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A Lower Bound on Unambiguous Context Free Grammars via Communication Complexity
No full textInternational audienceMotivated by recent connections to factorised databases, we analyse the efficiency of representations by context free grammars (CFGs). Concretely, we prove a recent conjecture by Kimelfeld, Martens, and Niewerth (ICDT 2025), that for finite languages representations by general CFGs can be doubly-exponentially smaller than those by unambiguous CFGs. To do so, we show the first exponential lower bounds for representation by unambiguous CFGs of a finite language that can efficiently be represented by ambiguous CFGs. Our proof first reduces the problem to proving a lower bound in a non-standard model of communication complexity. Then, we argue similarly in spirit to a recent discrepancy argument to show the required communication complexity lower bound. Our result also implies that a finite language may admit an exponentially smaller representation as a nondeterministic finite automaton than as an unambiguous CFG
Sum of Squares Circuits
No full textInternational audienceDesigning expressive generative models that support exact and efficient inference is a core question in probabilistic ML. Probabilistic circuits (PCs) offer a framework where this tractability-vs-expressiveness trade-off can be analyzed theoretically. Recently, squared PCs encoding subtractive mixtures via negative parameters have emerged as tractable models that can be exponentially more expressive than monotonic PCs, i.e., PCs with positive parameters only. In this paper we provide a more precise theoretical characterization of the expressiveness relationships among these models. First, we prove that squared PCs can be less expressive than monotonic ones. Second, we formalize a novel class of PCs – sum of squares PCs – that can be exponentially more expressive than both squared and monotonic PCs. Around sum of squares PCs, we build an expressiveness hierarchy that allows us to precisely unify and separate different tractable model classes such as Born Machines and PSD models, and other recently introduced tractable probabilistic models by using complex parameters. Finally, we empirically show the effectiveness of sum of squares circuits in performing distribution estimation
Study of rubber coating effect on the stability and fracture resistance of railway ballast
No full textInternational audienc
Analyzing Intraday Liquidity Dynamics in the Limit Order Book using MS Excel
No full textThe Case Centre, case study 125-0058-1 , teaching note 125-0058-8, teaching note supplement 125-0058-8
Understanding Bid and Offer Prices: A Trading Game Simulation Using Figgie by Jane Street
No full textThe Case Centre, case study 125-0052-1 , teaching note 125-0052-
Surface and structural improvements in PVA-modified expansive soils under cold conditions
No full textInternational audienc
Inter-municipal cooperation in drinking water supply: Trade-offs between transaction costs, efficiency and service quality
No full textInternational audienceInter-municipal cooperation (IMC) is frequently promoted as a solution to improve the management of local utilities such as drinking water. Yet its effectiveness remains ambiguous: while IMC can create economies of scale, it may also induce transaction costs that undermine its benefits. In France, drinking water services are managed at the municipal level, where local governments can decide whether to cooperate—and if so, whether to adopt a purely technical cooperative arrangement or a more politically integrated, supra-municipal governance structure. Using a comprehensive panel of French water utilities from 2008 to 2021, we investigate the factors that lead municipalities to remain independent. Our econometric analysis, based on a correlated random effects probit model with a control function approach, yields several key findings. First, while IMC is associated with higher water prices, these increased tariffs are offset by better network performance, as indicated by lower water loss indices and improved water quality. Second, we find that the more politically integrated form of cooperation is more common among publicly managed utilities and among municipalities seeking to reduce their dependence on imported water. These findings provide new insights into the governance of common-pool resources, suggesting that while cooperation can improve service provision, its institutional design must carefully balance organizational costs against expected efficiency gains
Hysteresis in addictive consumption depends on time preferences
No full textInternational audienceHow can individuals who have experienced a shock in their addictive consumption trajectories return to their habitual use? As part of a behavioral economics online survey conducted on a representative sample of the French population, we asked respondents to retrospectively quantify their consumption of tobacco, alcohol, and recreational screen use at three moments: before, during, and after the first Covid19 lockdown. Using a methodology that controls for inter-individual heterogeneity, we test for the presence of a hysteresis effect, i.e. whether the shocks in use that occurred during the lockdown last beyond the end of it and the return to a morenormal life. We find hysteresis for the three addictive goods. Studying this hysteresis effect in relation to time preferences, we find that, for tobacco, present-biased individuals exhibit more hysteresis. This hysteresis insight, related to time preferences, offers valuable perspectives for addiction research and policy design addressing population resilience to shocks
