Christopher Duffy

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The Critical Zone (CZ) incorporates all aspects of the earth's environment from the vegetation canopy to the bottom of groundwater. CZ researchers target processes that cross timescales from that of water fluxes (milliseconds to decades) to that of the evolution of landforms (thousands to tens of millions of years). Conceptual and numerical models are used(More)
We examine t-colourings of oriented graphs in which, for a fixed integer k ≥ 1, vertices joined by a directed path of length at most k must be assigned different colours. A homomorphism model that extends the ideas of Sherk for the case k = 2 is described. Dichotomy theorems for the complexity of the problem of deciding, for fixed k and t, whether there(More)
The Prisoner's Dilemma Process on a graph G is an iterative process where each vertex, with a fixed strategy (cooperate or defect), plays the game with each of its neighbours. At the end of a round each vertex may change its strategy to that of its neighbour with the highest pay-off. Here we study the spread of cooperative and selfish behaviours on a(More)
A (m, n)-colored mixed graph is a graph having arcs of m different colors and edges of n different colors. A graph homomorphism of a (m, n)-colored mixed graph G to a (m, n)-colored mixed graph H is a vertex mapping such that if uv is an arc (edge) of color c in G, then f (u)f (v) is also an arc (edge) of color c. The (m, n)-colored mixed chromatic number χ(More)
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