# Subgraphs and Paths and Cycles

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1 15 October, 2015

2 Subgraphs Definition Let G = (V, E) be a graph. Graph H = (V, E ) is a subgraph of G if V V and E E.

3 Subgraphs Definition Let G = (V, E) be a graph. Graph H = (V, E ) is a subgraph of G if V V and E E. C 5 is a subgraph of K 6, and K 1, K 2,..., K 5 are all subgraph of K 6.

4 Graph Isomorphism Definition Let G = (V, E), G = (V, E ) two graphs. Suppose f : V V is a one-to-one function. 1 f preserve adjacency if for every uv E, f (u)f (v) E.

5 Graph Isomorphism Definition Let G = (V, E), G = (V, E ) two graphs. Suppose f : V V is a one-to-one function. 1 f preserve adjacency if for every uv E, f (u)f (v) E. 2 f preserve non-adjacency if for every non adjacent vertices u, v then f (u), f (v) are non-adjacent.

6 Graph Isomorphism Definition Let G = (V, E), G = (V, E ) two graphs. Suppose f : V V is a one-to-one function. 1 f preserve adjacency if for every uv E, f (u)f (v) E. 2 f preserve non-adjacency if for every non adjacent vertices u, v then f (u), f (v) are non-adjacent. 3 f is a graph isomorphism from G to G if it is bijective and preserve both adjacency and non-adjacency. In this case we write G = G.

7 Graph Isomorphism Definition Let G = (V, E), G = (V, E ) two graphs. Suppose f : V V is a one-to-one function. 1 f preserve adjacency if for every uv E, f (u)f (v) E. 2 f preserve non-adjacency if for every non adjacent vertices u, v then f (u), f (v) are non-adjacent. 3 f is a graph isomorphism from G to G if it is bijective and preserve both adjacency and non-adjacency. In this case we write G = G. a b c d G G f(a)=c and f(b)=d f is isomorphism

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10 Cycles and Eulerian tour Definition Let G = (V, E) be a graph or multigraph with no isolated vertices. G has an Eulerian circuit (closed trail) if there is a circuit (closed trail) that traverses each edge exactly once.

11 Cycles and Eulerian tour Definition Let G = (V, E) be a graph or multigraph with no isolated vertices. G has an Eulerian circuit (closed trail) if there is a circuit (closed trail) that traverses each edge exactly once. a b d c

12 Theorem Let G = (V, E) be an undirected graph or multigraph with no isolated vertices. Then G has an Euler circuit if and only if G is connected and every vertex in G has even degree. Proof. If G has an Eulerian circuit then there is a trail from a to b for every two vertices a, b and hence by previous Theorem there is a path from a to b. This would imply that G is connected.

13 Theorem Let G = (V, E) be an undirected graph or multigraph with no isolated vertices. Then G has an Euler circuit if and only if G is connected and every vertex in G has even degree. Proof. If G has an Eulerian circuit then there is a trail from a to b for every two vertices a, b and hence by previous Theorem there is a path from a to b. This would imply that G is connected. The even degree follows from the fact that whenever we enter a vertex v (in the circuit) we leave v. Therefore number of times we visit a vertex is even (visit using different edges).

14 Proof. Now suppose G is connected and degree of each vertex is even. We use induction on the number of edges of G (for n = 1, 2 easy to see). Select vertex s and construct a circuit C 1 containing s there exists such a circuit (consider a longest trail starting at s). Remove C 1 and now in the remaining graph C each vertex has even degree. If C is not connected it has connected components and each connected components is Eulerian (by induction). Now we can traverse each circuit of each connected components as we traverse C 1. This way we can pick up all the edges and go back to s.

15 Theorem Let G = (V, E) be an undirected graph or multigraph with no isolated vertices. Then G has an Euler trail if and only if G is connected and has exactly two vertices of odd degree. Proof. Add edge a, b where a, b have odd degree. Now G is Eulerian and hence it has an Eulerian circuit. By removing edge ab we obtained the desired trail in G

16 Definition Let G = (V, A) be a directed graph or multigraph. For each vertex v (a) The incoming or in-degree of v is the number of arcs coming to v and denoted by id(v). (b) The outgoing or out-degree of v is the number of arcs leaving v and denoted by od(v).

17 Definition Let G = (V, A) be a directed graph or multigraph. For each vertex v (a) The incoming or in-degree of v is the number of arcs coming to v and denoted by id(v). (b) The outgoing or out-degree of v is the number of arcs leaving v and denoted by od(v). Theorem Let G = (V, A) be an directed graph or multigraph with no isolated vertices. Then G has a directed Euler circuit if and only if G is connected and for each vertex v, id(v) = od(v).

18 Hamiltonian cycles and paths Definition Let G = (V, E) be an undirected graph or multigraph. We say G has a Hamilton cycle if there is a cycle in G that contains every vertex of G. We say G has a Hamilton path if there is a path in G that contains every vertex of G.

19 Hamiltonian cycles and paths Definition Let G = (V, E) be an undirected graph or multigraph. We say G has a Hamilton cycle if there is a cycle in G that contains every vertex of G. We say G has a Hamilton path if there is a path in G that contains every vertex of G.

20

21 Theorem Let T be a tournament (between every pair u, v (u v) exactly one of the uv and vu is an arc). Show that T has a directed Hamilton path. Proof. Let P be a longest directed path in T. If P is a Hamilton path then we are done. Otherwise suppose there is a vertex u outside P. Let P = v 1, v 2,..., v m. Let i m be a maximum index such that v i u is an arc. If i does not exists then it means uv 1 is an arc and hence u, v 1, v 2,..., v m is a longer directed path. If i exists then it means uv i+1, i < m is an arc. If i = m then Pu is a longer directed path, a contradiction. If i < m then P = v 1, v 2,..., v i, u, v i+1, v i+2,..., v m is a longer path, a contradiction.

22 Theorem Let T be a strongly connected tournament (between every pair (u, v) (u v) there is a directed path). Show that T has a directed Hamilton cycle. Proof. Since T is strong it has a closed trail and hence it has a directed cycle. Let C = v 1, v 2,..., v m, v 1 be a longest directed cycle. If C = T then we are done. Otherwise suppose there is a vertex u T C. If there exists an index i such that v i u, uv i+1 are arcs of T then C = v 1, v 2,..., v i, u, v i+1, v i+2,..., v m, v 1 is a longer cycle, a contradiction. So for every vertex u we have either V (C) u or u V (C). Let I = {u u V (C)} and O = {u V (C) u}. Note that V (T ) = V (C) I O. If there is an arc from u O to v I then C = v 1, u, v, v 2,..., v m, v 1 is longer than C, contradiction. Therefore I O. But in this case T is not strong, a contradiction.

23 Theorem Let G = (V, E) be a loop free simple graph with V = n 2. If deg(x) + deg(y) n 1 for all x, y, x y, then G has a Hamilton path. Proof. We show that G is connected. Suppose G 1 and G 2 are two components of G. Let v i V (G i ), i = 1, 2. Now v 1 is adjacent to at most G 1 1 vertices and v 2 is adjacent to at most G 2 1 vertices and hence deg(v 1 ) + deg(v 2 ) G 1 + G 2 2 n 2, contradiction. Let P = v 1, v 2,..., v m be a longest path in G. Note that uv 1 E for u G P otherwise up is a longer path. Similarly v m u is not an edge as otherwise Pu is a longer path. Thus the neighbors of v 1, v m are on P.

24 Proof. We show there exists some i such that v 1 v i is an edge and v i 1 v m is also an edge. Suppose this is not the case. Then for each neighbor v j of v 1, v j 1 is not neighbor of v m. We also note that v 1 v m is not an edge (otherwise we are done) and hence the neighbors of v 1, v m are between v 2, v 3,..., v m 1. Note that if v 1 has r neighbors on P, v m has r 1 non-neighbors on P. This means v m has at most m 2 (r 1) neighbors. Therefore deg(v 1 ) + deg(v m ) m 1, a contradiction since m < n. So there exists such i and hence C = v 1, v 2,..., v i 1, v m, v m 1,..., v i+1, v i, v 1 is a cycle in G. Now since G is connected, there is an edge from C to a vertex outside C, say an edge v j u. Now u, v j, v j+1,..., v m, v 1, v 2,..., v j 1 is a longer path, a contradiction.

25 Theorem Let G = (V, E) be a loop free simple graph with V = n 2. If deg(x) + deg(y) n for all x, y, x y, then G has a Hamilton cycle. Proof. By the previous Theorem G has a Hamilton path. Let P = v 1, v 2,..., v n. Now similar to the previous proof there exists i such that v 1 v i is an edge and v i 1 v n is also an edge. Suppose this is not the case. Then for each neighbor v j of v 1, v j 1 is not neighbor of v n. We also note that v 1 v n is not an edge (otherwise we are done) and hence the neighbors of v 1, v n are between v 2, v 3,..., v n 1. Note that if v 1 has r neighbors on P, v n has r 1 non-neighbors on P. This means v n has at most n 2 (r 1) neighbors. Therefore deg(v 1 ) + deg(v n ) n 1, a contradiction.

26 Exercises Problem Suppose there are n students in each of the three schools A, B, C. Each student from each of these three universities has at least n + 1 friends from the other two universities altogether. Show that there are three students x, y, z from A, B, C (respectively) that are pairwise friend.

27 Exercises Problem Suppose there are n students in each of the three schools A, B, C. Each student from each of these three universities has at least n + 1 friends from the other two universities altogether. Show that there are three students x, y, z from A, B, C (respectively) that are pairwise friend. Proof. We consider a graph G with three partite sets A, B, C. We put an edge from x to y if they are friends and in different universities. We need to show there is a triangle in G. Consider a student u from one partite sets that has the maximum number of friends in one other part.

28 Proof. We may assume x A has a set S of r neighbors in B (r maximum). Let z C be a neighbor of x. If z is adjacent to a vertex in S then we are done. Otherwise z has at most n r neighbors in B. Since z has n + 1 neighbors in A B, we conclude that z must have at least n + 1 (n r) = r + 1 neighbors is A. Therefore deg A (z) = r + 1, a contradiction to the choice of x.

29 Problem Show that in every tournament T, there exists a vertex x such that for every other vertex y x, either xy is an arc or there exists z such that xz, zy are arcs.

30 Problem Show that in every tournament T, there exists a vertex x such that for every other vertex y x, either xy is an arc or there exists z such that xz, zy are arcs. Proof. Consider a vertex x with maximum out-degree. Let O be the set of out neighbors of x and I be the set of in neighbors of x. Observe that T = I + O + 1. Consider an arbitrary vertex y O. If there exists a vertex z I such that zy is an arc then we are done. Therefore for every z O, yz is an arc. Now od(y) = O + 1 and hence od(y) > od(x), contradiction.

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