Transcription of G V;E
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PlanarGraphsA graphG= (V; E)isplanarif it canbe drawn ontheplanewithoutedgescrossingexceptaten dpoints :thereis a 1-1functionf:V!R2andforeache2 Ethereexistsa 1-1continuousge:[0;1]!R2suchthat(a)e=xyi mpliesf(x) =ge(0)andf(y) =ge(1).(b)e6=e0impliesthatge(x)6=ge0(x0) forallx; x02(0;1).georitsimageis referredto Theorem(F ary)A simpleplanargraphhasanembeddingin Notallgraphsareplanar. Graphscanhave several planegraphG, a faceis a maximalregionSsuchthatx; y2 Simpliesthatx; ycanbejoinedbya curve whichdoesnotmeetany edgeof embeddinghas7 theouterorinfiniteface. (G)is thenumberof a 1-1continuousmapfromthecircleS1!R2thenfp artitionsR2nf(S1)intotwo disjointcon-nectedopensetsInt(f); Ext(f). Theformeris boundedandthelatteris consequence, ifx2 Int(f); y2 Ext(f)andx; yarejoinedby a closedcurveCinR2thenCmeetsf(S1).4K5is insideoroutsideofC noplacetoputv5 we placev5intoC1thenthev5v3curve crossestheboundary graphisembeddableintheplaneiffit isembed-dable onthesurfaceof a :R2!
We define its dual G = (V ;E )as follows: There is a vertex f correspond-ing to each face f of G. There is an edge e corresponding to each edge e of G. f and g are joined by edge e iff edge e is on the boundary of f and g. Cut edges yield loops. Theorem 1 (a) G is planar.
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