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The thesis consists of five chapters. The first of them contains introduction. Chapter 2 considers a broad class of two player symmetric games, which display a fundamental non-concavity when actions of both players are about to be the same. This implies that no symmetric equilibrium is possible. We distinguish different properties of the payoff functions, like strategic substitutes, complements and quasi-concavity, which are not necessarily imposed globally on the joint action space. For each of these cases we provide conditions to secure the existence of exclusively asymmetric equilibria. Moreover we consider the case of convex payoff functions. A number of applications from industrial organization and applied microeconomics literature are provided. In Chapter 3 we generalize to the extent possible the known results for the case of games with one-dimensional action sets to the general case of games with strategic complemantarities with action spaces that are complete lattices. One key issue addressed is the extent to which all equilibria tend to be symmetric for the general case of multi-dimensional (i.e. only partially ordered) strategy spaces. We find that the scope for asymmetric equilibrium behavior is definitely broader than in the one-dimensional case, though still quite limited. Another key question investigated here is whether asymmetric pure strategy Nash equilibria are always Pareto dominated by symmetric pure strategy Nash equilibria. While this need not hold in general for games with strategic complementarities, we identify different sufficient conditions that guarantee that such dominance holds. In Chapter 4 we deal with the effects of market transparency on prices in the Bertrand duopoly model. The analysis is intuitive and simple when we consider two types of strategic interaction between firms in an industry - strategic complementarities and substitutabilities. We present also traditional comparative statics analyses, demanding additionally some other regularity conditions, to cover those problems, when neither of these situations is the case. In the first case, the results are close to conventional wisdom, especially, when in the same time products are substitutes. Namely, equilibrium prices and profits are always decreasing in transparency level, while the consumer's surplus is increasing. For a special case when supermodularity holds, but products are not substitutes, the result on profits is not valid anymore. Considering price competition with strategic substitutes, an ambiguity in the direction of change of prices appears. This leads to ambiguity concerning equilibrium profits and surplus changes caused by increasing transparency. In Chapter 5 we provide general conditions for Cournot oligopoly with product differentiation to have monotonic reaction correspondences. We give a proof for the conditions stated by Vives (1999). Moreover we elaborate more general requirements. They allow for identifying increasing best responses even in case inverse demand is submodular, and similarly, decreasing best responses in case of supermodular inverse demand. Examples illustrating the scope of applicability of these results are provided.

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THESIS ABSTRACT The bulk of Internet Service Providers adopt two-part tariff. Two-part tariff means users should pay a fixed rental every month, and pay a varied fee base on their connecting hours. Internet Service Provider group their customers by analyzing customer¡¦s demand level, and provides different group different Pay-Rating. We call this situation Multiple Pay-Rating. If Internet Service Provider provide their customers single Pay-Rating. We call this situation Single Pay-Rating. People always analyze two-part tariff in monopoly market. In this thesis I will analyze two-part tariff in oligopoly market.Then compare with the former, and we will know whether two-part tariff erode customer¡¦s welfare. I will create a differentiate-product demand function base on Logit model, and build two-part tariff oligopoly competition model. Analyze the equilibrium in Single Pay-rating and in Multiple Pay-rating. And I drew several conclusions from the analysis. 1. In oligopoly market structure two-part tariff not necessarily deprive of customer¡¦s welfare. On the contrary if ISP group their customers, customer¡¦s welfare will improve. 2. If there are no differentiation between customers¡¦ utilities, nonlinear pricing will degraded to linear pricing. If customers¡¦ utilities are different, ISP will set up the unit connect fee equal to the unit cost, and the month rental will raised with the increasing differentiation between customers¡¦ utilities. 3. If Internet service provider group their customer and provide them different Pay-rating, the final equilibrium will be symmetrical equilibrium or non-symmetrical equilibrium.

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This thesis is an extension of a recent study into the relationship between merger size and profitability. It studies a class of Cournot oligopoly with linear cost and quadratic demand. Its focus is to analyze how a mergers profitability is affected by its size and by the demand elasticity. Such results have not yet been reported in previous studies, perhaps due to the complexity of the equilibrium equation involved. It shows an increase in the demand elasticity also raises a mergers profitability. Consequently, an increase in the demand elasticity reduces merged members critical combined per-merger market share for the merger to be profit enhancing. Comparing with 80% minimum market share requirement for a profitable merger in Salant, Switzer, and Reynolds (1983), a greater market share is needed when the demand function is concave (demand is relatively inelastic), while a smaller market share may still be profitable when the demand function is convex (demand is relatively elastic). In our model, mergers are generally detrimental to public interests by increasing market price and reducing output. However, the merger will be less harmful when the goods are very inelastic.

This thesis is an extension of a recent study into the relationship between merger size and profitability. It studies a class of Cournot oligopoly with linear cost and quadratic demand. Its focus is to analyze how a mergers profitability is affected by its size and by the demand elasticity. Such results have not yet been reported in previous studies, perhaps due to the complexity of the equilibrium equation involved. It shows an increase in the demand elasticity also raises a mergers profitability. Consequently, an increase in the demand elasticity reduces merged members critical combined per-merger market share for the merger to be profit enhancing. Comparing with 80% minimum market share requirement for a profitable merger in Salant, Switzer, and Reynolds (1983), a greater market share is needed when the demand function is concave (demand is relatively inelastic), while a smaller market share may still be profitable when the demand function is convex (demand is relatively elastic). In our model, mergers are generally detrimental to public interests by increasing market price and reducing output. However, the merger will be less harmful when the goods are very inelastic.