# GRAPH THEORY STUDY GUIDE

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1 GRAPH THEORY STUDY GUIDE 1. Definitions Definition 1 (Partition of A). A set A = A 1,..., A k of disjoint subsets of a set A is a partition of A if A of all the sets A i A and A i for every i. Definition 2 (Vertex set). The set of vertices in a graph denoted by V (G). Definition 3 (Edge set). The set of edges in a graph denoted by E(G). Definition 4 (Order). the number of vertices of a graph G written G. Definition 5 (Incident). A vertex v is incident with an edge e if v e, then e is an edge at v. Definition 6 (Adjacent). Two vertices x, y of G are adjacent if xy is an edge of G. Definition 7 (Complete). If all vertices of G are pairwise adjacent, then G is complete. Definition 8 (Independent). A set of vertices or edges is independent if no two of its elements are adjacent. Definition 9 (Isomorphic). We call G and H isomorphic, and write G H, if there exists a bijection φ : V V with xy E φ(x)φ(y) E for all x, y in V. Definition 10 (Property). A class of graphs that is closed under isomorphism is called a graph property. Definition 11 (Invariant). A map taking graphs as arguments is called a graph invariant if it assigns equal values to isomorphic graphs. Definition 12 (Induced). If G G and G contains all the edges xy E with x, y V, then G is an induced subgraph of G. We say that V induces G in G. Definition 13 (Spanning). G G is a spanning subgraph of G if V spans all of G or V = V. Definition 14 (Line Graph). The line graph L(G) of G is the graph of E in which x, y E are adjacent as vertices if and only if they are adjacent as edges in G. Definition 15 (N(G)). the set of neighbors of a vertex v. Definition 16 (Degree). The degree (d(v)) of a vertex v is the number E(v) of edges at v or the number of neighbors of v. Definition 17 (Isolated). A vertex of degree 0 is isolated. Definition 18 (Minimum degree). δ(g) is the minimum degree of G. Definition 19 (Maximum degree). (G) = maximum degree of G. Definition 20 (Regular). all the vertices of G have the same degree k, then G is k-regular, or regular. Definition 21 (Average degree). d(g) = 1 V Definition 22 (Number of Edges). E = 1 2 v V d(v) and δ(g) d(g) (G). v V d(v) = 1 2d(G) V. Definition 23 (Path). A path is a non-empty graph P = (V, E) of the form V = {x 0, x 1,..., x k } and E = {x 0 x 1,..., x k 1 x k } where the x i are all distinct. Definition 24 (H-path). Given a graph H, we call P an H-path if P is non-trivial and meets H exactly in its end points. Definition 25 (Cycle). If P = x 0,...x k 1 is a path and k 3, then the graph C = P + x k 1 x 0 is called a cycle. 1

2 Definition 26 (Girth). The minimum length of a cycle in a graph G is the girth (g(g)). Definition 27 (Circumference). The maximum length of a cycle in G is its circumference. Definition 28 (Chord). An edge which joins two vertices of a cycle but is not itself an edge of the cycle is a chord of the cycle. Definition 29 (Induced cycle). an induced cycle in G, a cycle in G forming an induced subgraph, is one that has no chords. Definition 30 (Distance). The distance d G (x, y) in G of two vertices x, y is the length of a shortest x y path in G. Definition 31 (Diameter). The greatest distance between any two vertices in G is the diameter of G, denoted by diam G. Definition 32 (Central). a vertex is central in G if its greatest distance from any other vertex is as small as possible. Definition 33 (Radius). the greatest distance between the central vertex and from any other vertex denoted rad G where radg = min x V (G) max y V (G) d G (x, y) where radg diamg 2radG. Definition 34 (Walk). A walk of length k is a non-empty alternating sequence of vertices and edges in G. Definition 35 (Connected). A non-empty graph G is called connected if any two of its vertices are linked by a path in G. Definition 36 (Component). A maximal connected subgraph of G is called a component. Definition 37 (Separator). If A, B V and X V E are such that every A B path in G contains a vertex or an edge from X, we say that X separates the set A and B in G. Definition 38 (Cutvertex). A vertex which separates two other vertices of the same component is a cutvertex. Definition 39 (Bridge). An edge separating its ends is a bridge. Definition 40 (k-connected). G is called k-connected if G > k and G X is connected for every set X V with X < k. (No vertices of G are separated by fewer than k other vertices.) Definition 41 (Connectivity). The greatest integer k such that G is k-connected is the connectivity, κ(g). Definition 42 (l-edge-connected). If G > 1 and G F is connected for every set F E of fewer than l edges, then G is called l-edge-connected. Definition 43 (Edge-connectivity). The greatest integer l such that G is l-edge-connected is λ(g). Definition 44 (Forest). An acyclic, graph, one not containing any cycles is called a forest. Definition 45 (Tree). A connected forest is called a tree. Definition 46 (Leaf). The vertices of degree 1 of a tree. Definition 47 (Tree-order). Writing x y for x rt y then define a partial ordering on V (T ), the tree -order associated with T and r. Definition 48 (Normal Tree). A rooted tree T contained in a graph G is called normal in G if the ends of every T -path in G are comparable in the tree order of T. Definition 49 (R-partite). A graph G = (V, E) is called r-partitie if V admits a partition into r classes such that every edge has its ends in different classes. Definition 50 (Complete Partite). An r-partite graph in which every two vertices from different partitions classes are adjacent is called complete. Definition 51 (Contraction). Let e = xy be an edge of the graph G. By G/e we denote the graph obtained from G by contracting the edge e into a new vertex v e.

3 Definition 52 (Minor). If G = MX is a subgraph of another graph Y, we call X a minor of Y. Definition 53 (Eulerian). A closed walk in a graph is an Euler tour if it traverses every edge of the graph exactly once. Its called Eulerian if it admits an Euler tour. Definition 54 (Vertex Space). The vertex space of G is the vector space over the 2-element field of all functions V F 2, V(G). Definition 55 (Edge Space). The function E F 2 form the edge space of G, E(G). Definition 56 (Cycle Space). The cycle space is the subspace of the edge space spanned by all the cycles in G. C = C(G). Definition 57 (Cut Space). The cut edges of G form a subspace of E(G) denoted by C (G). Definition 58 (Hypergraph). A hypergraph is a pair (V, E) of disjoint sets, where the elements of E are non-empty subset (of any cardinality) of V. Definition 59 (Directed). : A directed graph is a pair (V, E) of disjoint set (of vertices and edges ) together with two maps init : E V and ter : E V assigning to every edge e an initial vertex init(e) and a terminal vertex ter(e). Definition 60 (Orientation). A directed graph D is an orientation of an (undirected) graph G if V (D) = V (G) ande(d) = E(F ) and if {init(e), ter(e)} = {x, y} for every edge e = xy. Definition 61 (Multigraph). A multigraph is a pair (V, E) of disjoint sets (of vertices and edges) together with a map E V [V ] 2 assigning to every edge either one or two vertices, its ends. Definition 62 (Matching). A set M of independent edges in a graph G = (V, E)is called a matching. Definition 63 (Factor). A k-regular spanning subgraph is called k-factor. Definition 64 (Alternating Paths). 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. Definition 65 (Augmenting Path). An alternating path P that ends in an unmatched vertex of B is called an augmenting path. Definition 66 (Cover). Let us call a set U V a (vertex) covering of E if every edge of G is incident with a vertex in U. Definition 67 (q(g)). Given a graph G, let us denote by C G the set of its components and by q(g) the number of its odd components, those of odd order. Definition 68 (Block). A maximal connected subgraph without a cutvertex is called a block. Definition 69 (Plane Graph). Graph drawing in the plane. Definition 70 (Planar). If G can be drawn in the plane without a crossing, G is plannar. Definition 71 (Frontier). The frontier of a set X R 2 is the set Y of all points y R 2 such that every neighborhood of y meets both X and R 2 X. Definition 72 (Geometric Graph). A Geometric Graph is a planar embedding of G with straight lines. Definition 73 (Topological Isomorphism). We call σ a topological isomorphism between H and H plane graphs if there exists a homeomorphism ϕ : S 2 S 2 such that ψ := π ϕ π 1 induces σ on V E. Definition 74 (Combinatorial Isomorphism). σ can be extended to an incidence preserving map: σ : V E F V E F. (preserves incidence of not only vertices and edges but also of vertices and edges with faces.) Definition 75 (Graph Theoretical Isomorphism). σ is a graph theoretical isomorphism of plane H and H if {σ(h[f]) : f F } = {H [f ] : f F }. Definition 76 (Simple). We call a subset F of a graph s edge space E(G) simple if every edge of G lies in at most two sets of F.

4 Definition 77 (Visible). Given a polygon P, vertices w, v, w is visible from v, if line segment vw P c =. Definition 78 (Plane Multigraph). A plane multigraph is a pair G = (V, E) of finite sets (of vertices and edges, respectively) satisfying the following conditions: (1) V R 2 (2) every edge is either an arc between two vertices or a polygon containing exactly one vertex (its endpoints). (3) apart from its own endpoints(s), and edge contains no vertex and no point of any other edge. Definition 79 (Plane Dual). A plane dual (V, E ) = G if there are bijections F V, E E, V F, where f v (f), e e, v f (v) satisfying the following conditions (1) i. v (v) f for all f F (2) e G = e e = e G = 1 for all e E, and in each of e and e this point is an inner point of a straight line segment. (3) v f (v) for all v V. Definition 80 (Proper coloring). If c(v) c(w) for all v adjacent w, then c is a proper coloring. Definition 81 (k-colorable). G is k-colorable if there exist proper coloring of the vertices with k colors. Definition 82 (k-edge-colorable). G is k-edge-colorable if there exists a proper edge coloring with k colors or proper coloring of line graph L(G). Definition 83 (χ(g)). minimum k so that G is k-colorable or chromatic number of G. Definition 84 (χ (G)). minimum k so that G is k-edge colorable or chromatic index of G. Definition 85 (c 1 (j)). color class of all the vertices colored j. Definition 86 (k-constructible). For each k N, let us define the class of k-constructible graphs recursively as follows: (1) K k is k-constructible. (2) If G is k-constructible and x, y V (G) are non-adjacent, then also (G + xy)/xy is k-constructible. (3) If G 1, G 2 are k-constructible and there are vertices x, y 1, y 2 such that G 1 G 2 = {x} and xy 1 E(G 1 ) and xy 2 E(G 2 ), then also (G 1 G 2 ) xy 1 xy 2 + y 1 y 2 is k-constructible. 2. Theorems Proposition 87. The number of vertices of odd degree in a graph is always even. Proposition 88. Every graph G with at least one edge has a subgraph H with δ(h) > ɛ(h) ɛ(g). Proposition 89. Every graph G contains a path of length δ(g) and a cycle of length at least δ(g)+1(provided that δ(g) 2). Proposition 90. Every graph G containing a cycle satisfies g(g) 2diam(G) + 1. Proposition 91. A graph G of radius at most k and maximum degree at most d 3 has fewer than d d 2 (d 1)k vertices. Theorem 92. Let G be a graph. If d(g) d 2 and g(g) g N then G n 0 (d, g) where n 0 (d, g) = 1 + d r 1 i=0 (d 1)i if g = 2r + 1 is odd; 2 r 1 i=0 (d 1)i if g = 2r is even. Corollary 93. If δ(g) 3 then g(g) < 2log G. Proposition 94. The vertices of a connected graph G can always be enumerated say as v 1,..., v n so that G i = G[v 1,..., v i ] is connected for every i. Proposition 95. If G is non-trivial then κ(g) λ(g) δ(g). Theorem 96. Let 0 k N. Every graph G with d(g) 4k has (k + 1)-connected subgraph H such that ɛ(h) > ɛ(g) k.

5 Theorem 97. The following assertions are equivalent fr a graph T: (1) T is a tree. (2) Any two vertices of T are linked by a unique path in T. (3) T is minimally connected, i.e. T is connected by T e is disconnected for every edge e in T (4) T is maximally acyclic, i.e. T contains no cycle but T + xy does, for any tow non-adjacent vertices x, y T. Corollary 98. The vertices of a tree can always be enumerated, say as v 1,..., v n so that every v i with i 2 has a unique neighbor in {v 1,..., v n 1 }. Corollary 99. A connected graph with n vertices is a tree if and only if it has n 1 edges. Corollary 100. If T is a tree and G is any graph with δ(g) T 1, then T G, i.e. G has a subgraph isomorphic to T. Lemma 101. Let T be a normal tree in G. (1) Any two vertices x, y in T are separated in G by the set x y. (2) If S V (T ) = V (G) and S is down-closed, then the components of G S are spanned by the sets x with x minimal in T S. Proposition 102. Every connected graph contains a normal spanning tree, with any specified vertex as its root. Proposition 103. A graph is bipartite if and only if it contains no odd cycles. Proposition 104. (1) Every TX is also an MX; thus, every topological minor of a graph is also its (ordinary) minor. (2) If (X) 3, then every MX contains a TX; thus every minor with maximum degree at most 3 of a graph is also its topological minor. Proposition 105. The minor relation and the topological minor relation are partial orderings on the class of finite graphs, ie. that are reflexive, antisymmetric and transitive. Theorem 106. A connected graph is Eulerian if and only if Every vertex has even degree. Proposition 107. The induced cycles in G generate its entire cycle space. Proposition 108. The following assertions are equivalent for edge set F E (1) F C(G) (2) F is a disjoint union of (edge sets of ) cycles in G. (3) All vertex degrees of the graph (V,F) are even. Proposition 109. Together with, the cuts in G form a subspace C of E. This space is generated by cuts of the form E(v). Lemma 110. Every cut is a disjoint union of bonds. Theorem 111. The cycle space C and the cut space C of any graph satisfy C = C and C = C. Theorem 112. Let G be a connected graph and T G a spanning tree. Then the corresponding fundamental cycles and cuts form a basis of C(G) and of C (G), respectively. If G has n vertices and m edges, then dim(c(g)) = m n + 1 and dim(c (G)) = n 1. Theorem 113 (Konig). The maximum cardinality of a matching in G is equal to the minimum cardinality of a vertex cover of its edges. Theorem 114 (Hall). G contains a matching of A if and only if N(S) S for alls A. Corollary 115. If G is k regular, with k 1, then G has a 1-factor. Corollary 116. Every regular graph of positive even degree has a 2-factor. Theorem 117 (Tutte). A graph G has a 1-factor if and only if q(g S) S fors V (G). Corollary 118 (Petersen). Every bridgeless cubic graph has a 1-factor.

6 Theorem 119. Every graph G = (V, E) contains a vertex set S with the following two properties: (1) S is matchable to C G S. (2) Every component of G S is factor-critical. Given any such set S, the graph G contains a 1-factor if and only if S = C G S Theorem 120. G is 2-connected iff either i. G is a cycle or ii. G can be obtained from a 2-connected H by adding an H-path. Lemma 121. If G is 3-connected and G > 4, then G has one edge s.t. G e is 3-connected. Theorem 122. G is 3-connected iff there exists a sequence of graphs G 0,..., G n such that: (1) G 0 = K 4 ; G n = G (2) G i+1 has an edge xy with d(x), d(y) 3 and G i = G i+1 /xy, i < n. Theorem 123 (Tutte). The cycle space of a 3-connected graph is generated by non-separating induced cycle. Theorem 124 (Menger). Let G = (V, E) be a graph and A, B V. Then the minimum number of vertices separating A from B in G is equal to the maximum number of disjoint A B paths in G. Corollary 125. Let a, b be distinct vertices of G. (1) If a, b E, then minimum number of vertices ab separating a from b equal maximum number of independent a b path. (2) the minimum number of edges separating a from b in G is equal to the max number of edges disjoint a b paths. Theorem 126 (Global Menger s Theorem). (1) A graph is k-connected if and only if it contains k independent path between any two vertices. (2) A graph is k-edge-connected if and only if it contains k edge-disjoint paths between any two vertices. Theorem 127 (Jordan Curve Theorem for Polygons). For every polygon P R 2, the set R 2 P has exactly two regions. Each of these has the entire polygon P as its frontier. Lemma 128. Let P 1, P 2, P 3 be three arcs, between the same two endpoints but otherwise disjoint: (1) R 2 (P 1 P 2 P 3 ) has exactly three regions, with frontiers P 1 P 2, P 2 P 3, andp 1 P 3. (2) If P is an arc between a point in P 1 and a point in P 3 whose interior lies in the region of R 2 (P 1 P 3 ) that contains P 2, then P P 2. Lemma 129. Let X 1, X 2 R 2 be disjoint sets, each the union of finitely many points and arcs, and let P be an arc between a point in X 1 and one in X 2 whose interior P lies in a region O of R 2 (X 1 X 2 ). Then O\ P is a region of R 2 (X 1 P X 2 ). Theorem 130. Let ϕ : C 1 C 2 be a homeomorphism between two circles on S 2, let O 1 be a region of C 1, and let O 2 be a region of C 2. Then ϕ can be extended to a homeomorphism C 1 O 1 C 2 O 2. Lemma 131. Let G be a plane graph, and e an edge of G. (1) If X is the frontier of a face of G, then either e XorX e =. (2) If e lies on a cycle C G, then e lies on the frontier of exactly two faces of G, and these are contained in distinct faces of C. (3) If e lies on no cycle, then e lies on the frontier of exactly one face of G. Corollary 132. The frontier of a face is always the point set of a subgraph. Proposition 133. A plane forest has exactly one face. Lemma 134. If a plane graph has different face with the same boundary, then the graph is a cycle. Proposition 135. In a 2-connected plane graph, every face is bounded by a cycle. Proposition 136. The face boundaries in a 3-connected plane graph are precisely its non-separating induced cycles. Proposition 137. A plane graph of order at least 3 is maximally planar if and only if it is a plane triangulation.

7 Theorem 138 (Euler s Formula). Let G be a connected plane graph with n vertices, m edges, and l faces, then n m + l = 2. Corollary 139. A plane graph with n 3 vertices has at most 3n 6 edges. Every plane triangulation with n vertices has 3n 6. Corollary 140. A plane graph contains neither K 5 nor K 3,3 as a topological minor. Theorem 141. (1) Every graph-theoretical isomorphism between two plane graphs is combinatorial. Its extension to a face bijection is unique if and only if the graph is not a cycle. (2) Every combinatorial isomorphism between two 2-connected plane graphs is topological. Proposition 142. Every two embedding of a 3-connected Graph are equivalent. Theorem 143 (Kuratowski). The following are equivalent: (1) G is planar. (2) G has no K 5 and K 3,3 minor. (3) G has no T K 5 or T K 3,3 minor. Lemma 144. Every 3-connected G with no K 5 or K 3,3 minor is planar. Lemma 145. Let χ be a set of 3-connected graphs. Let G be a graph with κ(g) 2 and G 1, G 2 proper induced subgraph of G such that. G = G 1 G 2 and G 1 G 2 = κ(g). If G is edge maximal with respect to not having a topological minor in χ, then so are G 1 and G 2, and G 1 G 2 = K 2. Lemma 146. If G 4 and G is edge-maximal with respect to having no T K 5 or T K 3,3 minor, then G is 3-connected. Theorem 147 (Maclane). G is planar if and only if its cycle space has a simple basis. Theorem 148. A 3-connected graph is planar if and only if every edge lies on at most 2 non separating induced cycles.(equivalently: exactly). Proposition 149. For any connected plane multigraph G, an edge set E E(G) is the edge set of a cycle in G if and only if E := {e e E} is a minimal cut in G. Proposition 150. If G is an abstract dual of G, then the cut space of G is the cycle space of G, C (G ) = C(G). Theorem 151. A graph is planar if and only if it has an abstract dual. Theorem 152. All planar graphs are 4-colorable. Theorem 153. Every planar graph is 5-colourable. Theorem 154. Every planar graph not containing a triangle is 3-colourable. Proposition 155. Every graph G with m edges satisfies χ(g) (2m ). Proposition 156. Every graph G satisfies χ(g) col(g) = max δ(h) H G + 1. Corollary 157. Every graph G has a subgraph of minimum degree at least χ(g) 1. Theorem 158 (Brooks). Let G be a connected graph. χ(g) (G). If G is neither complete nor an odd cycle, then Theorem 159 (Erdős). For every integer k there exists a graph G with girth g(g) > k and chromatic number χ(g) > k. Theorem 160 (Hajós). Let G be a graph and k N. Then χ(g) k if and only if G has a k-constructible subgraph. Proposition 161 (Konig). Every bipartite graph G satisfies χ (G) = (G). Theorem 162 (Vizing). Every graph G satisfies (G) χ (G) (G) + 1.

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