Rotulações graciosas e rotulações semifortes em grafos

Author(s)

Atílio Gomes Luiz

• Document type: Doctoral Thesis
• Press: Campinas, SP.
• Institution: Universidade Estadual de Campinas (UNICAMP). Instituto de Computação
• Defense date: 2018-05-21

Examining board members

Christiane Neme Campos; Cláudia Linhares Sales; Daniel Morgato Martin; Célia Picinin de Mello; João Meidanis

• Advisor: Christiane Neme Campos

Abstract

This thesis addresses three labelling problems on graphs: the Graceful Tree Conjecture, the 1,2,3-Conjecture, and the 1,2-Conjecture. A graceful labelling of a simple graph G=(V(G),E(G)) is an injective function f from V(G) to {0,...,|E(G)|} such that {|f(u)-f(v)| : uv in E(G)} = {1,...,|E(G)|}. The Graceful Tree Conjecture, posed by Rosa and Kotzig in 1967, states that every tree has a graceful labelling. A problem connected with the Graceful Tree Conjecture consists of determining whether, for every vertex v of a tree T, there exists a graceful labelling of T that assigns label 0 to v. Trees with such a property are called 0-rotatable. In this thesis, we present infinite families of 0-rotatable caterpillars. Our results reinforce a conjecture that states that every caterpillar with diameter at least five is 0-rotatable. We also investigate a stronger type of graceful labelling, called alpha-labelling. A graceful labelling f of G is an alpha-labelling if there exists an integer k with 0<= k <= |E(G)| such that, for each edge uv in E(G), either f(u) <= k < f(v) or f(v) <= k < f(u). In this thesis, we prove that the following families of lobsters have alpha-labellings: lobsters with maximum degree three, without Y-legs and with at most one forbidden ending; and lobsters T with a perfect matching M such that the contracted tree T/M has a balanced bipartition. These results point towards a characterization of all lobsters with maximum degree three that have alpha-labellings. In the second part of the thesis, we focus on generalizations of the 1,2,3-Conjecture and the 1,2-Conjecture. Given a simple graph G=(V(G),E(G)) and a subset L of real numbers, we call a function f from E(G) to L an L-edge-labelling of G, and we call a function f from V(G) union E(G) to L an L-total-labelling of G. For each vertex v of G, the colour of v, C(v), is defined as the sum of the labels of its incident edges, if f is an L-edge-labelling. If f is an L-total-labelling, C(v) is the sum of the labels of the edges incident with vertex v plus the label f(v). The pair (f,C) is a neighbour-distinguishing L-edge-labelling (neighbour-distinguishing L-total-labelling) if f is an edge-labelling (total-labelling) and C(u) is different from C(v), for every edge uv in E(G). The 1,2,3-Conjecture, posed by Kar\'onski et al. in 2004, states that every connected simple graph with at least three vertices has a neighbour-distinguishing {1,2,3}-edge-labelling. The 1,2-Conjecture, posed by Przybylo and Wozniak in 2010, states that every simple graph has a neighbour-distinguishing {1,2}-total-labelling. Let a,b,c be distinct real numbers. In this thesis, we investigate neighbour-distinguishing {a,b,c}-edge-labellings and neighbour-distinguishing {a,b}-total labellings for five families of graphs: powers of paths, powers of cycles, split graphs, regular cobipartite graphs and complete multipartite graphs. We prove that these families have such labellings for some real values a, b, and c. As a corollary of our results, we obtain that the 1,2,3-Conjecture and the 1,2-Conjecture are true for these families. Furthermore, we also show that our results on neighbour-distinguishing edge-labellings imply similar results on a closely related problem called detectable edge-labelling of graphs (AU)

• FAPESP's process: 14/16861-8 - Problems on graph labelling
• Grantee: Atilio Gomes Luiz
• Support Opportunities: Scholarships in Brazil - Doctorate