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Abstract. Agent-based software engineering has been proposed in addition to object-oriented software engineering as a means of mastering the complexity associated with the development of large-scale distributed systems. However, there is still a poor understanding of the interplay between the notions of agents and objects from a software engineering perspective. Moreover, the many facets of agent-based software engineering are rarely used in the various phases of the software development lifecycle because of the lack of a comprehensive framework to provide the software designers with a clear understanding of the use of these two key abstractions. In this context, this paper presents TAO, an evolving innovative conceptual framework based on agent and object abstractions, which are the foundations for modeling large-scale software systems. The conceptual framework allows for the characterization of largescale software systems as organizations of passive components, the objects, and autonomous components, the agents, with each of these elements playing roles to interact with each other and to coordinate their actions in order to fulfill system goals.

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The anisotropic-fluid interpretation of a stress-energy tensor formed from the sum of three tensors, each of which is the energy-momentum tensor of a perfect fluid or a null fluid in the special case that the fluids four-velocities are linearly dependent, is studied. The anisotropic-fluid model formed by an arbitrary number of perfect fluids and null fluids is also studied in the particular case that all the fluids' four-velocities lay on a timelike two-plane. The anisotropic-fluid interpretation of the Bondi model of self-gravitating spheres is presented. The particular case of an anisotropic-fluid model formed with three perfect fluids with a stiff equation of state, and the particular case of two null and one perfect fluid with a p = p equation of state, are used as sources of the Einstein equations for a cylindrically symmetric spacetime, and these last equations are solved. Also, for these particular cases the generalization for an arbitrary number of fluid components is indicated.

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