hw1soln

# hw1soln - CME 305: Discrete Mathematics and Algorithms...

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CME 305: Discrete Mathematics and Algorithms Instructor: Professor Amin Saberi ([email protected]) HW#1 Solutions 1. Prove that at least one of G and G is connected. Here G is the complement of G i.e. a graph on the same vertex set such that two vertices in G are adjacent if and only if they are not adjacent in G . Solution: Let G be a disconnected graph in which case we can decompose it into k connected components C 1 ,C 2 ,...,C k . We want to show that G is connected i.e. there is a path between any u and v in G . In the case that u and v are in diﬀerent components we know that there exist an edge (a path of length one) between them in G . In the case that u and v are in the same component, say C i , we can construct a path of two edges between them in G as follows. Pick any vertex w from some other component C j for j 6 = i and note that edges { u,w } and { w,v } are in G . Thus u,w,v is a path in G and hence G is connected. 2. Recall the deﬁnition of a bipartite graph. Let G ( V,E ) be a graph and ( A,B ) be a partition of V . We say that G is bipartite if all edges in E have one end-point in A and the other in B . More precisely, for all ( u,v ) E either u A,v B or u B,v A . (a) Prove that a graph is bipartite if and only if it doesn’t have an odd cycle. (b) A graph is called k -regular if all vertices have degree k . Prove that if a bipartite G is also k -regular with k 1 then | A | = | B | . Solution: (a) The following is for a connected bipartite graph. The extension is trivial after the statement holds for each connected component. ” Assume G is bipartite and pick a cycle v 1 ,v 2 ,...,v k ,v 1 with v 1 A . Then v i A for i odd and v i B for i even. An odd cycle has and odd number of vertices, thus k is odd and so v k A . But we have { v k ,v 1 } ∈ E which contradicts the fact that G is bipartite since both vertices are in A . Therefore a bipartite G has no odd cycles. ” Let G be a graph with no odd cycles. Deﬁne d ( u,v ) as the shortest distance between the two vertices. Pick an arbitrary vertex u V and deﬁne A = { u }∪{ w | d ( u,w ) is even } . Let B = V - A to create a partition ( A,B ). We can show by contradiction that G ( A,B,E ) is a bipartite graph. Assume there is an edge { w,v } with w,v A ( B ). Then by construction d ( u,w ) and d ( u,v ) are even (odd). Thus, edge { w,v } creates an odd cycle, a contradiction. The above uses the fact that the sum of two even (odd) numbers is always even. (b) The number of edges leaving A is computed as k | A | . The number of edges leaving B is computed as k | B | . For a bipartite graph both quantities have to be equal to the total number of edges. Thus | E | = k | A | = k | B | , implies | A | = | B | .

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3. The stable roommate problem is similar to the stable marriage problem, except pairings are made within a single pool rather than between two genders. As in stable marriage, a rogue pair is a pair ( a,b ) such that
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hw1soln - CME 305: Discrete Mathematics and Algorithms...

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