Boolean approach to planar embeddings of a graph

The purpose of this paper which is a sequel of “ Boolean planarity characterization of graphs ” [9] is to show the following results.

1. Both of the problems of testing the planarity of graphs and embedding a planar graph into the plane are equivalent to finding a spanning tree in another graph whose order and size are bounded by a linear function of the order and the size of the original graph, respectively.
2. The number of topologically non-equivalent planar embeddings of a Hamiltonian planar graphG is τ(G)=2 ^c(H)?1, wherec (H) is the number of the components of the graphH which is related toG.

     

H-extension of graphs

We consider the problem of constructing a graphG* from a collection of isomorphic copies of a graphG in such a way that for every two copies ofG, either no vertices or a section graph isomorphic to a graphH is identified. It is shown that ifG can be partitioned into vertex-disjoint copies ofH, thenG* can be made to have at most |H| orbits. A condition onG so thatG* can be vertextransitive is also included.     

Sparse ramsey graphs

IfH is a Ramsey graph for a graphG thenH is rich in copies of the graphG. Here we prove theorems in the opposite direction. We find examples ofH such that copies ofG do not form short cycles inH. This provides a strenghtening also, of the following well-known result of Erdős: there exist graphs with high chromatic number and no short cycles. In particular, we solve a problem of J. Spencer. Dedicated to Paul Erdős on his seventieth birthday     

Greatest common divisors and least common multiples of graphs

A graphH divides a graphG, writtenH|G, ifG isH-decomposable. A graphG without isolated vertices is a greatest common divisor of two graphsG ₁ andG ₂ ifG is a graph of maximum size for whichG|G ₁ andG|G ₂, while a graphH without isolated vertices is a least common multiple ofG ₁ andG ₂ ifH is a graph of minimum size for whichG ₁|H andG ₂|H. It is shown that every two nonempty graphs have a greatest common divisor and least common multiple. It is also shown that the ratio of the product of the sizes of a greatest common divisor and least common multiple ofG ₁ andG ₂ to the product of their sizes can be arbitrarily large or arbitrarily small. Sizes of least common multiples of various pairsG ₁,G ₂ of graphs are determined, including when one ofG ₁ andG ₂ is a cycle of even length and the other is a star.G. C's research was supported in part by the Office of Naval Research, under Grant N00014-91-I-1060     

Packing paths in planar graphs

A generalization of P. Seymour's theorem on planar integral 2-commodity flows is given when the underlying graphG together with the demand graphH (a graph having edges that connect the corresponding terminal pairs) form a planar graph and the demand edges are on two faces ofG.     

Graphs isomorphic to subgraphs of their iterated line graphs

A graphG is said to be embeddable into a graphH, if there is an isomorphism ofG into a subgraph ofH. It is shown in this paper that every unicycle or tree which is neither a path norK _1,3 embeds in itsn-th iterated line graph forn1 or 2, 3, and that every other connected graph that embeds in itsn-th iterated line graph may be constructed from such an embedded unicycle or tree in a natural way. A special kind of embedding of graph into itsn-th iterated line graph, called incidence embedding, is studied. Moreover, it is shown that for every positive integerk, there exists a graphG such that (G) = , where (G) is the leastn1 for whichG embeds inL ⁿ(G).     

Greatest common subgraphs with specified properties

A graphG without isolated vertices is a greatest common subgraph of a setG of graphs, all having the same size, ifG is a graph of maximum size that is isomorphic to a subgraph of every graph inG. A number of results concerning greatest common subgraphs are presented. For several graphical propertiesP, we discuss the problem of determining, for a given graphG with propertyP, the existence of two non-isomorphic graphsG ₁ andG ₂ of equal size, also with propertyP, such thatG is the unique greatest common subgraph ofG ₁ andG ₂. In particular, this problem is solved whenP is the property of being 2-connected and whenP is the property of having chromatic numbern.     

Reconstructing graphs as subsumed graphs of hypergraphs,and some self-complementary triple systems

LetV be a set ofn elements. The set of allk-subsets ofV is denoted . Ak-hypergraph G consists of avertex-set V(G) and anedgeset , wherek≥2. IfG is a 3-hypergraph, then the set of edges containing a given vertexvεV(G) define a graphG _v . The graphs {G _v νvεV(G)} aresubsumed byG. Each subsumed graphG _v is a graph with vertex-setV(G) − v. They can form the set of vertex-deleted subgraphs of a graphH, that is, eachG _v ≽H −v, whereV(H)=V(G). In this case,G is a hypergraphic reconstruction ofH. We show that certain families of self-complementary graphsH can be reconstructed in this way by a hypergraphG, and thatG can be extended to a hypergraphG ^*, all of whose subsumed graphs are isomorphic toH, whereG andG ^* are self-complementary hypergraphs. In particular, the Paley graphs can be reconstructed in this way. This work was supported by an operating grant from the Natural Sciences and Engineering Research Council of Canada.     

RandomlyH-coverable graphs

A graph israndomly matchable if every matching of the graph is contained in a perfect matching. We generalize this notion and say that a graphG israndomly H-coverable if every set of independent subgraphs, each isomorphic toH, that does not cover the vertices ofG can be extended to a larger set of independent copies ofH. Various problems are considered for the situation whereH is a path. In particular, we characterize the graphs that are randomlyP ₃ -coverable.     

Opposition graphs are strict quasi-parity graphs

An opposition graph is a graph whose edges can be acyclically oriented in such a way that every chordless path on four vertices has its extreme edges both pointing in or pointing out. A strict quasi-parity graph is a graphG such that every induced subgraphH ofG either is a clique or else contains a pair of vertices which are not endpoints of an odd (number of edges) chordless path ofH. The perfection of opposition graphs and strict quasi-parity graphs was established respectively by Olariu and Meyniel. We show here that opposition graphs are strict quasi-parity graphs.The second author acknowledges the support of the Air Force Office of Scientific Research under grant number AFOSR 0271 to Rutgers University.     

On the maximal number of certain subgraphs inK r -free graphs

Given two graphsH andG, letH(G) denote the number of subgraphs ofG isomorphic toH. We prove that ifH is a bipartite graph with a one-factor, then for every triangle-free graphG withn verticesH(G) H(T ₂(n)), whereT ₂(n) denotes the complete bipartite graph ofn vertices whose colour classes are as equal as possible. We also prove that ifK is a completet-partite graph ofm vertices,r > t, n max(m, r – 1), then there exists a complete (r – 1)-partite graphG* withn vertices such thatK(G) K(G*) holds for everyK _r -free graphG withn vertices. In particular, in the class of allK _r -free graphs withn vertices the complete balanced (r – 1)-partite graphT _r–1(n) has the largest number of subgraphs isomorphic toK _t (t < r),C ₄,K _2,3. These generalize some theorems of Turán, Erdös and Sauer.Dedicated to Paul Turán on his 80th Birthday     

Relaxed chromatic numbers of graphs

Given any family of graphsP, theP chromatic number _p (G) of a graphG is the smallest number of classes into whichV(G) can be partitioned such that each class induces a subgraph inP. We study this for hereditary familiesP of two broad types: the graphs containing no subgraph of a fixed graphH, and the graphs that are disjoint unions of subgraphs ofH. We generalize results on ordinary chromatic number and we computeP chromatic number for special choices ofP on special classes of graphs.Research supported in part by ONR Grant N00014-85K0570 and by a grant from the University of Illinois Research Board.     

A generalization of menger's theorem for certain block—cactus graphs

A graphG is called a block—cactus graph if each block ofG is complete or a cycle. In this paper, we shall show that a block—cactus graphG has the property that the cardinality of a smallest set separating any vertex setJ ofG is the maximum number of internally disjoint paths between the vertices ofJ if and only if every block ofG contains at most two cut-vertices. This result extends two theorems of Sampathkumar [4] and [5].     

Finite modules over non-semisimple group rings

LetG be a fixed graph and letX _G be the number of copies ofG contained in the random graphG(n, p). We prove exponential bounds on the upper tail ofX _G which are best possible up to a logarithmic factor in the exponent. Our argument relies on an extension of Alon’s result about the maximum number of copies ofG in a graph with a given number of edges. Similar bounds are proved for the random graphG(n, M) too. Research of the second author supported by KBN grant 2 P03A 027 22. Research of the third author supported by KBN grant 2 P03A 15 23.     

The chromatic number of the product of two 4-chromatic graphs is 4

For any graphG and numbern≧1 two functionsf, g fromV(G) into {1, 2, ...,n} are adjacent if for all edges (a, b) ofG, f(a) ≠g(b). The graph of all such functions is the colouring graph ℒ(G) ofG. We establish first that χ(G)=n+1 implies χ(ℒ(G))=n iff χ(G ×H)=n+1 for all graphsH with χ(H)≧n+1. Then we will prove that indeed for all 4-chromatic graphsG χ(ℒ(G))=3 which establishes Hedetniemi’s [3] conjecture for 4-chromatic graphs. This research was supported by NSERC grant A7213     

Mortality of iterated Gallai graphs

For a finite or infinite graphG, theGallai graph (G) ofG is defined as the graph whose vertex set is the edge setE(G) ofG; two distinct edges ofG are adjacent in (G) if they are incident but do not span a triangle inG. For any positive integert, thetth iterated Gallai graph ^t (G) ofG is defined by ( ^t–1(G)), where ⁰(G):=G. A graph is said to beGallai-mortal if some of its iterated Gallai graphs finally equals the empty graph. In this paper we characterize Gallai-mortal graphs in several ways.     

On thel-connectivity of a graph

For an integerl 2, thel-connectivity of a graphG is the minimum number of vertices whose removal fromG produces a disconnected graph with at leastl components or a graph with fewer thanl vertices. A graphG is (n, l)-connected if itsl-connectivity is at leastn. Several sufficient conditions for a graph to be (n, l)-connected are established. IfS is a set ofl( 3) vertices of a graphG, then anS-path ofG is a path between distinct vertices ofS that contains no other vertices ofS. TwoS-paths are said to be internally disjoint if they have no vertices in common, except possibly end-vertices. For a given setS ofl 2 vertices of a graphG, a sufficient condition forG to contain at leastn internally disjointS-paths, each of length at most 2, is established.     

Weak embedding of planar graphs

A weak embedding of a graphG is an embedding ofG with all the faces being tours. In this paper, for some planar graphs the weak maximum genus of them are given. And for the planar graphG, a characterization of whetherG can be weakly embedded on the nonspherical surfaces is obtained.     

On the power of a perturbation for testing non-isomorphism of graphs

In this paper we prove an inverted version of A. J. Schwenk's result, which in turn is related to Ulam's reconstruction conjecture. Instead of deleting vertices from an undirected graphG, we add a new vertexv and join it to all other vertices ofG to get a perturbed graphG+v. We derive an expression for the characteristic polynomial of the perturbed graphG+v in terms of the characteristic polynomial of the original graphG. We then show the extent to which the characteristic polynomials of the perturbed graphs can be used in determining whether two graphs are non-isomorphic.This work was supported by the U.S. Army Research Office under Grant DAAG29-82-K-0107.     

Edge-clique graphs

The edge-clique graphK(G) of a graphG is that graph whose vertices correspond to the edges ofG and where two vertices ofK(G) are adjacent whenever the corresponding edges ofG belong to a common clique. It is shown that every edge-clique graph is a clique graph, and that ifG is either an interval graph or a line graph, then so too isK(G). An algorithm is provided for determining whether a graph is an edge-clique graph. A new graph called the STP graph is introduced and a relationship involving this graph, the edge-clique graph, and the line graph is presented. The STP graphs are also characterized.Research supported in part by Office of Naval Research Contract N00014-88-K-0018.Research supported in part by Office of Naval Research Contract N00014-88-K-0163.