# Vertex Degrees in Outerplanar Graphs

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1 Vertex Degrees in Outerplanar Graphs Kyle F. Jao, Douglas B. West October, n 6 k 4 Abstract For an outerplanar graph on n vertices, we determine the maximum number of vertices of degree at least k. For k = 4 (and n 7) the answer is n 4. For k = 5 (and n 4), the answer is 2(n 4) (except one less when n 1 mod 6). For k 6 (and n k + 2), the answer is. We also determine the maximum sum of the degrees of s vertices in an n-vertex outerplanar graph and the maximum sum of the degrees of the vertices with degree at least k. 1 Introduction For n > k > 0, Erdős and Griggs [1] asked for the minimum, over n-vertex planar graphs, of the number of vertices with degree less than k. For k 6, the optimal values follow from results of Grünbaum and Motzkin [4]. West and Will [5] determined the optimal values for k 12, obtained the best lower bounds for 7 k 11, and provided constructions achieving those bounds for infinitely many n when 7 k 10. Griggs and Lin [2] independently found the same lower bounds for 7 k 10 and gave constructions achieving the lower bounds when 7 k 11 for all sufficiently large n. We study the analogous question for outterplanar graphs, expressed in terms of largedegree vertices. Let β k (n) be the maximum, over n-vertex outerplanar graphs, of the number of vertices having degree at least k. For k 2, the problem is trivial; β k (n) = n, achieved by a cycle (or by any maximal outerplanar graph). When k {3, 4}, the square of a path shows that β 3 (n) n 2 and β 4 (n) n 4. Since every outerplanar graph with n 2 has at least two vertices of degree at most 2, β 3 (n) = n 2. We will prove β 4 (n) = n 4 when n 7 (Theorem 2.6). For k = 5 and Mathematics Department, University of Illinois, Urbana, IL 61820, Mathematics Department, University of Illinois, Urbana, IL 61820, Research supported by NSA under grant H

2 n 4, we prove β k (n) = 2(n 4)/3, except one less when n 1 mod 6 (Theorem 2.5). For k 6 and n k + 2, we prove β k (n) = (n 6)/(k 4) (Theorem 3.4). We close this introduction with a general upper bound that is optimal for k = 5 when n 1 mod 6. In Section 2 we improve the upper bound by 1 when n 1 mod 6 and provide the general construction that meets the bound; these ideas also give the upper bound for k = 4. In Section 3 we solve the problem for k 6. The bounds in [5] were obtained by first solving a related problem, which here corresponds to maximizing the sum of the degrees of the vertices with degree at least k. We use this approach in Section 3 to prove the upper bound on β k (n) when k 6. Adding edges does not decrease the number of vertices with degree at least k, so an n-vertex outerplanar graph with β k (n) vertices of degree at least k must be a maximal outerplanar graph, which we abbreviate to MOP. For a MOP with n vertices, let β be the number of vertices having degree at least k, and let n 2 be the number of vertices having degree 2. A MOP with n vertices has 2n 3 edges, so summing the vertex degrees yields 2n 2 + 3(n n 2 β) + kβ 4n 6. (1) This inequality simplifies to (k 3)β n + n 2 6. Using n 2 n β then yields β k (n) 2(n 3)/(k 3). To improve the bound, we need a structural lemma. Lemma 1.1. Let G be an n-vertex MOP with external cycle C. If n 4, then G has two vertices with degree in {3, 4} that are not consecutive along C. Proof. We use induction on n. Note that G contains n 3 chords of C. If every chord lies in a triangle with two external edges, then n 6 and (G) 4, and the two neighbors of a vertex of degree 2 are the desired vertices. This case includes the MOPs for n {4, 5}. Otherwise, a chord xy not in a triangle with two external edges splits G into two MOPs with at least four vertices, each with x and y consecutive along its external cycle. By the induction hypothesis, each has a vertex with degree 3 or 4 outside {x,y}. In G, those two vertices retain their degrees, and they are separated along C by x and y. Corollary 1.2. If k 5 and n 4, then β k (n) 2(n 4)/(k 2). Proof. Lemma 1.1 yields n n 2 β 2, and hence n 2 n β 2. Substituting this improved inequality into the inequality (k 3)β n + n 2 6 that follows from (1) yields β k (n) 2(n 4)/(k 2). Corollary 1.2 gives us a target to aim for in the construction for k = 5. For k 6 we will need further improvement of the upper bound. The argument of Corollary 1.2 is not valid for k = 4, since vertices of degree 4 are counted by β. The upper bound β 4 (n) n 4 (for n 7) will come as a byproduct of ideas in the next section. 2

4 4q 5, 4q 4, 4q 4, 4q 3, 4q 3, 4q 2, respectively. The induction hypothesis for an induction on q states that when n = 6q 4 = 6(q 1) + 2, the graph has 4(q 1) 2 vertices of degree 5. Starting from this point, we augmented one vertex to degree 5 when n is congruent to each of { 3, 2, 0, 2} modulo 6, matching the formula. The lower bound in Lemma 2.1 and upper bound in Corollary 1.2 are equal except when n 1 mod 6. In this case we improve the upper bound by showing that there is no outerplanar graph having the vertex degrees required to achieve equality in the upper bound. The construction shows β 5 (n) 2(n 5)/3. We used the existence of two vertices with degree 3 or 4 to improve the upper bound from 2(n 3)/3 to 2(n 4)/3, which differs from 2(n 5)/3 by 1 when n 1 mod 6. We begin by showing that slightly stronger hypotheses further reduce the bound. Lemma 2.2. If G is a MOP having a 6 + -vertex, or a 3-vertex and a 4-vertex, or two 4- vertices, or at least three 3-vertices, then G has at most (2n 9)/3 vertices of degree at least 5. Proof. We have proved that β (2n 8)/3. If we cannot improve the upper bound to (2n 9)/3, then equality must hold in all the inequalities that produced the upper bound (2n 8)/3. Thus n 2 = n β 2, which forbids a third vertex with degree 3 or 4. Also, the sum of the degrees must equal 2n 2 +3(n n 2 β)+5β (see (1)). This requires that the two vertices of degree 3 or 4 both have degree 3 and that no vertex has degree at least 6. Let T be the subgraph of the dual graph of G induced by the vertices corresponding to bounded faces of G; we call T the dual tree of G. Since G is a MOP, T is a tree. A triangular face in G having j edges on the external cycle corresponds to a (3 j)-vertex in T. The next lemma will reduce the proof of the theorem to the case where T is a special type of tree. Lemma 2.3. If in the dual tree T the neighbor of a leaf t has degree 2, then G has a 3- vertex on the triangle corresponding to t. If two leaves in T have a common neighbor, then the common vertex of the corresponding triangles in G is a 4-vertex in G. Proof. Let x,y,z be the vertices of the triangle in G corresponding to t, with x having degree 2 in G. The neighboring triangle t raises the degree of y and z to 3. If t has degree 2 in T, then in G only one of {y,z} can gain another incident edge. Two leaves in T having a common neighbor ˆt correspond to two triangles in G having a common vertex z. The vertex ˆt in T corresponds to a triangle in G that shares an edge with each of them. No further edges besides the four in these triangles are incident to z. A triangle in a MOP is internal if none of its edges lie on the external cycle. 4

5 Lemma 2.4. In a MOP with n vertices, let n 2 be the number of 2-vertices and t be the number of internal triangles. If n 4, then t = n 2 2. Proof. Let T be the dual tree. Note that T has n 2 vertices, of which n 2 have degree 1, t have degree 3, and the rest have degree 2. By counting the edges in terms of degrees, 2(n 3) = n 2 + 3t + 2(n 2 n 2 t), which yields n 2 2 = t. { 2(n 5)/3 if n 1 mod 6, Theorem 2.5. If n 4, then β 5 (n) = 2(n 4)/3 otherwise. Proof. It suffices to prove the upper bound when n 1 mod 6. Let n = 6q+1. Corollary 1.2 yields β 4q 2, and we want to improve this to 4q 3, which equals (2n 9)/3. If the upper bound cannot be improved, then by the computation in the proof of Lemma 2.2 we may assume that G has exactly two 3-vertices, exactly 4q 2 vertices of degree 5, and exactly 2q + 1 vertices of degree 2. A MOP with n vertices has n 2 bounded faces, so T has n 2 vertices. Since G has 2q + 1 vertices of degree 2, there are 2q + 1 leaves in T; by Lemma 2.4, T has 2q 1 vertices of degree 3. The remaining 2q 1 vertices of T have degree 2. A caterpillar is a tree such that deleting all the leaves yields a path, called its spine. A tree that is not a caterpillar contains as a subtree the graph Y obtained by subdividing each edge of the star K 1,3. If Y T, then consider longest paths in T starting from the central vertex v of Y along each of the three incident edges. Each such path reaches a leaf. By Lemma 2.3, each such path generates a vertex of degree 3 or 4 in G. Since G has at most two such vertices, T is a caterpillar. Furthermore, since G has no 4-vertices, Lemma 2.3 implies that each endpoint of the spine of T has degree 2 in T. Consider vertices a, b, c V (T) such that ab, bc E(T). The corresponding three triangles in G have a common vertex x. If a and c are 3-vertices in T, then x has degree 6 in G. Hence no two 3-vertices in T have a common neighbor. This implies that along the spine of T (which starts and ends with 2-vertices), there are at most two consecutive 3-vertices, and nonconsecutive 3-vertices are separated by at least two 2-vertices. In particular, every run of 3-vertices has at most two vertices, every run of 2-vertices has at least two vertices (except possibly the runs at the ends), and the number of runs of 2-vertices is one more than the number of runs of 3-vertices. The only way this can produce the same number of 2-vertices and 3-vertices is 2, 3, 3, 2, 2, 3, 3,...,2, 2, 3, 3, 2. However, in this configuration the number of vertices of each type is even and cannot equal 2q 1. We have proved that no outerplanar graph has the required vertex degrees. Theorem 2.6. If n 7, then β 4 (n) = n 4. 5

6 Proof. When the dual tree T is a path, the graph G has two 2-vertices, two 3-vertices, and n 4 vertices of degree 4; this proves the lower bound. For the upper bound, since leaves of T correspond to triangles in G having 2-vertices, we may assume that T has at most three leaves. If T has only two leaves, then the neighbor of each has degree 2 in T, and Lemma 2.3 provides two 3-vertices in G, matching the construction. If T has exactly three leaves, then T has one 3-vertex and at least four other vertices, since T has n 2 vertices and n 7. Hence at least one leaf in T has a neighbor of degree 2, and Lemma 2.3 provides one 3-vertex in addition to the three 2-vertices in G. (In fact, this case yields another construction having exactly n 4 vertices with degree at least 4.) 3 The solution for k 6 Trivially, β k (n) = 1 when k = n 1, which does not satisfy the general formula. We restrict our attention to n k + 2 and begin with the construction. Fix k with k 6. Form a graph B from B in Section 2 by respectively replacing edges yz and ux with paths P and Q, each having k 6 internal vertices. Make v adjacent to all of V (P) and w adjacent to all of V (Q) (see Fig. 2). Since P and Q have k 4 vertices each, B has 2k 6 vertices; also, B is a MOP. Its vertices v and w of maximum degree have degree k 2. z P v y z 1 z 0 v 0 v 1 v 2 v 3 y 1 z 2 y 2 z 3 y 3 u w Q x B u 1 w 1 w 2 w 3 x 1 x 2 x 3 B 1 u 2 B 2 u 3 B 3 Figure 2: The graph B and its use in constructing F. Lemma 3.1. If n k + 2, then β k (n) n 6. k 4 Proof. Letting n = 2(k 4)q r with 0 r < 2(k 4), the claim is equivalent to β k (n) 2q when 0 r < k 4 and β k (n) 2q + 1 when k 4 r < 2(k 4). Let F be the union of q copies B 1,...,B q of B, modifying the names of vertices in B i by adding the subscript i and taking y i = v i+1 and w i = u i+1 for i 1 (see the solid graph 6

8 an edge from a neighbor of x that is in S and its degree is no longer among the s largest, then it was replaced by a vertex of the same degree; thus we have increased the sum of the s largest degrees, which contradicts the choice of G.) The last neighbor of x is now farther along P. When it reaches y the sum of the m largest degrees increases. Since we started with a MOP maximizing this sum, the edge xy must have been present initially. We have shown that the outer boundary of G[S] is a cycle. Every bounded face is a face of G, since there are no vertices of G inside it. Hence G[S] is a MOP. In fact, when s < n 1 there is always a unique set of s vertices with largest degrees in a graph maximizing the sum of those degrees. Theorem 3.3. The maximum value D(n,s) of the sum of s vertex degrees in an n-vertex outerplanar graph is given by n 1 if s = 1, D(n,s) = n 6 + 4s if s < n/2, 2n 6 + 2s if s n/2. Proof. Let G be a MOP in which some set S of s vertices has degree-sum D(n,s). If s = 1, then n 1 is clearly an upper bound, achieved by a star. For s 2, let G be a MOP achieving the maximum; we know that G[S] is also a MOP and hence has 2s 3 edges. The question then becomes how the remaining n s vertices can be added to produce the maximum sum of the degrees in S. Consider an outerplanar embedding of G. The subgraph induced by S is also an outerplanar embedding of S. Since G[S] has 2s 3 edges, the outer boundary of the subgraph is a cycle. In the embedding of G, no vertex of V (G) S appears inside this cycle. Also, vertices outside S can be adjacent to only two vertices of S, and they can be adjacent to two only if those two are consecutive on the outer boundary of G[S]. This implies that at most s vertices of V (G) S can have two neighbors in S, and the rest have at most one neighbor in S. Furthermore, the vertices outside S can be added to achieve this bound. If s n/2, then we add 2(n s) to the degree-sum within G[S], obtaining D(n,s) = 2n 6 + 2s. If s n/2, then we add 2s + 1(n 2s), obtaining D(n,s) = n 6 + 4s. n 6 Theorem 3.4. If k 6, then β k (n). k 4 Proof. In an extremal graph, the β k (n) vertices with degree at least k have the largest degrees. With s = β k (n), we have sk D(n,s). Using the bound obtained in Theorem 3.3, 8

9 we have sk { n 6 + 4s if s < n/2, 2n 6 + 2s if s n/2. If k 6 and s n/2, then 6s ks 2n 6 + 2s 6s 6. Hence k 6 implies s < n/2, and therefore ks n 6 + 4s, which simplies to s n 6 k 4. Finally, we consider the maximum sum of the degrees of the vertices with degree at least k. Essentially, the point is that we cannot increase this sum by using fewer than β k (n) vertices with degrees larger than k. Corollary 3.5. For k 6, the maximum sum of the degrees of the vertices with degree at least k in an n-vertex outerplanar graph is n n 6 k 4. Proof. Let G be an n-vertex outerplanar graph, and let S = {v V (G): d(v) k}. Let s = S. Since these are the vertices of largest degree, v S d(v) D(n,s). For k 6, since D(n,s) is monotone increasing in s, we obtain a bound on the sum by using the bound on β k (n) obtained in Theorem 3.4. Since β k (n) < n/2, Theorem 3.3 yields v S d(v) D(n,s) D(n,β k(n)) = n n 6 k 4. We show that a modification of the construction in Lemma 3.1 achieves equality in the bound. When n = (k 4)q + 6 for some integer q, let G be the graph constructed in Lemma 3.1, having β k (n) vertices of degree at least k; here β k (n) = q. All q of these vertices have degree exactly k, so the sum of their degrees is q k, equaling the upper bound here. For each increase in n over the next k 5 vertices, adding one vertex of degree 2 can increase the degree-sum of these vertices by 1, again equaling the upper bound here. When the (k 4)th addition is reached, start over with the construction from Lemma 3.1 for the new value of q. Remark 3.6. Similar analysis solves the problem of maximizing the sum of the degrees of the vertices with degree at least 5. We remark that the maximum value when n 1 mod 6 is 2n 8 + 4β k (n), which is smaller by 2 than the former in terms of β k (n) when k 6. Writing the expression as 2(n β k (n)) 2+(4β k (n) 6), we begin with degree-sum 4β k 6 within the MOP H induced by the vertices of degree at least 5. Since β 5 (n) is roughly 2n/3, we should be able to augment the sum of the degrees by 2 for each of the remaining n β k (n) vertices. However, H has at least two vertices of degree 2. For each such vertex, raising its degree to 5 requires one of the added vertices to contribute only 1 instead of 2. With this adjustment, the improved upper bound meets the construction. When n 1 mod 6, there are additional technicalities we leave to the reader. 9

10 References [1] P. Erdős and J. R. Griggs, Problem Session, Workshop on Planar Graphs, DIMACS, Rutgers Univ., [2] J. R. Griggs and Y.-C. Lin, Planar graphs with few vertices of small degree. Discrete Math 143 (1995), [3] J. R. Griggs and Y.-C. Lin, The maximum sum of degrees above a threshold in planar graphs. Discrete Math 169 (1997), [4] B. Grünbaum and T. S. Motzkin, The number of hexagons and the simplicity of geodesics on certain polyhedra. Canad. J. Math. 15 (1963), [5] D. B. West and T. G. Will, Vertex degrees in planar graphs. In Planar graphs (New Brunswick, NJ, 1991), DIMACS Ser. Discrete Math. Theoret. Comput. Sci. 9 (Amer. Math. Soc., 1993),

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