Photo Ludovic A. Julien

Ludovic A. Julien

Professeur(e)
  • Email
  • Tél. professionnel 0140977543
  • Bureau à Paris Nanterre (Bât. + num.) G517C
  • Research group

      Droit, Institutions, Règlementation et Interactions Stratégiques

  • Theme(s)
    • Interactions stratégiques et équilibre général
    • Théorie des défauts de coordination
    • Variations conjecturales
    • Externalités et politiques publiques

2018-10 "Existence and Optimality of Cournot-Nash Equilibria in a Bilateral Oligopoly with Atoms and an Atomless Part"

Francesca Busetto, Giulio Codognato, Sayantan Ghosal, Ludovic A. Julien, Simone Tonin

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Abstract
We consider a bilateral oligopoly version of the Shapley window model with large traders, represented as atoms, and small traders, represented by an atomless part. For this model, we provide a general existence proof of a Cournot-Nash equilibrium that allows one of the two commodities to be held only by atoms. Then, we show, using a corollary proved by Shitovitz (1973), that a Cournot-Nash allocation is Pareto optimal if and only if it is a Walras allocation.
Classification-JEL
C72, D51
Mot(s) clé(s)
Shapley window model; Atoms; Atomless part; Cournot–Nash equilibrium; Optimality
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2017-22 "Hierarchical competition and heterogeneous behavior in noncooperative oligopoly markets"

Ludovic A. Julien

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Abstract
In this paper, we consider a sequential bilateral oligopoly market which embodies a finite number of leaders and followers who compete on quantities. We define a noncooperative equilibrium concept for this two-stage market game with complete and perfect information, namely the Stackelberg-Nash equilibrium (SNE). Then, we study the existence of a SNE with trade. The existence proof requires some steps as this market game displays a rich set of strategic interactions. In particular, to show the existence of a pure strategy subgame perfect Nash equilibrium, we have to determine the conditions under which there exist well defined continuously differentiable best responses. Some examples buttress the approach and discuss the assumptions made on the primitives.
Classification-JEL
C72, D51
Mot(s) clé(s)
Best responses, Stackelberg-Nash equilibrium, trade, autarky
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2016-13 "On Noncooperative Oligopoly Equilibrium in the Multiple Leader-Follower Game"

Ludovic A. Julien

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Abstract
In this paper, we provide new proofs of existence and uniqueness of a Stackelberg market equilibrium for a multiple leader-follower noncooperative oligopoly model in which heterogeneous firms compete on quantities. To this end, we consider a two-step game of perfect and complete information in which many leaders interact strategically with many followers. The Stackelberg market equilibrium constitutes a pure strategy subgame perfect Nash equilibrium of this game. The existence (and uniqueness) problem is complexified in this framework since strategic interactions occur within each partial game but also between both partial games through sequential decisions. Then, to prove existence, we notably provide a new procedure to determine (the conditions under which) the optimal behavior of the followers (may be written) as functions of the leaders'strategy profile only. Some examples outline our procedure and discuss our assumptions.
Classification-JEL
C61, C62, C72, D43.
Mot(s) clé(s)
Best response functions, existence, uniqueness.
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2011-10 "Do followers really matter in Stackelberg competition?"

Ludovic A. Julien, Olivier Musy, Aurélien Saïdi

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Abstract
In this note, we consider a generalized T−stage Stackelberg oligopoly. We provide
a proof and an interpretation that under the two necessary and sufficient conditions
of linear aggregate demand and identical constant marginal costs, followers
do not matter for leaders. Leaders act as rational myopic agents, voluntarily ignoring
the number of followers and remaining stages, thereby behaving as Cournotian
oligopolists. Strategies of incumbent firms are invariant to entry of new cohorts.
Their profits can be studied by the way of two discount factors: the first impacting
markup and the second impacting output supply. Some implications in terms of
welfare and convergence toward competitive equilibrium are derived.
Classification-JEL
L13 L20
Mot(s) clé(s)
Leader’s markup discount factor; linear economy; follower’s output discount factor; myopic behavior
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2008-29 "Market Price Mechanisms and Stackelberg General Equilibria"

Ludovic A. Julien, Fabrice Tricou

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Abstract
This paper considers Stackelberg competition in a general equilibrium framework with production. The working of market power and the confi...gurations of strategic interactions are complexi...ed by the presence of an active leader. Two market price mechanisms are here studied: one is associated with the Stackelberg-Walras equilibrium, the other is linked to the Stackelberg-Cournot equilibrium. In the context of an exchange economy with a production sector, several results are obtained about equilibria mergings and about welfare comparisons.
Classification-JEL
Mot(s) clé(s)
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2006-6 "Equilibres multiples avec chômage, coûts de transaction et concurrence monopolistique"

Ludovic A. Julien, Nicolas Sanz

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Abstract
The properties of WS-PS model are modified when trade activity is explicitly modelled. We introduce transaction costs on the output market to capture multiplicity whatever the degree of returns to scale. The price setting curve can become downward-sloping in the unemployment rate-real wage space. Trading externalities reinforce the effect of strategic complementarities between firms, leading to multiple unemployment equilibria. Moreover, a positive shock on the bargaining power of workers decreases the unemployment rate and the real wage at the low equilibrium, but increases both of them at the high equilibrium.
Classification-JEL
D43, E25, J64
Mot(s) clé(s)
multiple unemployment equilibria, transaction costs, monopolistic competition
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2006-3 "A note on price-taking and price-making behaviours in pure exchange economies"

Ludovic A. Julien, Fabrice Tricou

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Abstract
This paper explores the rationale of price-taking and price-making behaviours in the context of Walrasian and Cournotian pure exchange economies. Beside the influence of the number of agents, we underline the role of the structure of preferences in the definition and in the working of market power. Through three equilibrium variations of the same basic economy, we obtain several results about price manipulation, about asymptotic identifications for large economies and for degenerate preferences, and about welfare comparisons. Perfect competition does not only correspond to the case of large economies, but may also concern economies where fundamental market powers are more or less equivalent.
Classification-JEL
D43, D51
Mot(s) clé(s)
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