Letter from the guest editors

Agent organizations are an emergent area of multi-agent systems (MAS) that relies on the notion of openness and heterogeneity of MAS and poses new demands on traditional MAS models. These demands include the integration of organizational and individual perspectives and the dynamic adaptation of models to organizational and environmental changes.


INTRODUCTION
Agent organizations are an emergent area of multiagent systems (MAS) that relies on the notion of openness and heterogeneity of MAS and poses new demands on traditional MAS models.These demands include the integration of organizational and individual perspectives and the dynamic adaptation of models to organizational and environmental changes [6].Organizational self-design will play a critical role in the development of larger and more complex MAS.As systems grow to include hundreds or thousands of agents, we must move from an agent-centric view of coordination and control to an organization-centric one.However, in order to be able to adapt and evolve, this latter will need to coexist with a dynamic and (partially) emergent organization, based on the former.Practical appli- cations of agents to organizational modeling are being widely developed but formal theories are needed to describe interaction and organizational structure.Furthermore, it is necessary to get a closer look at the relation between organiza- tional roles and the agents that fulfill them.
The overall problem of analyzing the social, eco- nomic and technological dimensions of agent organizations, and the co-evolution of agent and human social and personal structures in the organization, provide theoretically demanding, interdisciplinary research questions at different levels of abstraction.Organizational research is increas-In closed domains, the design of MAS can suffice with the idea that agents are mere performers of organiza- tional roles or functions, interacting according to fixed protocols and unable to deviate from expected behavior [25].As such, agent autonomy is rather limited.In open do- mains, agents are self-governed autonomous entities that pursue their own individual goals based only on their own beliefs and capabilities.
Comprehensive models for MAS must, on the one hand, be able to specify global goals and requirements of organizations but, on the other hand, cannot assume that participating agents will act according to the needs and expectations of the system design.Concepts as organiza- tional rules [24], norms and institutions [5,7,8], and social structures [15] arise from the idea that the effective engi- neering of MAS needs high-level, agent-independent con- cepts and abstractions that explicitly define the organiza- tion in which agents live [25].These are the rules and global objectives that govern the activity of an enterprise, group, organization or nation.Given that agents might deviate from expected behavior, open societies need mechanisms to sys- tematize, defend and recommend right and wrong behavior, for instance by proposing a reputation mechanism which can inspire trust into the agents that will join them [4,17].Norms are commonly used means to describe such expected behavior.Finally, organizational models must provide means to represent concepts and relationships in the domain that are rich enough to cover the necessary contexts of agent interaction while keeping in mind the relevance of those concepts for the global aims of the system.
In the sequence, we will briefly describe some core notions of the field.First, we present a definition for the term organization.In [18], a possible taxonomy is proposed to better characterize the several dimensions associated with MAS organizations.These dimensions, formation, focus, description level, representation and adaptation d , are briefly described in the sequence.After defining these con- cepts, a relation between agents´ interactions and organi- zations is presented.

DEFINITION
As it occurs with other basic concepts in the MAS field, there is not a single, universally adopted semantics for the term organization.One of the most generic ones was proposed in [2]: " A MAS organization may be seen in a sim- plified manner as a set of constraints adopted by a group of agents in order to facilitate their goals achievements".
In other words, the very fact of belonging to an or- ganization limits the agent autonomy, since he has to cope with the organizational constraints if his behavior is organi- zation compliant.

ORGANIZATION FORMATION
The models used to describe or project an organiza- tion are classically divided in two points of view [14]: (i) agent centered, where the organization is functional and instrumental to the agents´ goals and (ii) organization cen- tered, where the agents are fungible and functionally sub- ordinated to the organization.While the former takes the agents as the engine for the organization formation, the latter sees the opposite direction: the organization exists apriori (defined by the designer or by the agents themselves) and the agents ought to follow it.A MAS organization maybe hence formed either by an emergent or by a predefined way.
In emergent organizations, the agents of the system do not have necessarily a previous common goal to achieve.Interactions among agents emerge dynamically, as a conse- quence of their behavior aiming to achieve their own goals.There is not a single objective description of an organiza- tion, this latter may be defined as a pattern of distributed mental notions relating each agent with others, like joint intentions or commitments.An example of this type of orga- nization genesis is dependence-based coalition formation [3,19].
In predefined organizations, agents have a common predefined goal, and hence cooperation occurs as a pattern of fixed, top-down interactions, that are sometimes called "orchestrated interaction" [3].These organization models, however, may be focused on different aspects, as described next.

ORGANIZATION FOCUS
Some models of MAS organizations stress the im- portance of its structural aspects.These aspects concern the definition of a set of prototypical functions, called roles, to be executed by agents in the organization.This dimension also defines the types of links among these roles (authority, communication, etc.), as well as some rules to form collective entities composed by a set of roles, usually named groups or divisions within the main organization.Examples of these models may be found in [9,10,11].
Another class of models is focused on organiza- tional functional aspects.These aspects main concern is the functioning of the organization, for instance, the speci- fication of global plans, policies to allocate tasks to agents, the coordination to execute a plan, and the quality (time consumption, resources usage, etc.) of a plan.In this group, the global purposes are better achieved because the MAS has a kind of organizational memory where the best plans to achieve a global goal are stored.Examples of these models are [16,20,22].
A third class of models concerns deontic aspects.The idea is that the global purpose of the organization is accomplished while the agents execute their plans/tasks, following the obligations and permissions entitled by the roles they are playing.These models define obligations and permissions that agents that play certain roles in an organization may have, regarding their knowledge (what do they have right to know?), their actions (what actions are they obliged to execute?) and resources (which re- sources may be used by them when trying to achieve their goals?).In some models, these aspects are not repre- sented separately [7], while in others these aspects are represented apart from the structural and functional di- mensions [12,13].
One should notice that these classes are not neces- sarily exclusive within an organizational model.There are some more comprehensive models where several of these focus are dealt with, as in [13,21].

ORGANIZATION DESCRIPTION LEVEL
In every organizational level that uses the role no- tion, there exist at least 2 different description levels.
The first one, that may be called abstract d organization, does not contain any reference to the real agents, i.e., it consists only of the organization roles, their links and groups, global plans and permissions/obligations.It may be seen as a kind of recipe of how should collective activity occur.On the other hand, when real agents start to play these organizational roles, a concrete organization is in- stantiated.This latter is effectively the one that is supposed to achieve the organizational top-level goals.

REPRESENTATION OF THE ORGANIZATION
As presented in section 2.2, MAS organizations may be divided in two points of view: agent centered and organi- zation centered.An important aspect of these points of view is the nature of the organization representation.While in an agent centered approach agents have subjective representations (within their minds), in an organization centered ap- proach there is quite always a single, objective description of the organization, which is independent of the subjective rep- resentations that agents may have of this organization.
In [12,13,18], it is presented an extension to this classification, by the introduction of a MAS observer.This extension leads to a classification of 4 different situations, depending whether the agents themselves observe the organization: -Agent centered, not observed by the agents: the organization formation is emergent, but the agents can not represent it, its detection is possible ex- clusively by an external observer of the MAS, like in cooperative behavior of ant-like agents; -Agent centered, observed by the agents: the organization formation is emergent, and the agents can represent it, for instance by joint intentions and commitments in their mental states, like in dependence-based coalition formation [3,19]; -Organization centered, not observed by the agents: the organization formation is predefined, but the agents can not represent it, the organization con- cepts are used in design time, like in [23,25]; -Organization centered, observed by the agents: the organization formation is predefined, and the agents can represent it, as the models presented in [6,11,13].

ORGANIZATION ADAPTATION
When considering predefined organizations, their temporal behavior may be static or dynamic.While the first ones maintain their structure and functionality over time (or these are changed off-line by a system designer), in the second group agents themselves have a meta-level reason- ing mechanism about their organization, which enables them to change either their structure or their functioning.A more comprehensive discussion of the several aspects of reor- ganization in MAS may be found in [12].

INTERACTIONS AND ORGANIZATIONS
As described in [18], agent's interactions and orga- nizations are not independent aspects of a MAS.If we con- sider emergent organizations, as described in section 2.2, a first result may be stated as:

"Agents interactions may eventually create dynamic organizations"
Whenever the same interaction patterns are repeated several times, involving the same agents, these interactions may be captured by pre-established structures, thus avoid- ing the inherent complexity of bottom-up emergent organization formation, like the model proposed in [19].As a con- sequence, collective behavior will be more efficient, since the organization formation is carried on a priori.
Hence, if one considers pre-defined organizations, as described in section 2.2, a second result may be stated as: "Agents organizations limits agents inter-" actions, aiming to optimize the achievement of global goals" Consequently, these dimensions of MAS make a virtuous circle: interactions build dynamic organizations, and pre-defined static organizations limit agents´ interactions in order to achieve more efficiently the MAS global goals.

OVERVIEW OF THE SPECIAL ISSUE
We have received 19 submissions for this special issue, from researchers currently working in many different countries such as Brazil, France, The Netherlands, Italy, Portugal, UK and USA.Each of these submissions was carefully revised by at least 3 different reviewers that were selected for their expertise and current work on multi-agent organizations.The papers appearing in this issue are there- fore the result of a very strict selection process.Of the 19 submissions, 5 high quality papers were accepted for pub- lication.After acceptance, the authors of these 5 papers were asked to enhance their final versions, based on the feedback provided by the referees.
These papers offer a broad perspective of the different issues and approaches to the field of Agent Organiza- tions: - At the high-level coordination, the specifi- cation of the organizational issues is crucial.However, in dynamic environments, agents must be able to adapt to the changing organizational goals, available resources, their relationships to another agents, and so on.This problem is a key one in multi-agent systems and relates to models of learn- ing and adaptation, such as those observed among social insects.The paper tackles the process of generating, adapting, and changing multi-agent organization dynamically at system runtime, us- ing a swarm inspired approach; -Automatic Formation of Agent Organizations: The paper "Automatic Formation and Analysis of Multi- Agent Virtual Organization" by Qinhe Zheng and Xiaoqin Zhang describes experience gained by implementing a multi-agent system that simulates an artificial marketplace, for which the authors have derived several decision-making mechanisms in various stages of a virtual organization.A virtual organization (VO) is defined as the temporary team- ing of enterprises.By sharing physical, human and knowledge resources via information technologies, a virtual organization enables member enterprises to share skills, costs, access to one another's mar- kets and, at the same time decrease the risk of investments.In order to realize this new generation of business model, the ability to form and operate virtual enterprises is very important.The paper pre- sents a negotiation protocol and a bid selection algorithm for agents to form a virtual organization.