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Intersection representations of graphs
Töpfer, Martin ; Jelínek, Vít (advisor) ; Pangrác, Ondřej (referee)
This thesis is devoted to the outer and grounded string representations of graphs and their subclasses. A string representation of a graph is a set of strings (bounded continuous curves in a plane), where each string corresponds to one vertex of the graph. Two strings intersect each other if and only if the two corresponding vertices are adjacent in the original graph. An outer string graph is a graph with a string representation where strings are realized inside a disk and one endpoint of each string lies on the boundary of the disk. Similarly, in case of grounded string graphs the strings lie in a common half- plane with one endpoint of each string on the boundary of the half-plane. We give a summary of subclasses of grounded string graphs and proves several results about their mutual inclusions and separations. To prove those, we use an order-forcing lemma which can be used to force a particular order of the endpoints of the string on the boundary circle or boundary line. The second part of the thesis contains proof that recognition of outer string graphs is NP-hard. 1
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Partial representation extension for subclasses of interval graphs
Onduš, Daniel ; Kratochvíl, Jan (advisor) ; Jelínek, Vít (referee)
The problem of extending partial representations for an interval graph asks, whether it is possible to extend a given representation of some vertices to a valid representation of the entire graph. In this thesis we extend the recent result of Klavík et al. who proved REPEXT can be decided for proper and unit interval graphs in polynomial time. We describe properties of PI± and U± graphs and their representations and present algorithms deciding REPEXT for these classes in polynomial time. In the process, we characterize relations between the K1,3's in a graph and show that we can decide the open vertex of every K1,3. We also define notions of representation of the same order type and locally similar representations as well as intervals forced and locally forced to be closed (open) that are essential for extending partial representations when multiple types of intervals can occur in the same representation. We characterize intervals forced and locally forced to be closed (open) in a U± graph using integer gaps in the pre-representation and we construct lower bounds for the rightmost endpoint of a component in polynomial time.
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Matrices without forbidden interval minors
Surma, David ; Jelínek, Vít (advisor) ; Klazar, Martin (referee)
In the thesis, we study the structure of binary matrices which do not contain a pat- tern P as an interval minor. We also deal with matrices that are critical for P, i.e., matrices without P which after changing any 0-entry to 1-entry contain the forbidden pattern P. First, we describe matrices critical for any one-line pattern. Then we deal with all patterns with two rows and three columns which contain at most four 1-entries. Finally, we characterize the matrices critical for the alternating pattern of size 2 × 4. 1
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Algorithmic aspects of intersection representations
Chmel, Petr ; Jelínek, Vít (advisor) ; Kratochvíl, Jan (referee)
As some problems are (NP-)hard to solve in the general case, a possible approach is to try to solve the problem on a restricted class of graphs. In the thesis, we focus on graphs induced by axis-aligned L-shapes, so-called L-graphs, and a similar class of axis- aligned L-shapes and L-shapes, referred to as {L, L}-graphs, with two vertices sharing an edge if and only if their respective curves intersect. We show that recognizing both L- graphs and {L, L}-graphs is NP-complete. The second part of the thesis focuses on other typical decision problems on L-graphs and their relatives: finding the clique number, the independence number or a 3-coloring.
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Computational and structural apects of interval graphs and their variants
Novotná, Jana ; Kratochvíl, Jan (advisor) ; Jelínek, Vít (referee)
Interval graphs, intersection graphs of segments on a real line (intervals), play a key role in the study of algorithms and special structural properties. Unit interval graphs, their proper subclass, where each interval has a unit length, has also been extensively studied. We study mixed unit interval graphs-a generalization of unit interval graphs where each interval has still a unit length, but intervals of more than one type (open, closed, semi-closed) are allowed. This small modification captures a much richer class of graphs. In particular, mixed unit interval graphs are not claw-free, compared to unit interval graphs. Heggernes, Meister, and Papadopoulos defined a representation of unit interval graphs called the bubble model which turned out to be useful in algorithm design. We extend this model to the class of mixed unit interval graphs. The original bubble model was used by Boyaci, Ekim, and Shalom for proving the polynomiality of the MaxCut problem on unit interval graphs. However, we found a significant mistake in the proof which seems to be hardly repairable. Moreover, we demonstrate advantages of such model by providing a subexponential-time algorithm solving the MaxCut problem on mixed unit interval graphs using our extended version of the bubble model. In addition, it gives us a polynomial-time...
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Intersection representations of graphs
Töpfer, Martin ; Jelínek, Vít (advisor) ; Pangrác, Ondřej (referee)
This thesis is devoted to the outer and grounded string representations of graphs and their subclasses. A string representation of a graph is a set of strings (bounded continuous curves in a plane), where each string corresponds to one vertex of the graph. Two strings intersect each other if and only if the two corresponding vertices are adjacent in the original graph. An outer string graph is a graph with a string representation where strings are realized inside a disk and one endpoint of each string lies on the boundary of the disk. Similarly, in case of grounded string graphs the strings lie in a common half- plane with one endpoint of each string on the boundary of the half-plane. We give a summary of subclasses of grounded string graphs and proves several results about their mutual inclusions and separations. To prove those, we use an order-forcing lemma which can be used to force a particular order of the endpoints of the string on the boundary circle or boundary line. The second part of the thesis contains proof that recognition of outer string graphs is NP-hard. 1
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The complexity of constrained graph drawing
Hora, Martin ; Jelínek, Vít (advisor) ; Fink, Jiří (referee)
A labeled embedding of a planar graph G is a pair (G, g) consisting of a planar drawing G of G and a function g assigning labels (colors) to the faces of G. We study the problem of Embedding Restriction Satisfiability (ERS) that investi- gates whether a given graph has a labeled embedding satisfying a provided set of conditions. ERS is a relatively new problem, so not much is known about it. Nevertheless, it has great potential. It generalizes several problems looking for a particular drawing of a planar graph, such as the problem of Partially Embedded Planarity. Therefore, ERS may become a focal point in the area of graph draw- ing. In this thesis, we examine the computational complexity of ERS. We show that ERS is NP-complete. After that, we look at the complexity of some specific classes of its instances. We try to locate the boundary between the NP-complete and the polynomial variants of the problem. 1
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Enumeration of polyomino fillings
Karpilovskij, Mark ; Jelínek, Vít (advisor) ; Klazar, Martin (referee)
We prove two new results about 0-1-fillings of skew diagrams avoiding long increasing and decreasing chains. In the first half of the thesis, we show that for a large class of skew diagrams, there is a bijection between sparse fillings avoiding an increasing chain of fixed length and sparse fillings avoiding a decreas- ing chain of the same length. In the second half, we extend a known inequality between the number of sparse 0-1-fillings of skew diagrams avoiding an increasing chain of length 2 and a decreasing chain of length 2 to all 0-1-fillings. 1
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