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離散數(shù)學(xué)課件第5章-展示頁

2025-01-25 20:16本頁面
  

【正文】 nly if 一個(gè)沒有孤立頂點(diǎn)的有向多重圖含有歐拉回路的充要條件是: the graph is weakly connected 弱連通的 the indegree and outdegree of each vertex are equal 每個(gè)頂點(diǎn)的出度和入度相等 24 2022/2/13 A directed multigraph having no isolated vertices has an Euler path but not an Euler circuit if and only if 一個(gè)沒有孤立頂點(diǎn)的有向多重圖含有歐拉通路但不含歐拉回路的充要條件是: the graph is weakly connected 弱連通的 the indegree and outdegree of each vertex are equal for all but two vertices, one that has indegree 1 larger than its outdegree and the other that has outdegree 1 larger than its indegree. 除去兩個(gè)頂點(diǎn)外每個(gè)頂點(diǎn)的出度和入度相等,其中一個(gè)頂點(diǎn)的出度比入度大 1,另一個(gè)頂點(diǎn)的入度比出度大 1. 25 2022/2/13 〖 Example 3〗 Determine whether the directed graph has an Euler path. Construct an Euler path if it exists. a c b d deg(v) deg+(v) a 1 2 b 2 2 c 2 2 d 3 2 Hence, the directed graph has an Euler path. Solution: 26 2022/2/13 Application A type of puzzle Draw a picture in a continuous motion without lifting a pencil so that no part of the picture is retraced. The equivalent problem: Whether the graph exist an Euler path or circuit. For example, 27 2022/2/13 二、 Hamilton paths and circuit 哈密頓通路和回路 Hamilton’s puzzle 28 2022/2/13 A Hamilton path in a graph G is a path which visits ever vertex in G exactly once. 哈密頓通路是一個(gè)訪問圖 G中每個(gè)頂點(diǎn)次數(shù)有且僅有一次的通路 A Hamilton circuit (or Hamilton cycle) is a cycle which visits every vertex exactly once, except for the first vertex, which is also visited at the end of the cycle. 哈密頓回路,僅訪問每個(gè)頂點(diǎn)一次,但除去始點(diǎn),這個(gè)始點(diǎn)同樣也是終點(diǎn)。 b、去某邊后不能造成圖形的不連通。 b、去某邊后不能造成圖形的不連通。 b、去某邊后不能造成圖形的不連通。 b、去某邊后不能造成圖形的不連通。 b、去某邊后不能造成圖形的不連通。 b、去某邊后不能造成圖形的不連通。 所以存在 Euler回路。 v1 v2 v5 v3 v4 v6 v7 v8 v9 解:首先看圖中是否有 Euler回路,即看每個(gè)頂點(diǎn)的度是否都是偶數(shù)。 b、去某邊后不能造成圖形的不連通。 C 8 2022/2/13 Necessary and sufficient condition for Euler circuit and paths 歐拉回路和歐拉通路的充要條件 【 Theorem 1】 連通多重圖具有歐拉回路當(dāng)且僅當(dāng)它的每個(gè)頂 點(diǎn)都有偶數(shù)度 Proof: (1) Necessary condition必要條件 G has an Euler circuit ? Every vertex in V has even degree Consider the Euler circuit. ? the vertex a which the Euler circuit begins with ? the other vertex 9 2022/2/13 (2) sufficient condition We will form a simple circuit that begins at an arbitrary vertex a of G. ? Build a simple path x0=a, x1, x2,…, xn=a. ? An Euler circuit has been constructed if all the edges have been used. otherwise, ? Consider the subgraph H obtained from G. Let w be a vertex which is the mon vertex of the circuit and H. Beginning at w, construct a simple path in H. 10 2022/2/13 【 Theorem 2】 連通多重圖具有歐拉通路而無歐拉回路, 當(dāng)且僅當(dāng)它恰有兩個(gè)奇數(shù)度頂點(diǎn) 〖 Example 1〗 Konigsberg Seven Bridge Problem 哥尼斯堡七橋問題 C A B D Solution: The graph has four vertices of odd degree. Therefore, it does not have an Euler 11 2022/2/13 〖 Example 2〗 Determine whether the following graph has an Euler path. Construct such a path if it 具有歐拉通路,如果存在構(gòu)建一條通路 B C D E F G H I J A Solution: The graph has 2 vertices of odd degree, and all of other vertices have even degree . Therefore, this graph has an Euler path. 12 2022/2/13 〖 Example 2〗 Determine whether the following graph has an Euler path. Construct such a path if it exists. B C D E F G H I J A Solution: The graph has 2 vertices of odd degree, and all of other vertices have even degree . Therefore, this graph has an Euler path. The Euler path: A,C,E,F,G,I,J,E,A,B,C,D,E,G, H,I,G,J
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