  We propose an extension of Hybrid I/O Automata (HIOAs) to model agent systems
and their implicit communication through perturbation of the environment, like
localization of objects or radio signals diffusion and detection. To this end
we decided to specialize some variables of the HIOAs whose values are functions
both of time and space. We call them world variables. Basically they are
treated similarly to the other variables of HIOAs, but they have the function
of representing the interaction of each automaton with the surrounding
environment, hence they can be output, input or internal variables. Since these
special variables have the role of simulating implicit communication, their
dynamics are specified both in time and space, because they model the
perturbations induced by the agent to the environment, and the perturbations of
the environment as perceived by the agent. Parallel composition of world
variables is slightly different from parallel composition of the other
variables, since their signals are summed. The theory is illustrated through a
simple example of agents systems.
