请输入您要查询的字词:

 

单词 MaximumFlowminimumCutTheorem
释义

maximum flow/minimum cut theorem


Let G be a finite digraphMathworldPlanetmath with nonnegative weights and withexactly one sink and exactly one source. Then

I) For any flow f on G and any cut C of G, the amountof flow for f is less than or equal to the weight of C.

II) There exists a flow f0 on G and a cut C0 of Gsuch that the amount of the flow of f0 equals the weight of C0.

Proof: (I) is easy, so we prove only (II).Write + for the set of nonnegative real numbers.Let V be the set of vertices of G.Define a matrix

κ:V×V+

where κ(x,y) is the sum of the weights (or capacities) ofall the directed edges from x to y.By hypothesisMathworldPlanetmathPlanetmath there is a unique vV (the source) such that

κ(x,v)=0  xV

and a unique wV (the sink) such that

κ(w,x)=0  xV.

We may also assume κ(x,x)=0 for all xV.Any flow f will correspond uniquely (see Remark below) to a matrix

φ:V×V+

such that

φ(x,y)κ(x,y)  x,yV
zφ(x,z)=zφ(z,x)  xv,w

Let λ be the matrix of any maximal flow, and let Abe the set of xV such that there exists a finite sequencePlanetmathPlanetmathx0=v,x1,,xn=xsuch that for all m from 1 to n-1, we have either

λ(xm,xm+1)<κ(xm,xm+1)(1)

or

λ(xm+1,xm)>0.(2)

Write B=V-A.

Trivially, vA.Let us show that wB.Arguing by contradictionMathworldPlanetmathPlanetmath, suppose wA, and let (xm) bea sequence from v to w with the properties we just mentioned.Take a real number ϵ>0 such that

ϵ+λ(xm,xm+1)<κ(xm,xm+1)

for all the (finitely many) m for which (1) holds, and suchthat

λ(xm+1,xm)>ϵ

for all the m for which (2) holds.But now we can define a matrix μ with a larger flowthan λ (larger by ϵ) by:

μ(xm,xm+1)=ϵ+λ(xm,xm+1)  if (1) holds
μ(xm+1,xm)=λ(xm+1,xm)-ϵ  if (2) holds
μ(a,b)=λ(a,b)  for all other pairs (a,b).

This contradiction shows that wB.

Now consider the set C of pairs (x,y) of vertices such thatxV and yW.Since W is nonempty, C is a cut.But also, for any (x,y)C we have

λ(x,y)=κ(x,y)(3)

for otherwise we would have yV.Summing (3) over C, we see that the amount of theflow f is the capacity of C, QED.

Remark: We expressed the proof rather informally, because the terminology of graph theoryMathworldPlanetmath is not very well standardized and cannot all be found yet here at PlanetMath. Please feel free to suggest any revision you think worthwhile.

随便看

 

数学辞典收录了18232条数学词条,基本涵盖了常用数学知识及数学英语单词词组的翻译及用法,是数学学习的有利工具。

 

Copyright © 2000-2023 Newdu.com.com All Rights Reserved
更新时间:2025/5/3 20:18:11