Semigroup approach to diffusion and transport problems on networks

Models describing transport and diffusion processes occurring along the edges of a graph and interlinked by its vertices have been recently receiving a considerable attention. In this paper we generalize such models and consider a network of transport or diffusion operators defined on one dimensional domains and connected through boundary conditions linking the end-points of these domains in an arbitrary way (not necessarily as the edges of a graph are connected). We prove the existence of $C_0$-semigroups solving such problems and provide conditions fully characterizing when they are positive

Long time behaviour of population models.

Thesis (M.Sc.)-University of KwaZulu-Natal, Westville, 2010.Non-negative matrices arise naturally in population models. In this thesis, we look at the theory of such matrices and we study the Perron-Frobenius type theorems regarding their spectral properties. We use these theorems to investigate the asymptotic behaviour of solutions to continuous time problems arising in population biology. In particular, we provide a description of long-time behaviour of populations depending on the nature of the associated matrix. Finally, we describe a few applications to population biology

The Eigen-chromatic Ratio of Classes of Graphs: Asymptotes, Areas and Molecular Stability

In this paper, we present a new ratio associated with classes of graphs, called the eigen-chromatic ratio, by combining the two graph theoretical concepts of energy and chromatic number. The energy of a graph, the sum of the absolute values of the eigenvalues of the adjacency matrix of a graph, arose historically as a result of the energy of the benzene ring being identical to that of the sum of the absolute values of the eigenvalues of the adjacency matrix of the cycle graph on n vertices (see [18]). The chromatic number of a graph is the smallest number of colour classes that we can partition the vertices of a graph such that each edge of the graph has ends that do not belong to the same colour class, and applications to the real world abound (see [30]). Applying this idea to molecular graph theory, for example, the water molecule would have its two hydrogen atoms coloured with the same colour different to that of the oxygen molecule. Ratios involving graph theoretical concepts form a large subset of graph theoretical research (see [3], [16], [48]). The eigen-chromatic ratio of a class of graph provides a form of energy distribution among the colour classes determined by the chromatic number of such a class of graphs. The asymptote associated with this eigen-chromatic ratio allows for the behavioural analysis in terms of stability of molecules in molecular graph theory where a large number of atoms are involved. This asymptote can be associated with the concept of graphs being hyper- or hypo- energetic (see [48])