片断: CHAPTER1 Fundamentals Thepurposeofthisintroductionistofamiliarisethereaderwiththebasic conceptsandresultsofgraphtheory.Thechapterinevitablycontainsa largenumberofdefinitionsandinordertopreventthereadergrowing wearyweprovesimpleresultsassoonaspossible.Thereaderisnotexpected tohavecompletemasteryofChapter1beforesamplingtherestofthe book,indeed,heisencouragedtoskipaheadsincemostoftheterminology isself-explanatory.Weshouldaddatthisstagethattheterminologyof graphtheoryisfarfrombeingstandard,thoughthatusedinthisbookis wellaccepted. ?Definitions AgraphGisanorderedpairofdisjointsets(V,E)suchthatEisasubset ofthesetofunorderedpairsofV.Unlessitisexplicitlystatedotherwise,we consideronlyfinitegraphs,thatisVandEarealwaysfinite.ThesetVis thesetofverticesandEisthesetofedges.IfGisagraphthenV=v(G) isthevertexsetofGandE=E(G)istheedgeset.Anedge{x,y}issaidto jointheverticesxandyandisdenotedbyxy.Thusxyandyxmeanexactly thesameedge;theverticesxandyaretheendverticesofthisedge.IfxyeE(6) thenxandyareadjacentorneighbouringverticesofGandtheverticesx andyareincidentwiththeedgexy.Twoedgesareadjacentiftheyhave exactlyonecommonendvertex. Astheterminologysuggests,wedonotusuallythinkofagraphasan orderedpair,butasacollectionofverticessomeofwhicharejoinedby edges.Itisthenanaturalsteptodrawapictureofthegraph.Infact,some- timestheeasiestwaytodescribeagraphistodrawit;thegraphG= ({1,2,3,4,5,6},{12,14,16,25,34,36,45,56})isimmediatelycomprehended bylookingatFigure1.1. WesaythatG'=(V',E')isasubgraphofG=(V,E)ifV'Vand E'E.InthiscasewewriteG'G.IfG'containsalledgesofGthatjoin twoverticesinV'thenG'issaidtobethesubgraphinducedorspannedby V'andisdenotedbyG[v'].AsubgraphG'ofGisaninducedsubgraphif G'=G(G')].IfV'=V,thenG'issaidtobeaspanningsubgraphofG. TheseconceptsareillustratedinFigure1.2. Weshalloftenconstructnewgraphsfromoldonesbydeletingoradding someverticesandedges.IfWv(G)thenG-W=G[V\W]isthesub- graphofGobtainedbydeletingtheverticesinWandalledgesincidentwith them.SimilarlyifE'E(G)thenG-E'=(v(G),E(G)\E').IfW={w} andE'={xy}thenthisnotationissimplifiedtoG-wandG-xy. Similarly,ifxandyarenon-adjacentverticesofGthenG xyisobtained fromGbyjoiningxtoy.
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