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EKİCİ, Özgün

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Özgün

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EKİCİ
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Now showing 1 - 10 of 12
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    ArticlePublicationOpen Access
    On optimal toll design for bosporus crossings
    (Sosyoekonomi Society, 2022-10) Ekici, Özgün; Economics; EKİCİ, Özgün
    For many years, two toll bridges served commuter demand to cross the strait called Bosporus in Istanbul, Turkey. An underground connection called the Eurasian tunnel had been recently launched to relieve the strait's traffic. We study a simple transportation model that incorporates the forces that have come into play after the opening of the Eurasian tunnel. We find that for welfare maximisation, the premium paid for using the tunnel should be fixed in the two directions and not excessive. The current toll regime violates these features, and we recommend its amendment in light of our findings.
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    On existence of equilibrium under social coalition structures
    (Springer, 2020) Caskurlu, B.; Ekici, Özgün; Kizilkaya, F. E.; Economics; Chen, J.; Feng, Q.; Xu, J.; EKİCİ, Özgün
    In a strategic form game, a strategy profile is an equilibrium if no viable coalition of agents benefits (in the Pareto sense) from jointly changing their strategies. Weaker or stronger equilibrium notions can be defined by considering various restrictions on coalition formation. In a Nash equilibrium, for instance, the assumption is that viable coalitions are singletons, and in a super strong equilibrium, every coalition is viable. Restrictions on coalition formation can be justified by communication, coordination or institutional constraints. In this paper, inspired by social structures in various real-life scenarios, we introduce certain restrictions on coalition formation, and on their basis, we introduce a number of equilibrium notions. We study our equilibrium notions in resource selection games (RSGs), and we present a complete set of existence and non-existence results for general RSGs and their important special cases.
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    ArticlePublicationOpen Access
    On approximate Nash equilibria of the two-source connection game
    (TÜBİTAK, 2022) Çaşkurlu, B.; Açikalin, U. U.; Kizilkaya, F. E.; Ekici, Özgün; Economics; EKİCİ, Özgün
    The arbitrary-sharing connection game is prominent in the network formation game literature [1]. An undirected graph with positive edge weights is given, where the weight of an edge is the cost of building it. An edge is built if agents contribute a sufficient amount for its construction. For agent i, the goal is to contribute the least possible amount while assuring that the source node si is connected to the terminal node ti. In this paper, we study the special case of this game in which there are only two source nodes. In this setting, we prove that there exists a 2-approximate Nash equilibrium that is socially optimal. We also consider the further special case in which there are no auxiliary nodes (i.e., every node is a terminal or source node). In this further special case, we show that there exists a 3/2 -approximate Nash equilibrium that is socially optimal. Moreover, we show that it is computable in polynomial time.
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    The Turkish appetite for gold: An Islamic explanation
    (Elsevier, 2016-06) Gülseven, O.; Ekici, Özgün; Economics; EKİCİ, Özgün
    A significant constituent of household wealth in Turkey is gold. Families accumulate gold especially on a variety of cultural occasions such as female-only gold days, circumcision feasts, and engagement and wedding ceremonies. This paper attempts to explain the gold appetite of Turkish households with a rational approach, although still rooted in culture. Many people in Turkey view “earning interest on money” as a transgression of Islamic rules and avoid investing their savings in fixed-income investment instruments. We study the implications of this investor behavior on portfolio gold holdings. Using the Markowitz mean-variance model and monthly return data from 1997 until 2015, we calculate optimal investment portfolios. We find that while the share of portfolio gold holdings is less than a meager 4% if the portfolio includes interest-earning deposits (in addition to the stock index, USD, EURO, and gold), this ratio may go up to more than 50% if interest-earning deposits are not included. Our results show that the role gold plays in an investment portfolio is greatly amplified when interest-earning deposits are not viable. We believe the key factor driving our pronounced findings has been Turkey's historically high rates of inflation. Avoiding fixed-income instruments, many Turkish investors may have turned to gold to shield their savings against inflation and manage portfolio risks caused by its high volatility. Our findings suggest that Turkish policymakers may find it useful to popularize Sukuk (Islam-compliant bonds) if they are to divert household savings away from gold into more productive uses.
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    An equilibrium analysis of the probabilistic serial mechanism
    (Springer Science+Business Media, 2016-08) Ekici, Özgün; Kesten, O.; Economics; EKİCİ, Özgün
    The prominent mechanism of the recent literature in the assignment problem is the probabilistic serial (PS). Under PS, the truthful (preference) proÖle always constitutes an ordinal Nash Equilibrium, inducing a random assignment that satisÖes the appealing ordinal e¢ ciency and envy-freeness properties. We show that both properties may fail to be satisÖed by a random assignment induced in an ordinal Nash Equilibrium where one or more agents are non-truthful. Worse still, the truthful proÖle may not constitute a Nash Equilibrium, and every non-truthful proÖle that constitutes a Nash Equilibrium may lead to a random assignment which is not ordinally e¢ cient, not even weakly envy-free, and which admits an ex-post ine¢ cient decomposition. A strong ordinal Nash Equilibrium may not exist, but when it exists, any proÖle that constitutes a strong ordinal Nash Equilibrium induces the random assignment induced under the truthful proÖle. The results of our equilibrium analysis of PS call for caution when implementing it in small assignment problems.
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    Random mechanisms for house allocation with existing tenants
    (Elsevier, 2020-08) Ekici, Özgün; Economics; EKİCİ, Özgün
    We study the house allocation problem with existing tenants: n houses (stand for "indivisible objects") are to be allocated to n agents; each agent needs exactly one house and has strict preferences; k houses are initially unowned; k agents initially do not own houses; the remaining n - k agents (the so-called "existing tenants") initially own the remaining n - k houses (each owns one). In this setting, we consider various randomized allocation rules under which voluntary participation of existing tenants is assured and the randomization procedure either treats agents equally or discriminates against some (or all) of the existing tenants. We obtain two equivalence results, which generalize the equivalence results in Abdulkadiroglu and Sonmez (1999) and Sonmez and Unver (2005).
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    The role of real estate and gold as inflation hedges: the Islamic influence
    (Emerald Publishing Limited, 2021-04-06) Gulseven, O.; Ekici, Özgün; Economics; EKİCİ, Özgün
    Purpose This paper aims to understand how aversion to interest income in Islam may influence the demand for real estate and gold when inflation is rampant. Design/methodology/approach According to Markowitz's mean-variance model, an optimal portfolio is one that blends maximum return with minimum variance. In investment portfolios, real estate and gold serve as inflation hedges. For religious reasons, many Muslims exclude interest-earning assets from their portfolios, however. This paper explores how this attitude influences the hedging role of real estate and gold when inflation is rampant. This paper compares optimal portfolios that include and do not include interest-earning assets. In the calculations, this study uses monthly Turkish data from 1997 until 2018. Findings The analysis shows that the best hedging instrument against inflation is an interest-earning asset. In its absence, the role of real estate and gold as inflation hedges markedly increases: For a medium-return and medium-risk portfolio, for instance, the portfolio share of gold holdings increases from 3.16% to 58.43% and that for real estate increases from 14.97% to 24.06%. Originality/value This paper is a pioneering work on the influence of Islam on the roles of real estate and gold as inflation hedges when inflation is rampant. It provides an explanation from financial theory for the strong real estate and gold demand in Turkey in the past two decades.
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    Characterizing the TTC rule via pair-efficiency: A short proof
    (Elsevier, 2024-01) Ekici, Özgün; Sethuraman, J.; Economics; EKİCİ, Özgün
    In the object reallocation problem, Ekici (2023) showed that Top Trading Cycles (TTC) is the unique rule that is strategyproof, individual-rational, and pair-efficient. We provide a short proof of this characterization result.
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    Reclaim-proof allocation of indivisible objects
    (Elsevier, 2013-09) Ekici, Özgün; Economics; EKİCİ, Özgün
    We study desirability axioms imposed on allocations in indivisible object allocation problems. The existing axioms in the literature are various conditions of robustness to blocking coalitions with respect to agentsʼ ex ante (individual rationality and group rationality) and ex post (Pareto efficiency) endowments. We introduce a stringent axiom that encompasses and strengthens the existing ones. An allocation is reclaim-proof if it is robust to blocking coalitions with respect to any conceivable interim endowments of agents. This is an appealing property in dynamic settings, where the assignments prescribed by an allocation to be implemented need to be made in multiple rounds rather than all in one shot. We show that an allocation is reclaim-proof if and only if it is induced by a YRMH–IGYT mechanism (introduced by Abdulkadiroğlu and Sönmez, 1999) and if and only if it is a competitive allocation.
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    On existence of equilibrium under social coalition structures
    (Cambridge University Press, 2022-02) Caskurlu, B.; Ekici, Özgün; Kizilkaya, F. E.; Economics; EKİCİ, Özgün
    In a strategic-form game, a strategy profile is an equilibrium if no viable coalition of agents (or players) benefits (in the Pareto sense) from jointly changing their strategies. Weaker or stronger equilibrium notions can be defined by considering various restrictions on coalition formation. For instance, in a Nash equilibrium, it is assumed that viable coalitions are singletons, and in a super strong equilibrium, it is assumed that every coalition is viable. Restrictions on coalition formation can be justified by communication limitations, coordination problems, or institutional constraints. In this paper, inspired by social structures in various real-life scenarios, we introduce certain restrictions on coalition formation, and on their basis, we introduce a number of equilibrium notions. As an application, we study our equilibrium notions in resource selection games (RSGs), and we present a complete set of existence and nonexistence results for general RSGs and their important special cases.