  Emergent behavior is a key feature defining a system under study as a complex
system. Simulation has been recognized as the only way to deal with the study
of the emergency of properties (at a macroscopic level) among groups of system
components (at a microscopic level), for the manifestations of emergent
structures cannot be deduced from analysing components in isolation. A
systems-oriented generalisation must consider the presence of feedback loops
(micro components react to macro properties), interaction among components of
different classes (modular composition) and layered interaction of subsystems
operating at different spatio-temporal scales (hierarchical organisation). In
this work we introduce Emergent Behavior-DEVS (EB-DEVS) a Modeling and
Simulation (M&S) formalism that permits reasoning about complex systems where
emergent behavior is placed at the forefront of the analysis activity. EB-DEVS
builds on the DEVS formalism, adding upward/downward communication channels to
well-established capabilities for modular and hierarchical M&S of heterogeneous
multi-formalism systems. EB-DEVS takes a minimalist stance on expressiveness,
introducing a small set of extensions on Classic DEVS that can cope with
emergent behavior, and making both formalisms interoperable (the modeler
decides which subsystems deserve to be expressed via micro-macro dynamics). We
present three case studies: flocks of birds with learning, population epidemics
with vaccination and sub-cellular dynamics with homeostasis, through which we
showcase how EB-DEVS performs by placing emergent properties at the center of
the M&S process.
