai
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GRAPH THEORY (III) Page 9
Third Proof.
let H be a spanning subgraph of G that satisfies the marriage condition and is edge-minimal
with this property.
Shanghai
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GRAPH THEORY (III) Page 9
Third Proof.
let H be a spanning subgraph of G that satisfies the marriage condition and is edge-minimal
with this property.
By the marriage condition, dH(a) ≥ 1 for every a ∈ A.
Shanghai
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GRAPH THEORY (III) Page 9
Third Proof.
let H be a spanning subgraph of G that satisfies the marriage condition and is edge-minimal
with this property.
By the marriage condition, dH(a) ≥ 1 for every a ∈ A. Suppose there is a vertex a ∈ A
with distinct neighbours b1, b2 in H .
Shanghai
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GRAPH THEORY (III) Page 9
Third Proof.
let H be a spanning subgraph of G that satisfies the marriage condition and is edge-minimal
with this property.
By the marriage condition, dH(a) ≥ 1 for every a ∈ A. Suppose there is a vertex a ∈ A
with distinct neighbours b1, b2 in H .
By the minimality of H , the graphs H − ab1 and H − ab2 violate the marriage condition.
Shanghai
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GRAPH THEORY (III) Page 9
Third Proof.
let H be a spanning subgraph of G that satisfies the marriage condition and is edge-minimal
with this property.
By the marriage condition, dH(a) ≥ 1 for every a ∈ A. Suppose there is a vertex a ∈ A
with distinct neighbours b1, b2 in H .
By the minimality of H , the graphs H − ab1 and H − ab2 violate the marriage condition.
So for i = 1, 2 there is a set Ai ⊆ A containing a such that |Ai| > |Bi| for
Bi := NH−abi(Ai).
Shanghai
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GRAPH THEORY (III) Page 9
Third Proof.
let H be a spanning subgraph of G that satisfies the marriage condition and is edge-minimal
with this property.
By the marriage condition, dH(a) ≥ 1 for every a ∈ A. Suppose there is a vertex a ∈ A
with distinct neighbours b1, b2 in H .
By the minimality of H , the graphs H − ab1 and H − ab2 violate the marriage condition.
So for i = 1, 2 there is a set Ai ⊆ A containing a such that |Ai| > |Bi| for
Bi := NH−abi(Ai).
We can show∣∣NH(A1 ∩ A2 \ {a})
∣∣ ≤ |B1 ∩ B2|= |B1| + |B2| − |B1 ∪ B2|≤ |A1| − 1 + |A2| − 1 − ∣
∣A1 ∪ A2
∣∣
=∣∣A1 ∩ A2 \ {a}
∣∣ − 1.
�
Shanghai
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GRAPH THEORY (III) Page 10
Corollary. If G is k-regular with k ≥ 1, then G has a 1-factor.
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GRAPH THEORY (III) Page 11
A set of preferences for G is a family (≤v)v∈V of linear ordering ≤v on E(G).
Shanghai
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GRAPH THEORY (III) Page 11
A set of preferences for G is a family (≤v)v∈V of linear ordering ≤v on E(G).
A matching M is stable if for every edge e ∈ E \ M there exists an edge f ∈ M such that e
and f have a common vertex v with e <v f .
Shanghai
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GRAPH THEORY (III) Page 11
A set of preferences for G is a family (≤v)v∈V of linear ordering ≤v on E(G).
A matching M is stable if for every edge e ∈ E \ M there exists an edge f ∈ M such that e
and f have a common vertex v with e <v f .
Given a matching M , a vertex a ∈ A is acceptable to a vertex b ∈ B if e = ab ∈ E \ M and
any edge f ∈ M at b satisfies f <b e.
Shanghai
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GRAPH THEORY (III) Page 11
A set of preferences for G is a family (≤v)v∈V of linear ordering ≤v on E(G).
A matching M is stable if for every edge e ∈ E \ M there exists an edge f ∈ M such that e
and f have a common vertex v with e <v f .
Given a matching M , a vertex a ∈ A is acceptable to a vertex b ∈ B if e = ab ∈ E \ M and
any edge f ∈ M at b satisfies f <b e.
A vertex a ∈ A is happy with M if a is unmatched or its matching edge f ∈ M satisfies
f >a e for all edges e = ab such that a is acceptable to b.
Shanghai
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GRAPH THEORY (III) Page 11
A set of preferences for G is a family (≤v)v∈V of linear ordering ≤v on E(G).
A matching M is stable if for every edge e ∈ E \ M there exists an edge f ∈ M such that e
and f have a common vertex v with e <v f .
Given a matching M , a vertex a ∈ A is acceptable to a vertex b ∈ B if e = ab ∈ E \ M and
any edge f ∈ M at b satisfies f <b e.
A vertex a ∈ A is happy with M if a is unmatched or its matching edge f ∈ M satisfies
f >a e for all edges e = ab such that a is acceptable to b.
Lemma. 1. If a matching M is stable, then every vertex a ∈ A is happy with M .
Shanghai
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GRAPH THEORY (III) Page 11
A set of preferences for G is a family (≤v)v∈V of linear ordering ≤v on E(G).
A matching M is stable if for every edge e ∈ E \ M there exists an edge f ∈ M such that e
and f have a common vertex v with e <v f .
Given a matching M , a vertex a ∈ A is acceptable to a vertex b ∈ B if e = ab ∈ E \ M and
any edge f ∈ M at b satisfies f <b e.
A vertex a ∈ A is happy with M if a is unmatched or its matching edge f ∈ M satisfies
f >a e for all edges e = ab such that a is acceptable to b.
Lemma. 1. If a matching M is stable, then every vertex a ∈ A is happy with M .
2. If every matched vertex a ∈ A is happy with M and every unmatched a ∈ A is not
acceptable to any b ∈ B, then M is stable.
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GRAPH THEORY (III) Page 12
Theorem.[Gale and Shapley 1962] For every set of preferences, G has a stable matching.
Shanghai
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GRAPH THEORY (III) Page 12
Theorem.[Gale and Shapley 1962] For every set of preferences, G has a stable matching.
Proof.
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GRAPH THEORY (III) Page 12
Theorem.[Gale and Shapley 1962] For every set of preferences, G has a stable matching.
Proof.
Starting with the empty matching, construct a sequence of matchings that each keeps all the
vertices in A happy.
Shanghai
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GRAPH THEORY (III) Page 12
Theorem.[Gale and Shapley 1962] For every set of preferences, G has a stable matching.
Proof.
Starting with the empty matching, construct a sequence of matchings that each keeps all the
vertices in A happy.
Given such a matching M , consider a vertex a ∈ A that is unmatched but acceptable to
some b ∈ B. (If no such a exists, terminate the sequence.)
Shanghai
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GRAPH THEORY (III) Page 12
Theorem.[Gale and Shapley 1962] For every set of preferences, G has a stable matching.
Proof.
Starting with the empty matching, construct a sequence of matchings that each keeps all the
vertices in A happy.
Given such a matching M , consider a vertex a ∈ A that is unmatched but acceptable to
some b ∈ B. (If no such a exists, terminate the sequence.)
Add to M the ≤a-maximal edge ab such that a is acceptable to b, and discard from M any
other edge at b.
Shanghai
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GRAPH THEORY (III) Page 12
Theorem.[Gale and Shapley 1962] For every set of preferences, G has a stable matching.
Proof.
Starting with the empty matching, construct a sequence of matchings that each keeps all the
vertices in A happy.
Given such a matching M , consider a vertex a ∈ A that is unmatched but acceptable to
some b ∈ B. (If no such a exists, terminate the sequence.)
Add to M the ≤a-maximal edge ab such that a is acceptable to b, and discard from M any
other edge at b.
The new matching M ′ is better than the matching M : every vertex b in an edge f ∈ M is
incident also with some f ′ ∈ M ′ with f ≤b f ′.
Shanghai
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GRAPH THEORY (III) Page 12
Theorem.[Gale and Shapley 1962] For every set of preferences, G has a stable matching.
Proof.
Starting with the empty matching, construct a sequence of matchings that each keeps all the
vertices in A happy.
Given such a matching M , consider a vertex a ∈ A that is unmatched but acceptable to
some b ∈ B. (If no such a exists, terminate the sequence.)
Add to M the ≤a-maximal edge ab such that a is acceptable to b, and discard from M any
other edge at b.
The new matching M ′ is better than the matching M : every vertex b in an edge f ∈ M is
incident also with some f ′ ∈ M ′ with f ≤b f ′. The improvement cannot continue forever.
�
Shanghai
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GRAPH THEORY (III) Page 13
Corollary.[Peterson 1891] Every regular graph (not necessarily bipartite) of positive even
degree has a 2-factor.
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GRAPH THEORY (III) Page 14
2.2 Matching in general graphs
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GRAPH THEORY (III) Page 14
2.2 Matching in general graphs
Let G be a graph. CG denotes the number of its components, and q(G) denotes its set of
odd components, those of odd order.
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GRAPH THEORY (III) Page 14
2.2 Matching in general graphs
Let G be a graph. CG denotes the number of its components, and q(G) denotes its set of
odd components, those of odd order.
Theorem.[Tutte 1947] A graph G has a 1-factor if and only if q(G − S) ≤ |S| for all
S ⊆ V (G).
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GRAPH THEORY (III) Page 14
2.2 Matching in general graphs
Let G be a graph. CG denotes the number of its components, and q(G) denotes its set of
odd components, those of odd order.
Theorem.[Tutte 1947] A graph G has a 1-factor if and only if q(G − S) ≤ |S| for all
S ⊆ V (G).
Proof. (=⇒) Every odd component in G − S must send a factor edge to S.
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GRAPH THEORY (III) Page 15
Proof. (Cont’d)
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GRAPH THEORY (III) Page 15
Proof. (Cont’d)
(⇐=) Let G = (V, E) be a graph without a 1-factor. Our task is to find a bad set S ⊆ V ,
one that violates Tutte’s condition.
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GRAPH THEORY (III) Page 15
Proof. (Cont’d)
(⇐=) Let G = (V, E) be a graph without a 1-factor. Our task is to find a bad set S ⊆ V ,
one that violates Tutte’s condition.
We can assume G is edge-maximal without a 1-factor. (Why?)
Shanghai
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GRAPH THEORY (III) Page 15
Proof. (Cont’d)
(⇐=) Let G = (V, E) be a graph without a 1-factor. Our task is to find a bad set S ⊆ V ,
one that violates Tutte’s condition.
We can assume G is edge-maximal without a 1-factor. (Why?)
Then for a bad S we have:
all the components of G − S are complete
and every vertex s ∈ S is adjacent to all the vertices of G − s.(1)
Otherwise, by adding the missing edges, S is still bad and hence G still has no 1-factor.
Shanghai
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GRAPH THEORY (III) Page 15
Proof. (Cont’d)
(⇐=) Let G = (V, E) be a graph without a 1-factor. Our task is to find a bad set S ⊆ V ,
one that violates Tutte’s condition.
We can assume G is edge-maximal without a 1-factor. (Why?)
Then for a bad S we have:
all the components of G − S are complete
and every vertex s ∈ S is adjacent to all the vertices of G − s.(1)
Otherwise, by adding the missing edges, S is still bad and hence G still has no 1-factor.
But also conversely, if a set S ⊆ V satisfies (1) then either S or the empty set must be bad:
Shanghai
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GRAPH THEORY (III) Page 15
Proof. (Cont’d)
(⇐=) Let G = (V, E) be a graph without a 1-factor. Our task is to find a bad set S ⊆ V ,
one that violates Tutte’s condition.
We can assume G is edge-maximal without a 1-factor. (Why?)
Then for a bad S we have:
all the components of G − S are complete
and every vertex s ∈ S is adjacent to all the vertices of G − s.(1)
Otherwise, by adding the missing edges, S is still bad and hence G still has no 1-factor.
But also conversely, if a set S ⊆ V satisfies (1) then either S or the empty set must be bad: if
S is not bad we can join the odd components of G − S disjointly to S and pair up all the
remaining vertices, unless |G| is odd, in which case ∅ is bad.
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GRAPH THEORY (III) Page 16
Proof. (Cont’d)
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GRAPH THEORY (III) Page 16
Proof. (Cont’d)
Let S be the set of vertices that are adjacent to every other vertex. If this set S does not
satisfy (1), then some component of G − S has nonadjacent vertices a, a′.
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GRAPH THEORY (III) Page 16
Proof. (Cont’d)
Let S be the set of vertices that are adjacent to every other vertex. If this set S does not
satisfy (1), then some component of G − S has nonadjacent vertices a, a′.
Let a, b, c be the first three vertices on a shortest a-a′ path in this component; then
ab, bc ∈ E but ac /∈ E.
Shanghai
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GRAPH THEORY (III) Page 16
Proof. (Cont’d)
Let S be the set of vertices that are adjacent to every other vertex. If this set S does not
satisfy (1), then some component of G − S has nonadjacent vertices a, a′.
Let a, b, c be the first three vertices on a shortest a-a′ path in this component; then
ab, bc ∈ E but ac /∈ E. Since b /∈ S, there is a vertex d ∈ V such that bd /∈ E.
Shanghai
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GRAPH THEORY (III) Page 16
Proof. (Cont’d)
Let S be the set of vertices that are adjacent to every other vertex. If this set S does not
satisfy (1), then some component of G − S has nonadjacent vertices a, a′.
Let a, b, c be the first three vertices on a shortest a-a′ path in this component; then
ab, bc ∈ E but ac /∈ E. Since b /∈ S, there is a vertex d ∈ V such that bd /∈ E.
By the maximality of G, there is a matching M1 of V in G + ac, and a matching M2 of V in
G + bd.
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GRAPH THEORY (III) Page 17
Proof. (Cont’d)
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GRAPH THEORY (III) Page 17
Proof. (Cont’d)
Let P = d . . . v be a maximal path in G starting at d with an edge from M1 and containing
alternately edges from M1 and M2.
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GRAPH THEORY (III) Page 17
Proof. (Cont’d)
Let P = d . . . v be a maximal path in G starting at d with an edge from M1 and containing
alternately edges from M1 and M2.
If the last edge of P lies in M1, then v = b, since otherwise we could continue P .
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GRAPH THEORY (III) Page 17
Proof. (Cont’d)
Let P = d . . . v be a maximal path in G starting at d with an edge from M1 and containing
alternately edges from M1 and M2.
If the last edge of P lies in M1, then v = b, since otherwise we could continue P . Let us
then set C := P + bd.
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GRAPH THEORY (III) Page 17
Proof. (Cont’d)
Let P = d . . . v be a maximal path in G starting at d with an edge from M1 and containing
alternately edges from M1 and M2.
If the last edge of P lies in M1, then v = b, since otherwise we could continue P . Let us
then set C := P + bd.
If the last edge of P lies in M2, then by the maximality of P the M1-edge at v must be ac,
so v ∈ {a, c};
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GRAPH THEORY (III) Page 17
Proof. (Cont’d)
Let P = d . . . v be a maximal path in G starting at d with an edge from M1 and containing
alternately edges from M1 and M2.
If the last edge of P lies in M1, then v = b, since otherwise we could continue P . Let us
then set C := P + bd.
If the last edge of P lies in M2, then by the maximality of P the M1-edge at v must be ac,
so v ∈ {a, c}; then let C be the cycle dPvbd.
In each case, C is an even cycle with every other edge in M2, and whose only edge not in E
is bd.
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GRAPH THEORY (III) Page 17
Proof. (Cont’d)
Let P = d . . . v be a maximal path in G starting at d with an edge from M1 and containing
alternately edges from M1 and M2.
If the last edge of P lies in M1, then v = b, since otherwise we could continue P . Let us
then set C := P + bd.
If the last edge of P lies in M2, then by the maximality of P the M1-edge at v must be ac,
so v ∈ {a, c}; then let C be the cycle dPvbd.
In each case, C is an even cycle with every other edge in M2, and whose only edge not in E
is bd.
Replacing in M2 its edges on C with the edges of C − M2, we obtain a matching of V
contained in E, a contradiction.
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GRAPH THEORY (III) Page 18
Corollary.[Petersen 1891] Every bridgeless cubic graph has a 1-factor.
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