# The 4-Choosability of Plane Graphs without 4-Cycles*

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1 Journal of Combinatorial Theory, Series B 76, (1999) Article ID jctb , available online at on The 4-Choosability of Plane Graphs without 4-Cycles* Peter Che Bor Lam Department of Mathematics, Hong Kong Baptist University, 224 Waterloo Road, Kowloon, Hong Kong cblammath.hkbu.edu.hk and Baogang Xu and Jiazhuang Liu Institute of Mathematics, Shandong University, Jinan, , People's Republic of China Received October 2, 1997 A graph G is called k-choosable if k is a number such that if we give lists of k colors to each vertex of G there is a vertex coloring of G where each vertex receives a color from its own list no matter what the lists are. In this paper, it is shown that each plane graph without 4-cycles is 4-choosable Academic Press Key Words: list coloring; choosability; plane graph; 4-cycle. 1. INTRODUCTION In this paper, we consider only finite and simple graphs. Let G be a plane graph. V(G), E(G), and F(G) shall denote the set of vertices, edges, and faces of G, respectively. Vertices u and v are adjacent, denoted by uv # E(G), if there is an edge in G joining them. N G (v), or N(v) if there is no possibility of confusion, denotes the set of vertices adjacent to v in G, and f denotes the set of vertices incident with the face f. Thedegree of u in G, written as d G (u), is the number of vertices in N G (v). A vertex u is called a k-vertex if d G (u)=k. The minimum degree of G, min[d G (v) v # V(G)], is denoted by \$(G). A face of a plane graph is said to be incident with all edges and vertices on its boundary. Two faces are adjacent if they have an edge in common. The degree of a face f of plane graph G, denoted by d G ( f ), is the number of edges incident with it, where each cut-edge is counted twice. A k-face is a face of degree k. Atriangle is synonymous with a 3-face. * Research is partially supported by Research Grant Council, Hong Kong; by Faculty Research Grant, Hong Kong Baptist University and by the Doctoral Foundation of the Education Commission of the The People's Republic of China Copyright 1999 by Academic Press All rights of reproduction in any form reserved.

2 118 LAM, XU, AND LIU A list coloring of G is an assignment of colors to V(G) such that each vertex v receives a color from a prescribed list L(v) of colors and adjacent vertices receive distinct colorssee [8]. L(G)=(L(v) v # V(G)) is called a color-list of G. G is called k-choosable if G admits a list-coloring for all color-lists L with k colors in each list. The choice number of G, denoted by / l (G), is the minimum k such that G is k-choosable. All 2-choosable graphs have been characterized by Erdo s et al. [4]. N. Alon and M. Tarsi [1] proved that every plane bipartite graph is 3- choosable. Thomassen [6, 7] proved that every plane graph is 5-choosable and every plane graph of girth at least 5 is 3-choosable, where the girth of a graph G is the length of the shortest cycle in G. Also, every plane graph G without 3-cycles is 4-choosable because \$(G)3. Examples of plane graphs which are not 4-choosable and plane graphs of girth 4 which are not 3-choosable were given by Voigt [9, 10]. Voigt and Wirth [11] also gave an example of a 3-colorable planar graph which is not 4-choosable. In 1976, Steinberg (see [5, p. 229] or [2]) conjectured that every plane graph without 4- and 5-cycles is 3-colorable. In 1990, Erdo s (also see [5, p. 229]) suggested the following relaxation of Steinberg's conjecture: Is there an integer k5 such that every plane graph without i-cycles, 4ik, is 3-colorable? In 1996, O. V. Borodin [2] proved that k=9 is suitable. It would be significant to find an integer k4 such that every plane graph without i-cycles, 4ik, is 4-choosable. In this paper, we shall show that k=4 is sufficient by proving the following theorem. Theorem 1. Let G be a C 4 -free plane graph. Then G is 4-choosable. In Section 2, we shall show that any C 4 -free plane graph contains a subgraph with a special configuration, and this fact will be used to prove Theorem 2 in Section SPECIAL CONFIGURATION F 3 5 We use F 3 5 to denote a special C 4-free plane graph consisting of a 5-face with an exterior adjacent triangle. A subgraph H of G is called an F 3 5 -subgraph if H is isomorphic to F 3 5 and d G(v)=4 for all v # V(H). We shall first prove that any C 4 -free plane graph G with \$(G)=4 contains an F 3 5 -subgraph. Let G be a C 4 -free plane graph with \$(G)4. The set of all 5-faces adjacent to exactly four triangles and the set of all 5-faces adjacent to five triangles are denoted by F 4 and F 5 respectively. The subset of F 4 (respectively of F 5 ) consisting only of faces incident with five 4-vertices is denoted by F 4 (respectively F 5).

3 THE 4-CHOOSABILITY OF PLANE GRAPHS 119 Let f # F 5 _ F 4, and let T be a triangle adjacent to f. The vertex u #(T )"( f ) is called an M*-vertex of f. Moreover, if d G (u)=5, then u is called an M-vertex of f. Since a 5-vertex u can be incident with at most two triangles, u can be an M-vertex of at most two faces in F 5 _ F 4. Ifu is an M-vertex of a face in F 5 _ F 4, we denote by m*(u) the number of faces in F 5 _ F 4 for which u is an M-vertex. It follows that m*(u)=1 or 2. A face f \$ is called an M*-face of f at the vertex u 0 if ( f ) & ( f \$)=[u 0 ] and there exist two triangles which are adjacent to both f and f \$. If in addition, d G ( f \$)=5, then f \$ is called an M-face of f. Iff \$isanm-face of f at u 0,then d G (u 0 )=4. If G contains no F 3 5 -subgraph and N(u 0) & f \$=[u 1, u 2 ],then d G (u i )5 for i=1 and 2. Therefore a 5-face f \$ may be the M-face of at most two faces f 1, f 2 # F 5 _ F 4. Iff \$isanm-face of a face in F 5 _ F 4, then we denote by m*( f \$) the number of faces in F 5 _ F 4 for which f \$isan M-face. It also follows that m*( f )=1 or 2. Moreover, if m*( f \$)=2, then f \$ is incident with three vertices of degree five or higher. In [2], Borodin proved that \$(G)4 for each plane graph without adjacent triangles. The following lemma is proved using the method of Borodin. Note that if a graph is C 4 -free, then it has no 4-face and no adjacent triangles. Lemma 1. Let G be a plane graph without 4-faces and without adjacent triangles. If \$(G)=4, then G contains an F 3 -subgraph. 5 Proof. Suppose there exists a plane graph satisfying all assumptions of the Lemma and containing no F 3 5-subgraph. Let G be a graph of minimum order among such graphs. Claim 1. Each f # F 5 has at least two M-vertices. If f has exactly two M-vertices which are incident with the same M*-face f \$ of f, then all M*-faces of f, with the possible exception of f \$, are M-faces. Proof of Claim 1. Let f # F 5 be surrounded by triangles as in Fig. 1a. Because G contains no F 3 5 -subgraph, we have d G(v i )5 for i=1,..., 5. FIGURE 1

4 120 LAM, XU, AND LIU FIGURE 2 If f has no M-vertex, then put H=G&[u 1,..., u 5 ]+[w, v 1 w, v 2 w, v 3 w, v 4 w, v 5 w]fig. 2a. If f has only one M-vertex v 1, then put H=G& [u 2, u 3, u 4 ]+[u 1 v 3, u 5 v 4 ]Fig. 2b. In both cases, \$(H)=4; V(H) < V(G) ; d H (v i )5 for i=1,..., 5; and H contains no 4-face, no adjacent triangles, and no F 3 5-subgraph; contradicting the choice of G. Therefore f contains at least two M-vertices. Suppose f has exactly two M-vertices v 1 and v 2 which are incident with the same M*-face f 1 of f. Ifd G ( f 3 )6, and w is the vertex other than u 3 such that wv 4 is an edge of f 3 Fig. 2c, then put H=G&[u 3, u 4 ]+[u 2 w, u 5 v 4 ].Ifd G ( f 2 )6, then put H=G&[u 2, u 3, u 4 ]+[u 5 v 4, u 1 v 3, v 2 v 3 ] Fig. 2d. The same contradiction as above establishes the latter part of Claim 1. Claim 2. Each f # F 4 has at least one M-vertex, and one of the two M*-faces of f denoted by f 1 and f 3 in Fig.3b is an M-face. The proof of Claim 2 is similar to that of Claim 1. If f has no M-vertex, then put H=G&[u 1, u 2, u 3 ]+[v 2 u 5, v 3 u 4 ]Fig. 3a. If both d G ( f 1 )6 FIGURE 3

5 THE 4-CHOOSABILITY OF PLANE GRAPHS 121 and d G ( f 3 )6, then put H=G&[u 1, u 2, u 3 ]+[v 2 v 1, v 2 u 5, v 3 u 4, v 3 v 4 ] Fig. 3b. This completes the proof of Claim 2. Claim 3. If u is an M-vertex of a face f # F 4 _ F 5 and is incident with a face f *#F 5, then f * must be an M-face of f. Proof of Claim 3. Since u is an M-vertex of f, it is incident with one triangle and two non-triangular faces f \$ and f ", all of which share at least one vertex with f. The remaining two faces incident with u must be adjacent to either f \$orf " and therefore cannot be in F 5. Therefore if a face f *#F 5 is incident with u, thenf * has to be either f \$orf ", and it is an M-face of f. This completes the proof of Claim 3. Now by Euler's formula, V(G) + F(G) & E(G) =2, we have : v # V(G)\ d G(v) 8 & : = 1 8\ : v # V(G) f # F(G)\ d G( f ) & d G (v)+ : f # F(G) d G ( f ) +&1 2 ( V(G) + F(G) ) = 1 8 (2 E(G) +2 E(G) )&1 2 ( E(G) +2)=&1. For each x # V(G) _ F(G), assigned to x. Then let _(x)=(d G (x)8)&(12) be a weight : x # V(G) _ F(G) _(x)=&1. (1) We shall modify _ to a new weight _* according to the following rules: (R-1) Every non-triangular face transfers 124 to each of its adjacent triangles. (R-2) Let k6. Every k-vertex v transfers 112 to each of its incident faces f \$ adjacent to two triangles, each of which shares with f \$ a common edge incident with v; and transfers 124 to each of its incident faces f " adjacent to exactly one triangle which shares with f " a common edge incident with v. (R-3) Let v be a 5-vertex of G. (a) If v is incident with one 5-face f 1 # F 5 and two 5-faces f 2, f 3 # F 4, then transfer 112 to f 1 from v and transfer 148 to each of f 2 and f 3.

6 122 LAM, XU, AND LIU (b) If v is incident with one 5-face f 1 # F 5 and up to one 5-face f 2 # F 4, then transfer 112 to f 1 from v and transfer 124 to f 2, if necessary. (c) If v is not incident with any 5-face in F 5, then transfer 124 from v to each face in F 4 which is incident with v. Note that a 5-vertex v cannot be incident with two 5-faces in F 5 because otherwise there are adjacent triangles. Now, it follows that : x # V(G) _ F(G) _*(x)= : x # V(G) _ F(G) _(x)=&1. (2) If v is a vertex of degree at least 6, then v is incident with lwd G (v)2x faces which receives 112 from v, and incident with at most w 2 3 (d G(v)&2l)x faces which receive 124 from v, where wxx is the largest integer not exceeding x. Therefore, _*(v)_(v)& l 12 & \ 2 3 (d G(v)&2l) } 1 24 _(v)& l 12 & 1 36 (d G(v)&2l) = 7d G(v)&36&2l 72 = (6d G(v)&36)+(d G (v)&2l) Let v be a 5-vertex. If v is incident with one 5-face in F 5 and two 5-faces in F 4, then _*(v)_(v)&(112)&2 } (148)=0. If v is incident with one 5-face in F 5 and at most one 5-face in F 4, then _*(v)_(v)&(112)& (124)=0. If v is not incident with any 5-face of F 5,thenv is incident with at most three 5-faces in F 4 and _*(v)_(v)&3 } (124)=0. Let f # F(G). If d G ( f )=3, then _*( f )=_( f )+3 } (124)=0. If d G ( f )6, then _*( f )_( f )&d G ( f )}(124)=(2d G ( f )&12)240. If d G ( f )=5 and f F 5 _ F 4, then _*( f )_( f )&3 } (124)=0. If f # F 5 "F 5, then there is at least one vertex of degree 5 or more incident with f and _*( f )_( f )+ (112)&5 } (124)=0. If f # F 4 "F 4, then there is also at least one vertex of degree 5 or higher incident with f. Suppose u is such a vertex. If 124 or more is transferred from u to f, which will be the case if d G (u)>5 or if transfer is done according to (R-3)(b) or (R-3)(c), then _*( f )_( f )+(124)&4 } (124)=0. Otherwise d G (u)=5 and u is incident with a face f 1 in F 5 and another face f 2 in F 4 in addition to f. Thus we have the situation of Fig. 4 and d G (w)5 because otherwise the triangles T and T\$ are adjacent. It follows that _*( f )_( f )+(148)+(148)&4 } (124)=0.

7 THE 4-CHOOSABILITY OF PLANE GRAPHS 123 FIGURE 4 Up to now, we have _*(x)0 for each x # V(G) _ F(G)&F 5 _ F 4, _*(x)=&112 for each x # F 5 and _*(x)=&124 for each x # F 4. For each f # F 5 _ F 4, let M*(f)=[vertex(face) which is an M-vertex(face) of f]. If we can prove that for each f # F 5 _ F 4, the inequality _*( f )+ : x # M*( f ) _*(x) 0, (3) m*(x) holds, then we have x # V(G) _ F(G) _*(x)0. This will contradict (2) and will complete the proof. Suppose f # F 5 as in Fig. 1a. Then _*( f )=&112. Let r i, j be the value transferred from v i to f j according to (R 2 ) and (R 3 ), where i, j=1,..., 5. By Claim 1, f has at least two M-vertices. Suppose v 1 is an M-vertex of f. In the sequel, we want to show that _*(v 1 ) m*(v 1 ) + _*( f 5) m*( f 5 ) + _*( f 1) m*( f 1 ) (4) (i) If f 1 # F 5, then r 1, 1 =r 2, 1 =112 and _*( f 1 )112. Hence (_*( f 1 )m*( f 1 ))(_*( f 1 )2)124. Similarly, if f 5 # F 5, then (_*( f 5 ) m*( f 5 ))124. In the following, we shall assume that f 1 and f 5 are both not in F 5, and hence by Claim 3, v 1 is not incident with any face in F 5. (ii) If f 1 # F 4 and f 5 F 4, then r 1, 1 =124, r 1, 5 =0, and at most 124 is transferred to one non-triangular face (other than f 1 and f 5 ) incident with v 1. Hence _*(v 1 )124 and (_*(v 1 )m*(v 1 ))(_*(v 1 )2)148. Because f 1 # F 4, we have r 2, If m*( f 1 )=1, then (_*( f 1 )m*( f 1 ))= _*( f 1 )148. If m*( f 1 )=2, then f 1 is incident with three vertices with degree 5 or higher. Hence there is vertex w [v 1, v 2 ] incident with f 1 and d G (w)5, so each of w and v 2 will transfer at least 148 to f 1 and (_*( f 1 ) m*( f 1 ))=(_*( f 1 )2)148. It follows that (_*(v 1 )m*(v 1 ))+(_*( f 1 ) m*( f 1 ))124. Similarly, if f 1 F 4 and f 5 # F 4, then (_*(v 1 )m*(v 1 ))+ (_*( f 5 )m*( f 5 ))124.

8 124 LAM, XU, AND LIU (iii) If f 1 F 4 and f 5 F 4,thenr 1, 1 =r 1, 5 =0 and v 1 will transfer at most 124 to one non-triangular face incident with it, and therefore _*(v 1 )112. It follows that (_*(v 1 )m*(v 1 ))(_*(v 1 )2)124. Thus the proof of (4) is complete. Similarly, if v 3 is an M-vertex of f, then (_*(v 3 )m*(v 3 ))+(_*( f 2 ) m*( f 2 ))+(_*( f 3 )m*( f 3 ))124. Combining this with (4), we get (3). An analogous conclusion can be made if v 4 is an M-vertex of f. Suppose that f has exactly two M-vertices v 1 and v 2. By Claim 1, f 2, f 3, f 4 and f 5 are all 5-faces. We also have d G (v j )6 for j=3, 4 and 5. If f 3 # F 5, then as in (i) above, we have _*( f 3 )112. If f 3 F 5,thenr 3, and r 4, 3 124, and hence _*( f 3 )124. Similarly, we have _*( f 4 )124. It follows that (_*( f 3 )m*( f 3 ))+(_*( f 4 )m*( f 4 ))(_*( f 3 )2)+(_*( f 4 )2) 124. Combining this with the argument made on v 1, we also have (3). Now suppose f # F 4 as in Fig. 1b. Then _*( f )=&124. Let r i, j be the weight transferred from v i to f j according to (R 2 ) and (R 3 ), where i=1,..., 4 and j=1,..., 5. By Claim 2, f has at least one M-vertex. If v 2 is an M-vertex of f, then we can show as before that (_*(v 2 )m*(v 2 ))+(_*( f 1 )m*( f 1 ))+(_*( f 2 ) m*( f 2 ))124, and (3) follows. Similarly if v 3 is an M-vertex of f, then (_*(v 3 )m*(v 3 ))+(_*( f 2 )m*( f 2 ))+(_*( f 3 )m*( f 3 ))124 and (3) follows also. If both v 2 and v 3 are not M-vertices of f, then without loss of generality, we may assume that v 1 is an M-vertex of f. Also by Claim 2, either f 1 or f 3 is an M-face of f. Suppose f 1 is an M-face of f. If f 1 # F 5 then r 1, 1 =r 2, 1 =112. So _*( f 1 )112 and (_*( f 1 )m*( f 1 ))(_*( f 1 )2)124. Suppose f 1 # F 4, then by Claim 3, v 1 is not incident with any face in F 5 and therefore r 1, 1 =124. If v 1 is not incident with two triangles adjacent to f 1, then v 2 is incident with two triangles adjacent to f 1. Since v 2 is not an M-vertex of f, d G (v 2 )6 and so r 2, 1 =112. Hence _*( f 1 )112. It follows that (_*( f 1 )m*( f 1 ))(_*( f 1 )2)124. If v 1 is incident with two triangles adjacent to f 1, then f 5 is adjacent to at least two non-triangular face and hence r 1, 5 =0. Since we transfer at most 124 from v 1 to one non-triangular face (other than f 1 and f 5 ) incident with v 1, we have _*(v 1 )124. Since r 2, 1 124, we have _*( f 1 )124. It follows that (_*( f 1 )m*( f 1 ))+ (_*(v 1 )m*(v 1 ))(_*( f 1 )2)+(_*(v 1 )2)124. If f 1 F 5 _ F 4, then r 1, 1 =0 and because d G (v 2 )6, r 2, It follows that _*(v 1 )124 and _*( f 1 )124, and that (_*(v 1 )m*(v 1 ))+(_*( f 1 )m*( f 1 ))124. As before, (3) holds. Suppose f 1 is not an M-face of f, then r 1, 1 =0 and _*(v 1 )124. By Claim 2, f 3 must be an M-face of f. Iff 3 # F 5,orifv 4 is also an M-vertex of f, the same argument as above leads to (3). Suppose f 3 F 5 and v 4 is not an M-vertex of f, thend G (v 4 )6 and therefore r 4, Because v 3 is

9 THE 4-CHOOSABILITY OF PLANE GRAPHS 125 not an M-vertex of f, we also have r 3, and hence _*( f 3 )124. It follows that (_*(v 1 )m*(v 1 ))+(_*( f 3 )m*( f 3 ))(_*(v 1 )2)+(_*( f 3 )2) 124 and that (3) again holds. 3. PROOF OF THEOREM 1 Suppose that G is a counterexample of minimum order, then \$(G)=4. Because G is C 4 -free, G has no adjacent triangles and has no 4-face. By Lemma 1, G has a F 3 5-subgraph H with V(H)=[u 1,..., u 6 ] and E(H)=[u 1 u 2, u 2 u 3, u 3 u 4, u 4 u 5, u 5 u 6, u 6 u 1, u 2 u 6 ]. Let L=(L(v) v # V(G)) be a color-list of G in which each list contains 4 colors. Then G\$=G&V(H) admits a list coloring,\$ with color-list L restricted to G\$. For all v # V(H), let L 0 (v)=l(v)"[,\$(u) u # V(G\$) and vu # E(G)]. Then, L 0 (u i ) 2, i=1,3,4,5, L 0 (u 2 ) 3 and L 0 (u 6 ) 3. Let L* bea subset of L 0 (u 3 ) with L* =2. We shall choose at u 2 a color c 2 # L 0 (u 2 )"L*, at u 1 a color c 1 # L 0 (u 1 )"[c 2 ],atu 6 a color c 6 # L 0 (u 6 )"[c 1, c 2 ], at u 5 a color c 5 # L 0 (u 5 )"[c 6 ],atu 4 a color c 4 # L 0 (u 4 )"[c 5 ] and at u 3 a color c 3 # L*"[c 4 ]. ACKNOWLEDGMENT The authors acknowledge the valuable comments and suggestions of the referees. REFERENCES 1. N. Alon and M. Tarsi, Colorings and orientations of graphs, Combinatorica 12, No. 2 (1992), O. V. Borodin, Structural properties of plane graphs without adjacent triangles and an application to 3-colorings, J. Graph Theory 12, No. 2 (1996), J. A. Bondy and U. S. R. Murty, ``Graph Theory with Applications,'' Macmillan, London, P. Erdo s, A. L. Rubin, and H. Taylor, Choosability in graphs, Congr. Numer. 26 (1979), R. Steinberg, The state of the three color problem, in ``Quo Vadis: Graph Theory?'' (J. Gimbel, J. W. Kennedy, and L. V. Quintas, Eds.), Ann. Discrete Math., Vol. 55, pp , North-Holland, Amsterdam, C. Thomassen, Every planar graph is 5-choosable, J. Combin. Theory Ser. B 62 (1994), C. Thomassen, 3-list-coloring planar graphs of girth 5, J. Combin. Theory Ser. B 64 (1995),

10 126 LAM, XU, AND LIU 8. V. G. Vizing, Vertex coloring with given colors, Diskret. Anal. 29 (1976), 310. [In Russian] 9. M. Voigt, List colouring of planar graphs, Discrete Math. 120 (1993), M. Voigt, A not 3-choosable planar graph without 3-cycles, Discrete Math. 146 (1995), M. Voigt and B. Wirth, On 3-colorable non-4-choosable planar graphs, J. Graph Theory 24, No. 3 (1997),

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