abstract: The primary focus of this dissertation lies in extremal combinatorics, in particular intersection theorems in finite set theory. A seminal result in the area is the theorem of Erdos, Ko and Rado which finds the upper bound on the size of an intersecting family of subsets of an n-element set and characterizes the structure of families which attain this upper bound. A major portion of this dissertation focuses on a recent generalization of the Erdos--Ko--Rado theorem which considers intersecting families of independent sets in graphs. An intersection theorem is proved for a large class of graphs, namely chordal graphs which satisfy an additional condition and similar problems are considered for trees, bipartite graphs and other special classes. A similar extension is also formulated for cross-intersecting families and results are proved for chordal graphs and cycles. A well-known generalization of the EKR theorem for k-wise intersecting families due to Frankl is also considered. A stability version of Frankl's theorem is proved, which provides additional structural information about k-wise intersecting families which have size close to the maximum upper bound. A graph-theoretic generalization of Frankl's theorem is also formulated and proved for perfect matching graphs. Finally, a long-standing conjecture of Chvatal regarding structure of maximum intersecting families in hereditary systems is considered. An intersection theorem is proved for hereditary families which have rank 3 using a powerful tool of Erdos and Rado which is called the Sunflower Lemma. / Dissertation/Thesis / Ph.D. Mathematics 2011

With the publication of the famous Erdős-Ko-Rado Theorem in 1961, intersection problems became a popular area of combinatorics. A family of combinatorial objects is t-intersecting if any two of its elements mutually t-intersect, where the latter concept needs to be specified separately in each instance. This thesis is split into two parts; the first is concerned with intersecting injections while the second investigates intersecting posets. We classify maximum 1-intersecting families of injections from {1, ..., k} to {1, ..., n}, a generalisation of the corresponding result on permutations from the early 2000s. Moreover, we obtain classifications in the general t>1 case for different parameter limits: if n is large in terms of k and t, then the so-called fix-families, consisting of all injections which map some fixed set of t points to the same image points, are the only t-intersecting injection families of maximal size. By way of contrast, fixing the differences k-t and n-k while increasing k leads to optimal families which are equivalent to one of the so-called saturation families, consisting of all injections fixing at least r+t of the first 2r+t points, where r=|_ (k-t)/2 _|. Furthermore we demonstrate that, among injection families with t-intersecting and left-compressed fixed point sets, for some value of r the saturation family has maximal size . The concept that two posets intersect if they share a comparison is new. We begin by classifying maximum intersecting families in several isomorphism classes of posets which are linear, or almost linear. Then we study the union of the almost linear classes, and derive a bound for an intersecting family by adapting Katona's elegant cycle method to posets. The thesis ends with an investigation of the intersection structure of poset classes whose elements are close to the antichain. The overarching theme of this thesis is fixing versus saturation: we compare the sizes and structures of intersecting families obtained from these two distinct principles in the context of various classes of combinatorial objects.

Estudamos o problema de famílias intersectantes extremais em um subconjunto aleatório da família dos subconjuntos com exatamente k elementos de um conjunto dado. Obtivemos uma descrição quase completa da evolução do tamanho de tais famílias. Versões semelhantes do problema foram estudadas por Balogh, Bohman e Mubayi em 2009, e por Hamm e Kahn, e Balogh, Das, Delcourt, Liu e Sharifzadeh de maneira concorrente a este trabalho. / We studied the problem of maximal intersecting families in a random subset of the family of subsets with exactly k elements from a given set. We obtained a nearly complete description of the evolution of the size of such families. Similar versions of this problem have been studied by Balogh, Bohman and Mubayi in 2009, and by Hamm and Kahn, and Balogh, Das, Delcourt, Liu and Sharifzadeh concurrently with this work.

Estudamos o problema de famílias intersectantes extremais em um subconjunto aleatório da família dos subconjuntos com exatamente k elementos de um conjunto dado. Obtivemos uma descrição quase completa da evolução do tamanho de tais famílias. Versões semelhantes do problema foram estudadas por Balogh, Bohman e Mubayi em 2009, e por Hamm e Kahn, e Balogh, Das, Delcourt, Liu e Sharifzadeh de maneira concorrente a este trabalho. / We studied the problem of maximal intersecting families in a random subset of the family of subsets with exactly k elements from a given set. We obtained a nearly complete description of the evolution of the size of such families. Similar versions of this problem have been studied by Balogh, Bohman and Mubayi in 2009, and by Hamm and Kahn, and Balogh, Das, Delcourt, Liu and Sharifzadeh concurrently with this work.