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雅思阅读
2016-08-26 14:18
来源：新东方整理
作者：上海新东方
Revision: Graphs and Networks
Definitions
In a complete graph every node is connected by an arc to each of the other nodes. There are 1/2*n (n-1) arcs in a complete graph with n nodes.
In a connected graph there are no isolated nodes.
A trail is a sequence of arcs such that the end node of one arc is the start node of the next.
A closed trail (or cycle) is a route through the nodes which starts and finishes in the same place. No arc is used more than once. Only the start node is used more than once.
A path is a trail where no node is passed more than once.
The order of a node is the number of arcs meeting at that node.
An Eulerian graph is a connected graph which has a closed trail containing every arc precisely once. This can occur if and only if every node is even.
A semi-Eulerian graph is a connected graph which has a trail (not closed) containing every arc precisely once. It occurs when a graph has 2 odd nodes: the trail starts at one odd node and ends at the other.
A planar graph is one which can be drawn so that arcs do not cross each other.
Matrix formulation
Networks can be represented by matrices. If the network has only 2 way links (no arrows) the matrix is symmetrical about a diagonal drawn from top left to bottom right.
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