# Basic Combinatorics. Math 40210, Section 01 Fall Basic graph definitions

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1 Basic Combinatorics Math 40210, Section 01 Fall 2012 Basic graph definitions It may seem as though the beginning of graph theory comes with a lot of definitions. This may be so, but hopefully by repeatedly using them you will very quickly become accustomed to them all. This document serves as a cheat-sheet for all the basic definitions that will be important throughout the course. A graph (usually denoted by G) consists of two objects: a finite, non-empty set V of vertices, and a set E of edges. The set V can be anything, but the set E must consist of unordered pairs of distinct elements of V. Think of V as a set of points (called vertices), and think of E as specifying which pairs of vertices are joined. Properly, what has just been described is a simple graph. The simple here indicates that we do not allow an edge to go from a vertex to itself, nor do we allow multiple edges to join the same pair of vertices. (Other types of graphs are briefly considered in Section 1.1.1, but will not play a major role in the course.) An single element v of V is a called a vertex. (NB the unusual singular/plural: vertex/vertices. It s just the same as index/indices, or appendix/appendices.) Properly we should write an edge as {u, v} (an unordered set of two distinct vertices), but for the sake of preserving our sanity we will typically just write uv, or e = uv. The edge e = uv is said to join u and v, and u and v, the endvertices of e, are adjacent. Also, both u and v are said to be incident with e, and we say that u is a neighbor of v, and vice versa. If uv is not an edge, then u and v are non-adjacent. The order of a graph G is the number of vertices, often denoted by n, so n = V. The size of G is the number of edges, often denoted by m, so m = E. The open neighborhood of a vertex v, denoted N(v), is the set of vertices that are joined to v by an edge. The degree of v, denoted deg(v), is the size of the open neighborhood of v, that is, deg(v) = N(v) is the number of neighbors that v has. The closed neighborhood of v, denoted N[v], consists of N(v) together with v itself. When we say neighborhood without any adjective, we always mean the open neighborhood. The degree sequence of G is the n-term sequence listing all of the degrees of vertices of G (most typically in decreasing order). The maximum degree of G, denoted by 1

2 or (G), is the largest degree of any vertex in G (so the largest entry in the degree sequence). The minimum degree, denoted by δ or δ(g), is the smallest degree of any vertex in G. A walk is a list v 1,..., v k of vertices (not necessarily distinct) with the property that v 1 v 2, v 2 v 3, etc., are all edges. If there is no repetition among these edges, then the walk is a trail. If there are no repetitions among the vertices, the walk is a path. So a walk is any kind of continuous perambulation; a trail doesn t visit the same edge twice (but may visit a vertex multiple times) and a path neither visits an edge nor a vertex twice. Adding the edge v k v 1 to a path results in a closed path or cycle (we will usually say cycle). If a trail begins and ends at the same vertex, it is called a closed trail or a circuit (we will usually say circuit). The length of a walk, trail, path, cycle circuit is the number of edges (counted with repetitions, if necessary) involved. A subgraph of G is obtained by deleting some (perhaps no) vertices, and then some (perhaps no) edges. When a vertex is deleted, all edges which have that vertex as an endvertex must be deleted too. We write H G to indicate that H is a subgraph of G, and sometimes say G contains H. An induced subgraph of G is obtained by only deleting some (perhaps no) vertices. Such a subgraph is completely determined by the vertices that we choose not to delete. If S is that set of vertices, we write S to indicate the induced subgraph. If H is an induced subgraph of G, we sometimes say that G contains H as an induced subgraph. The graph on n vertices that has all possible edges is called the complete graph on n vertices, and it is denoted K n. The graph on n vertices that has no edges is called the empty graph on n vertices, and it is denoted E n. The graph on n vertices v 1,..., v n that has exactly the edges v 1 v 2, v 2 v 3,..., v n 1 v n is called the path on n vertices, and it is denoted P n. If in addition it has the edge v n v 1 then it is called the cycle on n vertices, and it is denoted C n. The complement of G, denoted G, is the graph on the same vertex set as G that has uv as an edge if and only if G does not have uv as an edge. A graph is said to be bipartite is it is possible to partition the vertex set as V (G) = X Y in such a way that all edges have exactly one end vertex in X and one endvertex in Y. The sets X and Y (which are not necessarily unique) are called the partite sets of G, or sometimes the bipartition classes. If in a bipartite graph we have all possible edges between X and Y present, then the graph is said to be a complete bipartite graph. If X = a and Y = b then such a graph is denoted K a,b. A graph is regular if every vertex has the same degree. If the degree is given, say d, the graph is said to be d-regular. 2

3 Two graphs G and H are said to be isomorphic if there is a bijection f : V (G) V (H) such that uv E(G) if and only if f(u)f(v) E(H). Another way to say this is that two graphs are isomorphic if they can be made to look identical by a relabeling of the vertices. We write G = H, or G = H, to indicate that G is isomorphic to H, and sometimes say that G is H. If G and H are isomorphic then any statement that is true for G and that does not mention the names of the vertices is also true for H. An unlabeled graph is a graph in which we do not name the vertices. Strictly speaking, when we talk about an unlabeled graph we are talking simultaneously about a particular labeled graph, and every other graph that is isomorphic to it (for example, there are infinitely many graphs that satisfy the description of a cycle on four vertices; when we talk about the cycle on four vertices we mean the whole class of graphs that consist of four vertices and four edges arranged in a cycle, all of which are isomorphic to one another). The distance between two vertices u, v in a graph is the length of the shortest path join them, or if they are not in the same component. It is denoted d(u, v). For a connected graph G, the eccentricity of a vertex v, denoted by ecc(v), is the length of the longest path that starts at v. The radius of G, denoted by rad(g), is the value of the smallest eccentricity, and the diameter of G, denoted by diam(g), is the value of the largest eccentricity. So the diameter is the length of the longest path that can be found in G. The adjacency matrix A = A(G) of a graph G with vertices {1,..., n} is the n by n matrix with ij entry 0 if ij is not an edge, and 1 if it is an edge. The degree matrix D = D(G) of G is the n by n matrix with ii entry equal to the degree of vertex i for each i, and all other entries 0. The Laplacian of G is the n by n matrix L = D A. A tree is a connected graph that has no cycles (also called an acyclic graph). Equivalently, it is a connected graph with n 1 edges (where n is the number of vertices), or an acyclic graph with n 1 edges. A forest is a graph with no cycles, or equivalently, a graph all of whose components are trees. A tree is considered to be an example of a forest. A vertex of degree 1 in a tree is called a leaf. A spanning tree of a connected graph is a minimal connected subgraph. Equivalently, it is a subgraph that is a tree, and includes all of the vertices of the graph. If w : E(G) R + is a function that assigns a non-negative weight w(e) to each edge e of a graph G, then the weight of a subgraph is the sum of the weights of the edges in that subgraph. A spanning tree that has the property that there is no other spanning tree of smaller weight is called a minimum weight spanning tree. An Eulerian trail (or Euler trail) in a graph is a trail that visits every edge (once and only once, by definition of a trail). An Eulerian circuit (or Euler circuit) is a circuit that visits every edge (and so, by definition of circuit, it begins and ends at the same vertex and visits each edge once and only once). A graph with an Euler circuit is said to be Eulerian. 3

4 A Hamiltonian path (or Hamilton path) in a graph is a path that visits every vertex (once and only once, by definition of a path). A Hamiltonian cycle (or Hamilton cycle) is a cycle that visits every vertex (and so, by definition of cycle, it begins and ends at the same vertex and visits each other vertex once and only once). A graph with a Hamilton path is said to be traceable; a graph with a Hamilton cycle is said to be Hamiltonian (or a Hamilton graph). A graph is planar if it can be drawn in the plane without any crossing edges, and such a representation is called a planar representation. A region of a planar representation is a maximal connected piece of the plane, when the drawing of the graph is removed. A bounding edge for a region is an edge whose removal increases the area of the region. The bounding degree of a region, denoted b(r), is the number of bounding edges that region has. A graph H is a subdivision of a graph G if it possible to obtain H from G by replacing each edge uv of G with a path, with all the non-end vertices of the paths being new vertices, and with each new vertex being involved in just one path. A proper k-coloring of a graph G is a function K : V (G) {1,..., k} satisfying K(x) K(y) whenever xy E(G); in other words, it is an assignment of colors to the vertices, using a palette of k colors, such that adjacent vertices receive distinct colors. A proper k-coloring is sometimes just called a k-coloring. A graph is said to be k-colorable if there is a k-coloring of it. The chromatic number of G is the smallest k for which there exists a k-coloring. Vertices mapped to the same color in a coloring are called color classes. A clique in a graph G is a set of mutually adjacent vertices (i.e., a complete graph). The clique number of G, denoted ω(g), is the number of vertices in the largest clique in G. An independent set in G is a set of mutually non-adjacent vertices (i.e., an empty graph). The independence number of G, denoted α(g), is the number of vertices in the largest independent set in G. A matching M in a graph G is a set of disjoint edges, that is, a set of edges that have no end vertices in common. Sometimes such a set of edges is called an independent set of edges. If a vertex appears as an endvertex of some edge in M, it is said to be M-saturated; otherwise, it is said to be M-unsaturated. A matching is maximal if it cannot be extended by the addition of more edges, and maximum if there is no matching of larger size (i.e., having more edges) in the graph. A matching is said to perfect if it saturates all the vertices of the graph. For a matching M in a graph G, an M-alternating path is a path in G every second vertex of which lies in M. An M-augmenting path is an M-alternating path with the property that the two endvertices of the path are M-unsaturated. If G is a bipartite graph with partite sets X and Y, we say that X can be matched into Y if there is a matching in G that saturates X, that is, one in which all vertices from X appear as an endvertex of some edge in the matching. 4

5 If G is a bipartite graph with partite sets X and Y, the set X is said to satisfy Hall s condition if for every S X, N(S) S, where N(S) is the set of vertices in Y adjacent to something in S. If G is an arbitrary graph, it is said to satisfy Tutte s condition if for every S V (G) we have Ω(G S) S, where Ω(H) is the number of components of a graph H with odd order. For a family X = {S 1,..., S n } of sets, a system of distinct representatives is a set {x 1,..., x n } of distinct elements with x i S i for each i = 1,..., n. A set C of vertices in a graph G is said to cover the edges of G if every edge of G is incident with at least one vertex of C. Such a set C is called an edge cover of G. It is also often called a vertex cover. 5

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