ALGEBRAIC AND GEOMETRIC ASPECTS OF BIPARTITE PLANAR GRAPHS

Forcing faces in plane bipartite graphs

Finding Paths between graph colourings: PSPACE-completeness and superpolynomial distances

We investigate the space complexity of certain perfect matching problems over bipartite graphs embedded on surfaces of constant genus (orientable or non-orientable). We show that the problems of deciding whether such graphs have (1) a perfect matching or not and (2) a unique perfect matching or not, are in the logspace complexity class SPL. Since SPL is contained in the logspace counting classes oplus L (in fact in mod_k for all k >= 2), C=L, and PL, our upper bound places the above-mentioned matching problems in these counting classes as well. We also show that the search version, computing a perfect matching, for this class of graphs is in FL^SPL. Our results extend the same upper bounds for these problems over bipartite planar graphs known earlier. As our main technical result, we design a logspace computable and polynomially bounded weight function which isolates a minimum weight perfect matching in bipartite graphs embedded on surfaces of constant genus. We use results from algebraic topology for proving the correctness of the weight function.

In 1969, Barnette conjectured that every 3-connected cubic planar bipartite graph is Hamiltonian. We obtain two results to help under-stand Barnette’s conjecture. The first result is inspired by the generation theorem of 3-connected cubic planar bipartite graphs, which is the work of Holton, Manvel and McKay. We define two operations called VO and EOR and prove that all graphs which are generated by the two operations from a unique graph of order 8 are Hamiltonian. We deduce also an equivalent description for the 3-connectivity of simple cubic plane bipartite graphs by the recent research hotspots of quasi spanning tree of faces. We prove that every simple cubic plane bipartite graph G is 3-connected if and only if the contraction graph HR(G) is 2-connected and 3-edge-connected, which meaning that if every 2-connected, 3-edge-connected planar graph of whose all vertex degrees are even and more than four has one quasi spanning tree of faces, then every 3-connected cubic planar bipartite graph is Hamiltonian.

The graph coloring problem is one of the most famous problems in graph theory and has a large range of applications. It consists in coloring the vertices of an undirected graph with a given number of colors such that two adjacent vertices get different colors. This thesis deals with some variations of this basic coloring problem which are related to scheduling and discrete tomography. These problems may also be considered as partitioning problems. In Chapter 1 basic definitions of computational complexity and graph theory are presented. An introduction to graph coloring and discrete tomography is given. In the next chapter we discuss two coloring problems in mixed graphs (i.e., graphs having edges and arcs) arising from scheduling. In the first one (strong mixed graph coloring problem) we have to cope with disjunctive constraints (some pairs of jobs cannot be processed simultaneously) as well as with precedence constraints (some pairs of jobs must be executed in a given order). It is known that this problem is NP-complete in mixed bipartite graphs. In this thesis we strengthen this result by proving that for k = 3 colors the strong mixed graph coloring problem is NP-complete even if the mixed graph is planar bipartite with maximum degree 4 and each vertex incident to at least one arc has maximum degree 2 or if the mixed graph is bipartite and has maximum degree 3. Furthermore we show that the problem is polynomially solvable in partial p-trees, for fixed p, as well as in general graphs with k = 2 colors. We also give upper bounds on the strong mixed chromatic number or even its exact value for some classes of graphs. In the second problem (weak mixed graph coloring problem), we allow jobs linked by precedence constraints to be executed at the same time. We show that for k = 3 colors this problem is NP-complete in mixed planar bipartite graphs of maximum degree 4 as well as in mixed bipartite graphs of maximum degree 3. Again, for partial p-trees, p fixed, and for general graphs with k = 2 colors, we prove that the weak mixed graph coloring problem is polynomially solvable. We consider in Chapter 3 the problem of characterizing in an undirected graph G = (V, E) a minimum set R of edges for which maximum matchings M can be found with specific values of p = |M ∩ R|. We obtain partial results for some classes of graphs and show in particular that for odd cacti with triangles only and for forests one can determine in polynomial time whether there exists a minimum set R for which there are maximum matchings M such that p= |R ∩ M|, for p= 0,1, ..., ν(G). The remaining chapters deal with some coloring (or partitioning) problems related to the basic image reconstruction problem in discrete tomography. In Chapter 4 we consider a generalization of the vertex coloring problem associated with the basic image reconstruction problem. We are given an undirected graph and a family of chains covering its vertices. For each chain the number of occurrences of each color is given. We then want to find a coloring respecting these occurrences. We are interested in both, arbitrary and proper colorings and give complexity results. In particular we show that for arbitrary colorings the problem is NP-complete with two colors even if the graph is a tree of maximum degree 3. We also consider the edge coloring version of both problems. Again we present some complexity results. We consider in Chapter 5 some generalized neighborhoods instead of chains. For each vertex x we are given the number of occurrences of each color in its open neighborhood Nd(x) (resp. closed neighborhood Nd+(x)), representing the set of vertices which are at distance d from x (resp. at distance at most d from x). We are interested in arbitrary colorings as well as proper colorings. We present some complexity results and we show in particular that for d = 1 the problems are polynomially solvable in trees using a dynamic programming approach. For the open neighborhood and d = 2 we obtain a polynomial time algorithm for quatrees (i.e. trees where all internal vertices have degree at least 4). We also examine the bounded version of these problems, i.e., instead of the exact number of occurrences of each color we are given upper bounds on these occurrences. In particular we show that the problem for proper colorings is NP-complete in bipartite graphs of maximum degree 3 with four colors and each color appearing at most once in the neighborhood N(x) of each vertex x. This result implies that the L(1,1)-labelling problem is NP-complete in this class of graphs for four colors. Finally in Chapter 6 we consider the edge partitioning version of the basic image reconstruction problem, i.e., we have to partition the edge set of a complete bipartite graph into k subsets such that for each vertex there must be a given number of edges of each set of the partition incident to this vertex. For k = 3 the complexity status is still open. Here we present a new solvable case for k = 3. Then we examine some variations where the union of two subsets E1, E2 has to satisfy some additional constraints as for example it must form a tree or a collection of disjoint chains. In both cases we give necessary and sufficient conditions for a solution to exist. We also consider the case where we have a complete graph instead of a complete bipartite graph. We show that the edge partitioning problem in a complete graph is at least as difficult as in a complete bipartite graph. We also give necessary and sufficient conditions for a solution to exist if E1 ∪ E2 form a tree or if they form a Hamiltonian cycle in the case of a complete graph. Finally we examine for both, complete and complete bipartite graphs, the case where each one of the sets E1 and E2 is structured (two disjoint Hamiltonian chains, two edge disjoint cycles) and present necessary and sufficient conditions.

On the Fiedler value of large planar graphs

Temperley–Lieb algebras have been generalized to 𝔰𝔩(3,ℂ) web spaces. Since a cubic bipartite planar graph with suitable directions on edges is a web, the quantum 𝔰𝔩(3) invariants naturally extend to all cubic bipartite planar graphs. First we completely classify them as a connected sum of prime webs. We provide a method to find all prime webs. Using the quantum 𝔰𝔩(3) invariants, we provide a criterion which determines the symmetry of cubic bipartite planar graphs. We also provide an example that our criterions for graphs is different from that of links.

We characterize plane bipartite graphs whose resonance graphs are daisy cubes, and therefore generalize related results on resonance graphs of benzenoid graphs, catacondensed even ring systems, as well as 2-connected outerplane bipartite graphs. Firstly, we prove that if G is a plane elementary bipartite graph other than K 2 , then the resonance graph of G is a daisy cube if and only if the Fries number of G equals the number of finite faces of G . Next, we extend the above characterization from plane elementary bipartite graphs to plane bipartite graphs and show that the resonance graph of a plane bipartite graph G is a daisy cube if and only if G is weakly elementary bipartite such that each of its elementary component G i other than K 2 holds the property that the Fries number of G i equals the number of finite faces of G i . Along the way, we provide a structural characterization for a plane elementary bipartite graph whose resonance graph is a daisy cube, and show that a Cartesian product graph is a daisy cube if and only if all of its nontrivial factors are daisy cubes.

We strengthen a result by Laskar and Lyle (Discrete Appl. Math. (2009), 330-338) by proving that it is NP-complete to decide whether a bipartite planar graph can be partitioned into three independent dominating sets. In contrast, we show that this is always possible for every maximal outerplanar graph with at least three vertices. Moreover, we extend their previous result by proving that deciding whether a bipartite graph can be partitioned into $k$ independent dominating sets is NP-complete for every $k \geq 3$. We also strengthen a result by Henning et al. (Discrete Math. (2009), 6451-6458) by showing that it is NP-complete to determine if a graph has two disjoint independent dominating sets, even when the problem is restricted to triangle-free planar graphs. Finally, for every $k \geq 3$, we show that there is some constant $t$ depending only on $k$ such that deciding whether a $k$-regular graph can be partitioned into $t$ independent dominating sets is NP-complete. We conclude by deriving moderately exponential-time algorithms for the problem.

We strengthen a result by Laskar and Lyle (Discrete Appl. Math. (2009), 330–338) by proving that it is \(\textsc {NP}\)-complete to decide whether a bipartite planar graph can be partitioned into three independent dominating sets. In contrast, we show that this is always possible for every maximal outerplanar graph with at least three vertices. Moreover, we extend their previous result by proving that deciding whether a bipartite graph can be partitioned into k independent dominating sets is \(\textsc {NP}\)-complete for every \(k \ge 3\). We also strengthen a result by Henning et al. (Discrete Math. (2009), 6451–6458) by showing that it is \(\textsc {NP}\)-complete to determine if a graph has two disjoint independent dominating sets, even when the problem is restricted to triangle-free planar graphs. Finally, for every \(k \ge 3\), we show that there is some constant t depending only on k such that deciding whether a k-regular graph can be partitioned into t independent dominating sets is \(\textsc {NP}\)-complete. We conclude by deriving moderately exponential-time algorithms for the problem.

We present a deterministic Logspace procedure, which, given a bipartite planar graph on n vertices, assigns O(log n) bits long weights to its edges so that the minimum weight perfect matching in the graph becomes unique. The Isolation Lemma as described in Mulmuley et al. (Combinatorica 7(1):105–131, 1987) achieves the same for general graphs using randomness, whereas we can do it deterministically when restricted to bipartite planar graphs. As a consequence, we reduce both decision and construction versions of the perfect matching problem in bipartite planar graphs to testing whether a matrix is singular, under the promise that its determinant is 0 or 1, thus obtaining a highly parallel $\mathsf{SPL}$algorithm for both decision and construction versions of the bipartite perfect matching problem. This improves the earlier known bounds of non-uniform $\mathsf{SPL}$by Allender et al. (J. Comput. Syst. Sci. 59(2):164–181, 1999) and $\mathsf{NC}$2 by Miller and Naor (SIAM J. Comput. 24:1002–1017, 1995), and by Mahajan and Varadarajan (Proceedings of the Thirty-Second Annual ACM Symposium on Theory of Computing (STOC), pp. 351–357, 2000). It also rekindles the hope of obtaining a deterministic parallel algorithm for constructing a perfect matching in non-bipartite planar graphs, which has been open for a long time. Further we try to find the lower bound on the number of bits needed for deterministically isolating a perfect matching. We show that our particular method for isolation will require Ω(log n) bits. Our techniques are elementary.

A proper coloring of a graph is an assignment of colors to the vertices so that no two adjacent vertices have the same color. A list coloring is a generalization of this concept, where each vertex is assigned a list of colors and a proper coloring is found so that the color assigned to each vertex must come from its list. The choice number, ch(G) of a graph G is then the smallest integer k such that G can be list colored given any lists of size k assigned to the vertices. We focus our research on a number of interesting questions about the choice number of bipartite graphs. First, what happens to the choice number of the complete bipartite graph $K\\sb{m,n}$ when edges are removed? We prove that if $n=m\\sp{m}$ then removing an edge lowers the choice number of $K\\sb{m,n}.$ When $n\\not= m\\sp{m}$ we prove that in certain cases we can remove several edges of $K\\sb{m,n}$ without lowering the choice number. We also investigate several examples to illustrate that these certain cases seem to be in the majority. Second, we introduce the concept of defective list colorings. That is, in a proper coloring the graph induced by each color class (say, all the red vertices) is an independent set. What if we relax this condition and allow each color class to induce a graph of maximum degree d? This is called a defective list coloring with defect d. Our main question then is: does the defective choice number, $ch\\sb{d}(K\\sb{m,n})$ of complete bipartite graphs behave like the choice number? We prove that $ch\\sb{d}(K\\sb{m,n})$ behaves asymptotically like $ch(K\\sb{m,n}).$ Also, as is the case for $ch(K\\sb{m,n}),$ we find formulas for $ch\\sb{d}(K\\sb{m,n})$ when n is much larger than m. Third, we investigate the defective choice number of various classes of planar graphs. We say a graph G is (k, d)-choosable if $ch\\sb{d}(G)\\le k.$ We prove that all planar graphs are (4,2)-choosable, all outerplanar graphs are (2,2)-choosable, all triangle-free outerplanar graphs are (2,1)-choosable and there is no d such that all planar bipartite graphs are (2,d)-choosable.

Raspaud and Sopena showed that the oriented chromatic number of a graph with acyclic chromatic number $k$ is at most $k2^{k-1}$. We prove that this bound is tight for $k \geq 3$. We also show that some improper and/or acyclic colorings are $\mathrm{NP}$-complete on a class $\mathcal{C}$ of planar graphs. We try to get the most restrictive conditions on the class $\mathcal{C}$, such as having large girth and small maximum degree. In particular, we obtain the $\mathrm{NP}$-completeness of $3$-$\mathrm{ACYCLIC \space COLORABILITY}$ on bipartite planar graphs with maximum degree $4$, and of $4$-$\mathrm{ACYCLIC \space COLORABILITY}$ on bipartite planar graphs with maximum degree $8$.

Forcing faces in plane bipartite graphs (II)

Motivated by the concept of reverse signed domination, we introduce the reverse minus domination problem on graphs, and study the reverse minus and signed domination problems from the algorithmic point of view. In this paper, we show that both the reverse minus and signed domination problems are polynomial-time solvable for strongly chordal graphs and distance-hereditary graphs, and are linear-time solvable for trees. For chordal graphs and bipartite planar graphs, however, we show that the decision problem corresponding to the reverse minus domination problem is NP-complete. For doubly chordal graphs and bipartite planar graphs, we show that the decision problem corresponding to the reverse signed domination problem is NP-complete. Furthermore, we show that even when restricted to bipartite planar graphs or doubly chordal graphs, the reverse signed domination problem is not fixed parameter tractable.