430 research outputs found
Sensitivity of wardrop equilibria
We study the sensitivity of equilibria in the well-known game theoretic traffic model due to Wardrop. We mostly consider single-commodity networks. Suppose, given a unit demand flow at Wardrop equilibrium, one increases the demand by Δ or removes an edge carrying only an Δ-fraction of flow. We study how the equilibrium responds to such an Δ-change.
Our first surprising finding is that, even for linear latency functions, for every Δ>â0, there are networks in which an Δ-change causes every agent to change its path in order to recover equilibrium. Nevertheless, we can prove that, for general latency functions, the flow increase or decrease on every edge is at most Δ.
Examining the latency at equilibrium, we concentrate on polynomial latency functions of degree at most p with nonnegative coefficients. We show that, even though the relative increase in the latency of an edge due to an Δ-change in the demand can be unbounded, the path latency at equilibrium increases at most by a factor of (1â+âΔ) p . The increase of the price of anarchy is shown to be upper bounded by the same factor. Both bounds are shown to be tight.
Let us remark that all our bounds are tight. For the multi-commodity case, we present examples showing that neither the change in edge flows nor the change in the path latency can be bounded
Matroids are Immune to Braess Paradox
The famous Braess paradox describes the following phenomenon: It might happen
that the improvement of resources, like building a new street within a
congested network, may in fact lead to larger costs for the players in an
equilibrium. In this paper we consider general nonatomic congestion games and
give a characterization of the maximal combinatorial property of strategy
spaces for which Braess paradox does not occur. In a nutshell, bases of
matroids are exactly this maximal structure. We prove our characterization by
two novel sensitivity results for convex separable optimization problems over
polymatroid base polyhedra which may be of independent interest.Comment: 21 page
Capacity and Price Competition in Markets with Congestion Effects
We study oligopolistic competition in service markets where firms offer a
service to customers. The service quality of a firm - from the perspective of a
customer - depends on the congestion and the charged price. A firm can set a
price for the service offered and additionally decides on the service capacity
in order to mitigate congestion. The total profit of a firm is derived from the
gained revenue minus the capacity investment cost. Firms simultaneously set
capacities and prices in order to maximize their profit and customers
subsequently choose the services with lowest combined cost (congestion and
price). For this basic model, Johari et al. (2010) derived the first existence
and uniqueness results of pure Nash equilibria (PNE) assuming mild conditions
on congestion functions. Their existence proof relies on Kakutani's fixed-point
theorem and a key assumption for the theorem to work is that demand for service
is elastic (modeled by a smooth and strictly decreasing inverse demand
function).
In this paper, we consider the case of perfectly inelastic demand, i.e. there
is a fixed volume of customers requesting service. This scenario applies to
realistic cases where customers are not willing to drop out of the market, e.g.
if prices are regulated by reasonable price caps. We investigate existence,
uniqueness and quality of PNE for models with inelastic demand and price caps.
We show that for linear congestion cost functions, there exists a PNE. This
result requires a completely new proof approach compared to previous
approaches, since the best response correspondences of firms may be empty, thus
standard fixed-point arguments are not directly applicable. We show that the
game is C-secure (see McLennan et al. (2011)), which leads to the existence of
PNE. We furthermore show that the PNE is unique, and that the efficiency
compared to a social optimum is unbounded in general.Comment: A one-page abstract of this paper appeared in the proceedings of the
15th International Conference on Web and Internet Economics (WINE 2019
Nash and Wardrop equilibria in aggregative games with coupling constraints
We consider the framework of aggregative games, in which the cost function of
each agent depends on his own strategy and on the average population strategy.
As first contribution, we investigate the relations between the concepts of
Nash and Wardrop equilibrium. By exploiting a characterization of the two
equilibria as solutions of variational inequalities, we bound their distance
with a decreasing function of the population size. As second contribution, we
propose two decentralized algorithms that converge to such equilibria and are
capable of coping with constraints coupling the strategies of different agents.
Finally, we study the applications of charging of electric vehicles and of
route choice on a road network.Comment: IEEE Trans. on Automatic Control (Accepted without changes). The
first three authors contributed equall
Path deviations outperform approximate stability in heterogeneous congestion games
We consider non-atomic network congestion games with heterogeneous players
where the latencies of the paths are subject to some bounded deviations. This
model encompasses several well-studied extensions of the classical Wardrop
model which incorporate, for example, risk-aversion, altruism or travel time
delays. Our main goal is to analyze the worst-case deterioration in social cost
of a perturbed Nash flow (i.e., for the perturbed latencies) with respect to an
original Nash flow. We show that for homogeneous players perturbed Nash flows
coincide with approximate Nash flows and derive tight bounds on their
inefficiency. In contrast, we show that for heterogeneous populations this
equivalence does not hold. We derive tight bounds on the inefficiency of both
perturbed and approximate Nash flows for arbitrary player sensitivity
distributions. Intuitively, our results suggest that the negative impact of
path deviations (e.g., caused by risk-averse behavior or latency perturbations)
is less severe than approximate stability (e.g., caused by limited
responsiveness or bounded rationality). We also obtain a tight bound on the
inefficiency of perturbed Nash flows for matroid congestion games and
homogeneous populations if the path deviations can be decomposed into edge
deviations. In particular, this provides a tight bound on the Price of
Risk-Aversion for matroid congestion games
US Highway Privatization and Heterogeneous Preferences
Abstract: We assess the welfare effects of highway privatization accounting for governmentâs behavior in setting the sale price, firmsâ strategic behavior in setting tolls in various competitive environments, and motoristsâ heterogeneous preferences for speedy and reliable travel. We conclude motorists can benefit from privatization if they are able to negotiate aggressively with a private provider to obtain tolls and service that align with their varying preferences. Surprisingly, motorists are likely to be better off negotiating with a monopolist than with duopoly providers or under public-private competition. Toll regulation may be counterproductive because it would treat motorists as homogeneous. Revised June 2009.Security Breach Costs; Financial Distress; Insurance; Resource Allocation.
On the Price of Anarchy of Highly Congested Nonatomic Network Games
We consider nonatomic network games with one source and one destination. We
examine the asymptotic behavior of the price of anarchy as the inflow
increases. In accordance with some empirical observations, we show that, under
suitable conditions, the price of anarchy is asymptotic to one. We show with
some counterexamples that this is not always the case. The counterexamples
occur in very simple parallel graphs.Comment: 26 pages, 6 figure
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