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This thesis includes three chapters on the conflicts of interest of intermediaries acting as M&A advisors. In Chapter 1, we provide a synthesis of the academic literature on the dual proprietary and client trading of market intermediaries. First, we provide an overview of this activity, and then explore questions such as who engages in this activity, why, and key concerns about conflicts of interest. In Chapter 2, we conduct an empirical analysis focusing on the mergers and acquisitions aspect of the research problem. Finally, in Chapter 3 we study the role of intermediaries who act as financial advisors in M&A transactions. We do this through an empirical analysis of a sample where we can identify, around the time of the announcement, in which transactions an advisor was involved. The results show value creation in M&A targets on the day of the event, the presence of informed trading and pre-announcement information leakage particularly strong for deals in which an advisor was involved. Our analysis allows us to highlight the importance of taking into account the regulatory aspect of M&A transactions.

This doctoral work aims to bring the French grain farmer closer to the financial markets through a serious game. For an agricultural producer, the uncertainty on price and quantities induces an important risk. Negotiating a contract with an intermediary (stockholder or broker) makes it possible to transfer part of the risk, but increases the dependence of grain farmers on third parties. Financial hedging against the risk of price variations is studied in the case of the European market Euronext, in order to evaluate its relevance: futures and options are analyzed from a micro-structural point of view (volume, open position, volatility, information and volatility transfers, contract holders, position rollovers) and compared. The markets are then set aside to focus on the characteristics of grain farmers through qualitative surveys. Finally, a digital tool in the form of a serious game is developed to introduce grain farmers to the financial markets.

This thesis studies the main transformations in the microstructure of financial markets since the generalization of market automation. Today, the structural change in financial markets, combined with the evolution of information technology, has led to upheavals in both market practices and in the instruments used to measure market quality. The cost of liquidity has continued to improve between 2010 and 2019, notably by reducing the spreads of smaller SBF 120 companies. On the other hand, effective spreads narrow significantly less showing the shallow depth of the order book at best limits for smaller firms. The work presents the changes in trading platforms and the technological developments that have enabled the deployment of high-frequency trading. The research team developed a financial market replication tool, VirteK. The library has enabled a simulation replicating the stylized facts of the May 6, 2010 flash-crash illustrating order book imbalances using VPIN.

The last decade has been marked by several new regulations in response to the financial crisis of 2007-2008. In order to contribute to the debate, this thesis consists of three chapters that address different aspects of the regulation of the stock and derivatives markets. The first chapter provides a clear assessment of the impact of increasing the KOSPI 200 option multiplier on investor participation and market efficiency for the period 2011-2013. We use two measures of market efficiency: the participation share of noise traders and asymmetric volatility. Along the same lines, Chapter 2 examines the impact of the capital gains tax (CGT) on the quality and efficiency of the KOSPI 200 options market. We use various measures of market liquidity: trading volume, trading value, and the bid-ask spread over the period August 2015 to December 2016. The third chapter assesses the impact of the French financial transaction tax (STT) on market liquidity and volatility. Unlike previous studies, the format of the French STT allows us to test its effect over a longer period 2012-2019.

The CAC40 index is the first thing you think of when you talk about the stock market in France. However, it is not tradable. This is why derivative contracts have appeared such as the FCE future contract whose underlying is the CAC40 which can be bought and sold. Their values are very close but not equal. So what is the relationship between the FCE contract and the CAC40? With the help of data that can be downloaded daily from the Internet and accessible to everyone, an answer will be given. In the first part we present the CAC40 futures contract, the reasons for trading it and compare it to other stock index futures contracts in the world. We then study the FCE/CAC40 relationship in terms of informational efficiency. To do so, we will study different basic notions and try to model them. Finally, in a last part, we study this same relationship from a microstructure point of view, by studying in particular non-price variables (volume and open position), and volatility. We will finally try to model the volatility as a function of these variables.

Technological and regulatory developments have favored the rise of two major phenomena associated with greater opacity: high frequency trading (HFT) and dark pools. One out of every two transactions is now made by HFT. Dark pools attract 10% of trading volume in Europe and 20% in the United States, notably because of the high frequency traders targeting uninformed orders that are exchanged there. Academic studies show that transaction costs have never been so low under the competitive pressure of HFT. However, they have been stable since 2009 even as speed continues to increase. Market efficiency has improved thanks to the arbitrage activity of THF. However, as trading volume shifts to dark pools, the risk of a deterioration in liquidity and efficiency increases. Add to this the risk of competitive distortion and operational and technological risk, and regulators are faced with increasingly complex issues.

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This thesis contains a theoretical paper on banking regulation and two empirical papers on the bond market. The first chapter studies the optimal allocation of lender of last resort responsibilities in a context where the collapse of a systemic bank due to a liquidity shock affects the profitability of a non-systemic bank. If the liquidity shock is small, the central bank should be the lender of last resort, but in the case of a large liquidity shock an unconditional bank bailout should be implemented. Moreover, the existence of a systemic risk reinforces the weight of the central bank for non-systemic banks. Finally, systemic risk leads to ambiguous results for systemic banks because the provision of liquidity to the top tier as well as the provision of liquidity by the central bank is less stringent. The second chapter defines six liquidity indicators based on information contained in transactions in the European OTC corporate bond market. This study then measures the ability of these indicators to detect a liquidity premium. The number of brokers, the size and the volume of transactions are the most significant data. They are also complementary since they cover different aspects of the multidimensional concept of liquidity. The third chapter studies the market power of brokers in the same market. The specific position of brokers and their small number allow them to influence price adjustments in their favor, particularly on the supply side of the market.

The introduction of order books in the U.S. and London markets, the adoption of the nsc trading system in some twenty countries, and the introduction of specialists to provide liquidity in order-driven markets illustrate the convergence toward a mixed structure. The way exchanges are organized and the characteristics of liquidity providers influence the quality of the market. A number of questions arise, however, regarding the market-making practices of the market's member traders. This is why we have mainly highlighted the existence of a new category of agents present on a market structure based on a central order book and also put their behavior into perspective with regard to the state of the order book and transaction costs. The volume traded by proprietary traders represents 27%. The agent-principal duality of traders reflects a situation of competition between retail investors and these dual traders for the supply of liquidity. The results show that the order flow of dual traders accelerates inside the range to gain time priority during an abnormal widening of the range. They change the composition of the order flow by placing more limit orders than immediate execution orders to guarantee the counterparty to retail investors' orders in exchange for a larger fee. Dual traders are more active during the opening and closing fixings than during the continuous trading phase. Even though dual traders do not practice front-running, they obtain more favorable prices and execution times for their orders than client orders. Dual traders therefore control the book in order to meet the demand for liquidity. The mixed structure of the Paris market is thus highlighted.