584 I. Benjamini, O. Schrammg : |G| →D, by letting g map each I e into the path f ∗ (e). It is easy to verifythat the number of crossings for each edge in the drawing is bounded by C times2κ + 1 times the maximal degree of G ◦ . Hence there is a drawing of G in theplane in which the number of crossings in any edge is bounded.Now suppose that there is a drawing g : |G| →R 2 of G in the plane so thatevery edge e ∈ E has at most κ
<str<strong>on</strong>g>Harm<strong>on</strong>ic</str<strong>on</strong>g> <str<strong>on</strong>g>functi<strong>on</strong>s</str<strong>on</strong>g> 5857.1. Theorem Let G =(V,E)be a finite genus, c<strong>on</strong>nected, locally finite graph.Then G can be drawn in the plane with finitely many crossings. In particular, ifG has bounded valence, then it is <strong>almost</strong> <strong>planar</strong>.Proof. Let S be a closed (compact, without boundary) surface such that G embeddsin S . We assume, with no loss of generality, that G is a triangulati<strong>on</strong> of adomain D ⊂ S . Recall that the genus of any boundaryless surface X is the maximumcardinality of a collecti<strong>on</strong> {γ 1 ,...,γ g } of disjoint simple closed curvesin X such that X −∪ j γ j is c<strong>on</strong>nected. Since the genus of D is bounded by thegenus of S , it is finite. Hence there is a finite collecti<strong>on</strong> of edges E 0 ⊂ E suchthat any simple closed path in G − E 0 separates D. Since G is a triangulati<strong>on</strong>of D, this implies that there is no simple closed n<strong>on</strong>-separating path in D − E 1 ,where E 1 is the set of all edges in G that share some vertex with some edgein E 0 . Therefore every comp<strong>on</strong>ent of D − E 1 has zero genus, <strong>and</strong> is <strong>planar</strong>. Inparticular, G − E 1 is <strong>planar</strong>. Let E ′ ⊂ E 1 be a minimal set of edges subject tothe property that G − E ′ is <strong>planar</strong>. (Here, G − E ′ c<strong>on</strong>tains all the vertices of G.)Clearly, G − E ′ is c<strong>on</strong>nected, <strong>and</strong> E ′ is finite.Let f be an embedding of G −E ′ in the plane so that for every edge or vertexj of G − E ′ there is an open set A c<strong>on</strong>taining f (j ) that intersects at most finitelymany of the vertices <strong>and</strong> edges of f (G − E ′ ). Now suppose that e is some edgein E ′ , <strong>and</strong> let v, w be its vertices. Since G − E ′ is c<strong>on</strong>nected, there is a path γin f (G − E ′ ) joining f (v) <strong>and</strong> f (w). We may define f (e) to be a simple path thatfollows al<strong>on</strong>gside γ <strong>and</strong> intersects f (G − E ′ ) in finitely many points. Similarly,all the edges in E ′ may be drawn in such a way that there are finitely manycrossings. ⊓⊔Almost <strong>planar</strong> <strong>graphs</strong> <strong>and</strong> separati<strong>on</strong> propertiesIt is known [18] that any <strong>planar</strong> graph G =(V,E) has the √ n separati<strong>on</strong> property.That is, there are c 0 < ∞, c 1 < 1, so that for any finite set W ⊂ V of nvertices, there is a subset W ′√⊂ W c<strong>on</strong>taining at most c 0 n vertices, suchthat any c<strong>on</strong>nected subset of W − W ′ has at most c 1 n vertices. Clearly <strong>almost</strong><strong>planar</strong> <strong>graphs</strong> have this √ n separati<strong>on</strong> property too. A natural questi<strong>on</strong> is whether√ n separati<strong>on</strong> is equivalent to <strong>almost</strong> <strong>planar</strong>ity. And if not, do transient <strong>graphs</strong>with the √ n separti<strong>on</strong> property always admit n<strong>on</strong> c<strong>on</strong>stant bounded harm<strong>on</strong>ic<str<strong>on</strong>g>functi<strong>on</strong>s</str<strong>on</strong>g>?Examples. Let T be a tree, <strong>and</strong> denote by T × Z the product of T by Z. InLemma 7.2 below, we show that T × Z has the √ n separati<strong>on</strong> property. Couplingshows that any bounded harm<strong>on</strong>ic functi<strong>on</strong> h <strong>on</strong> T ×Z must be c<strong>on</strong>stant <strong>on</strong> everyfibre of the form {t}×Z. Hence the <strong>on</strong>ly harm<strong>on</strong>ic Dirichlet <str<strong>on</strong>g>functi<strong>on</strong>s</str<strong>on</strong>g> <strong>on</strong> T × Zare the c<strong>on</strong>stants. By Theorem 1.9 it then follows that T × Z is not <strong>almost</strong> <strong>planar</strong>,if it is transient. In particular, when T is the binary tree, T × Z is a graph havingthe √ n separati<strong>on</strong> property, which is not <strong>almost</strong> <strong>planar</strong>. Hence, √ n separati<strong>on</strong>does not imply <strong>almost</strong> <strong>planar</strong>ity.
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