97 research outputs found
Constrained Consensus
We present distributed algorithms that can be used by multiple agents to
align their estimates with a particular value over a network with time-varying
connectivity. Our framework is general in that this value can represent a
consensus value among multiple agents or an optimal solution of an optimization
problem, where the global objective function is a combination of local agent
objective functions. Our main focus is on constrained problems where the
estimate of each agent is restricted to lie in a different constraint set.
To highlight the effects of constraints, we first consider a constrained
consensus problem and present a distributed ``projected consensus algorithm''
in which agents combine their local averaging operation with projection on
their individual constraint sets. This algorithm can be viewed as a version of
an alternating projection method with weights that are varying over time and
across agents. We establish convergence and convergence rate results for the
projected consensus algorithm. We next study a constrained optimization problem
for optimizing the sum of local objective functions of the agents subject to
the intersection of their local constraint sets. We present a distributed
``projected subgradient algorithm'' which involves each agent performing a
local averaging operation, taking a subgradient step to minimize its own
objective function, and projecting on its constraint set. We show that, with an
appropriately selected stepsize rule, the agent estimates generated by this
algorithm converge to the same optimal solution for the cases when the weights
are constant and equal, and when the weights are time-varying but all agents
have the same constraint set.Comment: 35 pages. Included additional results, removed two subsections, added
references, fixed typo
Towards an Algebra for Cascade Effects
We introduce a new class of (dynamical) systems that inherently capture
cascading effects (viewed as consequential effects) and are naturally amenable
to combinations. We develop an axiomatic general theory around those systems,
and guide the endeavor towards an understanding of cascading failure. The
theory evolves as an interplay of lattices and fixed points, and its results
may be instantiated to commonly studied models of cascade effects.
We characterize the systems through their fixed points, and equip them with
two operators. We uncover properties of the operators, and express global
systems through combinations of local systems. We enhance the theory with a
notion of failure, and understand the class of shocks inducing a system to
failure. We develop a notion of mu-rank to capture the energy of a system, and
understand the minimal amount of effort required to fail a system, termed
resilience. We deduce a dual notion of fragility and show that the combination
of systems sets a limit on the amount of fragility inherited.Comment: 31 page
Dynamics in Near-Potential Games
Except for special classes of games, there is no systematic framework for
analyzing the dynamical properties of multi-agent strategic interactions.
Potential games are one such special but restrictive class of games that allow
for tractable dynamic analysis. Intuitively, games that are "close" to a
potential game should share similar properties. In this paper, we formalize and
develop this idea by quantifying to what extent the dynamic features of
potential games extend to "near-potential" games. We study convergence of three
commonly studied classes of adaptive dynamics: discrete-time better/best
response, logit response, and discrete-time fictitious play dynamics. For
better/best response dynamics, we focus on the evolution of the sequence of
pure strategy profiles and show that this sequence converges to a (pure)
approximate equilibrium set, whose size is a function of the "distance" from a
close potential game. We then study logit response dynamics and provide a
characterization of the stationary distribution of this update rule in terms of
the distance of the game from a close potential game and the corresponding
potential function. We further show that the stochastically stable strategy
profiles are pure approximate equilibria. Finally, we turn attention to
fictitious play, and establish that the sequence of empirical frequencies of
player actions converges to a neighborhood of (mixed) equilibria of the game,
where the size of the neighborhood increases with distance of the game to a
potential game. Thus, our results suggest that games that are close to a
potential game inherit the dynamical properties of potential games. Since a
close potential game to a given game can be found by solving a convex
optimization problem, our approach also provides a systematic framework for
studying convergence behavior of adaptive learning dynamics in arbitrary finite
strategic form games.Comment: 42 pages, 8 figure
On the behavior of threshold models over finite networks
We study a model for cascade effects over finite networks based on a deterministic binary linear threshold model. Our starting point is a networked coordination game where each agent's payoff is the sum of the payoffs coming from pairwise interaction with each of the neighbors. We first establish that the best response dynamics in this networked game is equivalent to the linear threshold dynamics with heterogeneous thresholds over the agents. While the previous literature has studied such linear threshold models under the assumption that each agent may change actions at most once, a study of best response dynamics in such networked games necessitates an analysis that allows for multiple switches in actions. In this paper, we develop such an analysis. We establish that agent behavior cycles among different actions in the limit, we characterize the length of such limit cycles, and reveal bounds on the time steps required to reach them. We finally propose a measure of network resilience that captures the nature of the involved dynamics. We prove bounds and investigate the resilience of different network structures under this measure.Irwin Mark Jacobs and Joan Klein Jacobs Presidential FellowshipSiebel ScholarshipUnited States. Air Force Office of Scientific Research (Grant FA9550-09-1-0420)United States. Army Research Office (Grant W911NF-09-1-0556
The Reliability Value of Storage in a Volatile Environment
Author's final manuscript: September 29, 2011This paper examines the value of storage in securing reliability of a system with uncertain supply and demand, and supply friction. The storage is frictionless as a supply source, but it cannot be filled up instantaneously. The focus is on application to an energy network in which the nominal supply and demand are assumed to match perfectly, while deviations from the nominal values are modeled as random shocks with stochastic arrivals. Due to friction, the random shocks cannot be tracked by the main supply sources. Storage, when available, can be used to compensate, fully or partially, for the surge in demand or sudden drop in supply. The problem of optimal utilization of storage with the objective of maximizing system reliability is formulated as minimization of the expected discounted cost of blackouts over an infinite horizon. It is shown that when the stage cost is linear in the size of the blackout, the optimal policy is myopic in the sense that all shocks will be compensated by storage up to the available level of storage. However, when the stage cost is strictly convex, it may be optimal to curtail some of the demand and allow a small blackout in the interest of maintaining a higher level of reserve, which may help avoid a large blackout in the future. The value of storage capacity in improving reliability, as well as the effects of the associated optimal policies under different stage costs on the probability distribution of blackouts are examined.National Science Foundation (U.S.)Siemens-MIT Allianc
Near-Optimal Power Control in Wireless Networks: A Potential Game Approach
We study power control in a multi-cell CDMA wireless system whereby self-interested users share a common spectrum and interfere with each other. Our objective is to design a power control scheme that achieves a (near) optimal power allocation with respect to any predetermined network objective (such as the maximization of sum-rate, or some fairness criterion). To obtain this, we introduce the potential-game approach that relies on approximating the underlying noncooperative game with a "close" potential game, for which prices that induce an optimal power allocation can be derived. We use the proximity of the original game with the approximate game to establish through Lyapunov-based analysis that natural user-update schemes (applied to the original game) converge within a neighborhood of the desired operating point, thereby inducing near-optimal performance in a dynamical sense. Additionally, we demonstrate through simulations that the actual performance can in practice be very close to optimal, even when the approximation is inaccurate. As a concrete example, we focus on the sum-rate objective, and evaluate our approach both theoretically and empirically.National Science Foundation (U.S.) (DMI-05459100)National Science Foundation (U.S.) (DMI-0545910)United States. Defense Advanced Research Projects Agency (ITMANET program)7th European Community Framework Programme (Marie Curie International Fellowship
Bayesian Learning in Social Networks
We study the perfect Bayesian equilibrium of a model of learning over a general social network. Each individual receives a signal about the underlying state of the world, observes the past actions of a stochastically-generated neighborhood of individuals, and chooses one of two possible actions. The stochastic process generating the neighborhoods defines the network topology (social network). The special case where each individual observes all past actions has been widely studied in the literature. We characterize pure-strategy equilibria for arbitrary stochastic and deterministic social networks and characterize the conditions under which there will be asymptotic learning -- that is, the conditions under which, as the social network becomes large, individuals converge (in probability) to taking the right action. We show that when private beliefs are unbounded (meaning that the implied likelihood ratios are unbounded), there will be asymptotic learning as long as there is some minimal amount of "expansion in observations". Our main theorem shows that when the probability that each individual observes some other individual from the recent past converges to one as the social network becomes large, unbounded private beliefs are sufficient to ensure asymptotic learning. This theorem therefore establishes that, with unbounded private beliefs, there will be asymptotic learning an almost all reasonable social networks. We also show that for most network topologies, when private beliefs are bounded, there will not be asymptotic learning. In addition, in contrast to the special case where all past actions are observed, asymptotic learning is possible even with bounded beliefs in certain stochastic network topologies.
Separable and Low-Rank Continuous Games
In this paper, we study nonzero-sum separable games, which are continuous
games whose payoffs take a sum-of-products form. Included in this subclass are
all finite games and polynomial games. We investigate the structure of
equilibria in separable games. We show that these games admit finitely
supported Nash equilibria. Motivated by the bounds on the supports of mixed
equilibria in two-player finite games in terms of the ranks of the payoff
matrices, we define the notion of the rank of an n-player continuous game and
use this to provide bounds on the cardinality of the support of equilibrium
strategies. We present a general characterization theorem that states that a
continuous game has finite rank if and only if it is separable. Using our rank
results, we present an efficient algorithm for computing approximate equilibria
of two-player separable games with fixed strategy spaces in time polynomial in
the rank of the game
Flows and Decompositions of Games: Harmonic and Potential Games
In this paper we introduce a novel flow representation for finite games in
strategic form. This representation allows us to develop a canonical direct sum
decomposition of an arbitrary game into three components, which we refer to as
the potential, harmonic and nonstrategic components. We analyze natural classes
of games that are induced by this decomposition, and in particular, focus on
games with no harmonic component and games with no potential component. We show
that the first class corresponds to the well-known potential games. We refer to
the second class of games as harmonic games, and study the structural and
equilibrium properties of this new class of games. Intuitively, the potential
component of a game captures interactions that can equivalently be represented
as a common interest game, while the harmonic part represents the conflicts
between the interests of the players. We make this intuition precise, by
studying the properties of these two classes, and show that indeed they have
quite distinct and remarkable characteristics. For instance, while finite
potential games always have pure Nash equilibria, harmonic games generically
never do. Moreover, we show that the nonstrategic component does not affect the
equilibria of a game, but plays a fundamental role in their efficiency
properties, thus decoupling the location of equilibria and their payoff-related
properties. Exploiting the properties of the decomposition framework, we obtain
explicit expressions for the projections of games onto the subspaces of
potential and harmonic games. This enables an extension of the properties of
potential and harmonic games to "nearby" games. We exemplify this point by
showing that the set of approximate equilibria of an arbitrary game can be
characterized through the equilibria of its projection onto the set of
potential games
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