# Introduction III. Graphs. Motivations I. Introduction IV

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1 Introduction I Graphs Computer Science & Engineering 235: Discrete Mathematics Christopher M. Bourke cbourke@cse.unl.edu Graph theory was introduced in the 18th century by Leonhard Euler via the Königsberg bridge problem. In Königsberg (old Prussia), a river ran through town that created an island and then split off into two parts. Seven bridges were built so that people could easily get around. Euler wondered, is it possible to walk around Königsberg, crossing every bridge exactly once? Introduction II Introduction III To solve this problem, we need to model it mathematically. Specifically, we can define a graph whose vertices are the land areas and whose edges are the bridges. v 1 b 0 b 1 b 4 b 2 b 3 v 2 b 5 v 4 v 3 b 6 Introduction IV Motivations I The question now becomes, does there exist a path in the following graph such that every edge is traversed exactly once? v 1 b 0 b 1 v 2 b 2 b 3 v 3 b 4 b 5 b 6 v 4 Graph Theory has lots of applications (many problems can be modeled as graphs). Problems involving geographic/physical relations Networking Relations between entities Workflow applications Linguistics Chemistry, Physics, Biology, etc.

2 s I s II A simple undirected graph G = (V, E) is a 2-tuple with V = {v 1, v 2,..., v n } a finite set of vertices. E V V = {e 1, e 2,..., e m } an unordered set of edges where each e i = (v, v ) is an unordered pair of vertices, v, v V. Since V and E are sets, it makes sense to consider their cardinality. As is standard, V = n denotes the number of vertices in G and E = m denotes the number of edges in G. A multigraph is a graph in which the edge set E is a multiset. Multiple distinct (or parallel) edges can exist between vertices. A pseudograph is a graph in which the edge set E can have edges of the form (v, v) called loops A directed graph is one in which E contains ordered pairs. The orientation of an edge (v, v ) is said to be from v to v. A directed multigraph is a multigraph whose edges set consists of ordered pairs. s III Terminology Adjacency If we look at a graph as a relation then, among other things, Undirected graphs are symmetric. Non-pseudographs are irreflexive. Multigraphs have nonnegative integer entries in their matrix; this corresponds to degrees of relatedness. Other types of graphs can include labeled graphs (each edge has a uniquely identified label or weight), colored graphs (edges are colored) etc. For now, we will concern ourselves with simple, undirected graphs. We now look at some more terminology. Two vertices u, v in an undirected graph G = (V, E) are called adjacent (or neighbors) if e = (u, v) E. We say that e is incident with or incident on the vertices u and v. Edge e is said to connect u and v. u and v are also called the endpoints of e. Terminology Degree Terminology Handshake Lemma The degree of a vertex in an undirected graph G = (V, E) is the number of edges incident with it. The degree of a vertex v V is denoted deg(v) In a multigraph, a loop contributes to the degree twice. A vertex of degree 0 is called isolated. Lemma Let G = (V, E) be an undirected graph. Then 2 E = v V deg(v) The handshake lemma applies even in multi and pseudographs. proof By definition, each e = (v, v ) will contribute 1 to the degree of each vertex, deg(v), deg(v ). If e = (v, v) is a loop then it contributes 2 to deg(v). Therefore, the total degree over all vertices will be twice the number of edges.

3 Terminology Handshake Lemma Terminology - Directed Graphs I In a directed graph (digraph), G = (V, E), we have analogous definitions. Corollary An undirected graph has an even number of vertices of odd degree. Let e = (u, v) E. u is adjacent to or incident on v. v is adjacent from or incident from u. u is the initial vertex. v is the terminal vertex. For a loop in a pseudograph, these are the same. Terminology - Directed Graphs II Terminology - Directed Graphs III We make a distinction between incoming and outgoing edges with respect to degree. Let v V. The in-degree of v is the number of edges incident on v deg (v) The out-degree of v is the number of edges incident from v. Every edge e = (u, v) contributes 1 to the out-degree of u and 1 to the in-degree of v. Thus, the sum over all vertices is the same. Theorem Let G = (V, E) be a directed graph. Then deg (v) = deg + (v) = E v V v V deg + (v) More Terminology I More Terminology II A path in a graph is a sequence of vertices, v 1 v 2 v k such that (v i, v i+1 ) E for all i = 1,..., k 1. We can denote such a path by p : v 1 v k. The length of p is the number of edges in the path, p = k 1 A cycle in a graph is a path that begins and ends at the same vertex. v 1 v 2 v k v 1 Cycles are also called circuits. We define paths and cycles for directed graphs analogously. A path or cycle is called simple if no vertex is traversed more than once. From now on we will only consider simple paths and cycles.

4 Classes Of Graphs Bipartite Graphs Complete Graphs Denoted K n are simple graphs with n vertices where every possible edge is present. Cycle Graphs Denoted C n are simply cycles on n vertices. Wheels Denoted W n are cycle graphs (on n vertices) with an additional vertex connected to all other vertices. n-cubes Denoted Q n are graphs with 2 n vertices corresponding to each bit string of length n. Edges connect vertices whose bit strings differ by a single bit. Grid Graphs finite graphs on the N N grid. A graph is called bipartite if its vertex set V can be partitioned into two disjoint subsets L, R such that no pair of vertices in L (or R) is connected. We often use G = (L, R, E) to denote a bipartite graph. Bipartite Graphs Bipartite Graphs Theorem A graph is bipartite if and only if it contains no odd-length cycles. Another way to look at this theorem is as follows. A graph G can be colored (here, we color vertices) by at most 2 colors such that no two adjacent vertices have the same color if and only if G is bipartite. A bipartite graph is complete if every u L is connected to every v R. We denote a complete bipartite graph as K n1,n 2 which means that L = n 1 and R = n 2. s? Planar Graphs Decomposing & Composing Graphs I Planar graphs are graphs that can be drawn on the plane (2-space) without any edges crossing. s: K 4 is planar, but K 5 is not Many efficient algorithms exist for planar graphs Theorem (Kuratowski s Theorem) A graph G = (V, E) is planar if and only if it does not contain any K 5 nor K 3,3 minors. A minor is a subgraph that can obtained by expanding/contracting edges to paths and vice versa. We can (partially) decompose graphs by considering subgraphs. A subgraph of a graph G = (V, E) is a graph H = (V, E ) where V V and E E. Subgraphs are simply part(s) of the original graph.

6 Sparse vs Dense Graphs Graph Isomorphism I We say that a graph is sparse if E O( V ) and dense if E O( V 2 ). A complete graph K n has precisely E = n(n 1) 2 edges. Thus, for sparse graphs, Adjacency lists tend to be better while for dense graphs, adjacency matrices are better in general. An isomorphism is a bijection (one-to-one and onto) that preserves the structure of some object. In some sense, if two objects are isomorphic to each other, they are essentially the same. Most properties that hold for one object hold for any object that it is isomorphic to. An isomorphism of graphs preserves adjacency. Graph Isomorphism II Graph Isomorphism III Two graphs G 1 = (V 1, E 1 ) and G 2 = (V 2, E 2 ) are isomorphic if there exists a bijection ϕ : V 1 V 2 such that (u, v) E 1 if and only if ( ϕ(u), ϕ(v) ) E2 for all vertices u, v V 1. Lemma Isomorphism of graphs is an equivalence relation. Proof? If G 1 is isomorphic to G 2 we use the notation G 1 = G2 Graph Isomorphism I Computability Graph Isomorphism II Computability Problem Given: Two graphs, G 1, G 2. Question: Is G 1 = G2? The obvious way of solving this problem is to simply try to find a bijection that preserves adjacency. That is, search through all n! of them. Wait: Do we really need to search all n! bijections? There are smarter, but more complicated ways. However, the best known algorithm for general graphs is still only The graph isomorphism problem is of great theoretical interest because it is believed to be a problem of intermediate complexity. Conversely, it is sometimes easier (though not in general) to show that two graphs are not isomorphic. In particular, it suffices to show that the pair (G 1, G 2 ) do not have a property that isomorphic graphs should. Such a property is called invariant wrt isomorphism. O(exp( n log n))

7 Graph Isomorphism III Computability Graph Isomorphism I s of invariant properties: V 1 = V 2 E 1 = E 2 Degrees of vertices must be preserved. Lengths of paths & cycles. Such properties are a necessary condition of being isomorphic, but are not a sufficient condition. (8.3.35) Are the following two graphs isomorphic? v u 2 2 v u 1 u 1 3 u 5 u 4 v 5 v 4 v 3 Graph Isomorphism II Graph Isomorphism III All of the invariant properties previously mentioned hold. However, we still need to give an explicit bijection ϕ if they are isomorphic. Consider the following bijection. ϕ(u 1 ) = v 1 ϕ(u 2 ) = v 3 ϕ(u 3 ) = v 5 ϕ(u 4 ) = v 2 ϕ(u 5 ) = v 4 The original edges were (u 1, u 2 ) (ϕ(u 1 ), ϕ(u 2 )) = (v 1, v 3 ) E 2? (u 2, u 3 ) (ϕ(u 2 ), ϕ(u 3 )) = (v 3, v 5 ) E 2? (u 3, u 4 ) (ϕ(u 3 ), ϕ(u 4 )) = (v 4, v 2 ) E 2? (u 4, u 5 ) (ϕ(u 4 ), ϕ(u 5 )) = (v 2, v 4 ) E 2? (u 5, u 1 ) (ϕ(u 5 ), ϕ(u 1 )) = (v 4, v 1 ) E 2? Thus, they are isomorphic. Note that there are several bijections that show these graphs are isomorphic. We still need to verify that ϕ preserves adjacency. Graph Isomorphism Brute Force Algorithm Algorithm (Brute Force Isomorphism Testing) Input : Two graphs G 1 = (V 1, E 1), G 2 = (V 2, E 2) Output : True if G 1 = G2, false otherwise 1 foreach bijection, ϕ : V 1 V 2 do 2 iswitness true 3 foreach e = (u, v) E 1 do 4 if e = (ϕ(u), ϕ(v)) E 2 then 5 iswitness false 6 end 7 end 8 if iswitness then 9 output true 10 end 11 end 12 output false Connectivity I An undirected graph is called connected if for every pair of vertices, u, v there exists a path connecting u to v. A graph that is not connected is the union of two or more subgraphs called connected components. We have analogous (but more useful) notions for directed graphs as well. A directed graph is strongly connected if for every pair of vertices u, v There exists p 1 : u v and There exists p 2 : v u.

8 Connectivity II Even if a graph is not strongly connected, it can still be (graphically) one piece. A directed graph is weakly connected if there is a path between every two vertices in the underlying undirected graph (i.e. the symmetric closure). The subgraphs of a directed graph that are strongly connected are called strongly connected components. Such notions are useful in applications where we want to determine what individuals can communicate in a network (here, the notion of condensation graphs is useful).? Using Paths & Cycles in Isomorphisms I Recall that the lengths of paths & cycles are invariant properties for isomorphisms. Moreover, they can be used to find potential isomorphisms. For example, say there is a path of length k in G 1 v 0 v 1 v k Now consider the degree sequence of each vertex; deg(v 0 ), deg(v 1 ),..., deg(v k ) Since both of these properties are invariants, we could try looking for a path (of length k) in G 2 that has the same degree sequence. Using Paths & Cycles in Isomorphisms II Counting Paths I Often, we are concerned as to how connected two vertices are in a graph. If we can find such a path, say u 0 u 1 u k it may be a good (partial) candidate for an isomorphic bijection. That is, how many unique, paths (directed or undirected, but not necessarily simple) there are between two vertices, u, v? An easy solution is to use matrix multiplication on the adjacency matrix of a graph. Theorem Let G be a graph with adjacency matrix A. The number of distinct paths of length r from v i v j equals the entry a ij in the matrix A r. The proof is a nice proof by induction. Counting Paths II Euler Paths & Cycles I Furthermore, if we add up all such matrices, k i=1 we can compute the total number of paths between all pairs of vertices up to length k. This method can also be used to find the length of the shortest path, test connectivity, etc. A i Recall the Königsberg Bridge Problem. In graph theory terminology, the question can be translated as follows. Given a graph G, does there exist a cycle traversing every edge exactly once? Such a cycle is known as an Euler cycle. An Euler cycle in a graph G is a cycle that traverses every edge exactly once. An Euler path is a path in G that traverses every edge exactly once.

9 Euler Paths & Cycles II Euler Paths & Cycles III Theorem (Euler) A graph G contains an Euler cycle if and only if every vertex has even degree. This theorem also holds more generally for multigraphs. Therefore, the answer to the Königsberg Bridge problem is, no, does there does not exist an Euler cycle. In fact, there is not even an Euler path. Theorem A graph G contains an Euler path (not a cycle) if and only if it has exactly two vertices of odd degree. Constructing Euler Cycles I Constructing Euler Cycles II Constructing Euler paths is simple. Given a (multi)graph G, we can start at an arbitrary vertex. We then find any arbitrary cycle c 1 in the graph. Once this is done, we can look at the induced subgraph; the graph created by eliminating the cycle c 1. We can repeat this step (why?) until we have found a collection of cycles that involves every edge; c 1,..., c k. The Euler cycle can then be constructed from these cycles as follows. Starting with c 1, traverse the cycle until we reach a vertex in common with another cycle, c i ; then we continue our tour on this cycle until we reach a vertex in common with another cycle, etc. We are always guaranteed a way to return to the original vertex by completing the tour of each cycle. Hamiltonian Paths & Circuits I Hamiltonian Paths & Circuits II Euler cycles & paths traverse every edge exactly once. Cycles and paths that traverse every vertex exactly once are Hamiltonian cycles and paths. Exercise Show that K n has a Hamiltonian Cycle for all n 3. A path v 0, v 1,..., v n in a graph G = (V, E) is called a Hamiltonian Path if V = {v 0,..., v n } and v i v j for i j. A Hamiltonian cycle is a Hamiltonian path with (v n, v 0 ) E.

10 Hamiltonian Paths & Circuits III Hamiltonian Paths & Circuits IV Nevertheless, there are sufficient conditions. For general graphs, however, there is no known simple necessary and sufficient condition for a Hamiltonian Cycle to exist. This is a stark contrast with Euler Cycles: we have a simple, efficiently verifiable condition for such a cycle to exist. There are no known efficient algorithms for determining whether or not a graph G contains a Hamiltonian Cycle. Similar to the Traveling Salesman Problem, this problem is NP-hard. Theorem (Dirac Theorem) If G is a graph with n vertices with n 3 such that the degree of every vertex in G is at least n/2, then G has a Hamiltonian cycle. Theorem (Ore s Theorem) If G is a graph with n vertices with n 3 such that deg(u) + deg(v) n for every pair of nonadjacent vertices u, v in G then G has a Hamiltonian cycle. Application: Gray Codes I Application: Gray Codes II Electronic devices often report state by using a series of switches which can be thought of as bit strings of length n. (corresponding to 2 n states) If we use the usual binary enumeration, a state change can take a long time going from to for example. It is much better to use a scheme (a code) such that the change in state can be achieved by flipping a single bit. A Gray Code does just that. Recall Q n, the cube graph Each edge connects bit strings that differ by a single bit. To define a Gray Code, it suffices to find a Hamiltonian cycle in Q n. Application: Gray Codes III Application: Gray Codes IV So our code is as follows A Hamiltonian Path

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