Bharat Adsul

Nash equilibria in fisher market

Much work has been done on the computation of market equilibria. However due to strategic play by buyers, it is not clear whether these are actually observed in the market. Motivated by the observation that a buyer may derive a better payoff by feigning a different utility function and thereby manipulating the Fisher market equilibrium, we formulate the Fisher market game in which buyers strategize by posing different utility functions. We show that existence of a conflict-free allocation is a necessary condition for the Nash equilibria (NE) and also sufficient for the symmetric NE in this game. There are many NE with very different payoffs, and the Fisher equilibrium payoff is captured at a symmetric NE. We provide a complete polyhedral characterization of all the NE for the two-buyer market game. Surprisingly, all the NE of this game turn out to be symmetric and the corresponding payoffs constitute a piecewise linear concave curve. We also study the correlated equilibria of this game and show that third-party mediation does not help to achieve a better payoff than NE payoffs.

Complete and Tractable Local Linear Time Temporal Logics over Traces

We present the first expressively complete and yet tractable temporal logics, Past-TrLTL and TrLTL, to reason about distributed behaviours, modelled as Mazurkiewicz traces. Both logics admit singly exponential automata-theoretic decision procedures. General formulas of Past-TrLTL are boolean combinations of local formulas which assert rich properties of local histories of the behaviours. Past-TrLTL has the same expressive power as the first order theory of finite traces. TrLTL provides formulas to reason about recurring local Past-TrLTL properties and equals the complete first order theory in expressivity. The expressive completeness criteria are based on new local normal forms for the first order logic. We illustrate the use of our logics for specification of global properties.

A Generalization of the Łoś-Tarski Preservation Theorem over Classes of Finite Structures

We present a logic-based combinatorial property of classes of finite structures that allows an effective generalization of the Łoś-Tarski preservation theorem to hold over classes satisfying the property. The well-studied classes of words and trees, and structures of bounded tree-depth are shown to satisfy the property. We also show that starting with classes satisfying this property, the classes generated by applying composition operations like disjoint union, cartesian and tensor products, inherit the property. We finally show that all classes of structures that are well-quasi-ordered under the embedding relation satisfy a natural generalization of our property.

Preservation under Substructures modulo Bounded Cores

We investigate a model-theoretic property that generalizes the classical notion of preservation under substructures. We call this property preservation under substructures modulo bounded cores, and present a syntactic characterization via \(\Sigma_2^0\) sentences for properties of arbitrary structures definable by FO sentences. Towards a sharper characterization, we conjecture that the count of existential quantifiers in the \(\Sigma_2^0\) sentence equals the size of the smallest bounded core. We show that this conjecture holds for special fragments of FO and also over special classes of structures. We present a (not FO-definable) class of finite structures for which the conjecture fails, but for which the classical Łoś-Tarski preservation theorem holds. As a fallout of our studies, we obtain combinatorial proofs of the Łoś-Tarski theorem for some of the aforementioned cases.

A simplex-like algorithm for fisher markets

We propose a new convex optimization formulation for the Fisher market problem with linear utilities. Like the Eisenberg-Gale formulation, the set of feasible points is a polyhedral convex set while the cost function is non-linear; however, unlike that, the optimum is always attained at a vertex of this polytope. The convex cost function depends only on the initial endowments of the buyers. This formulation yields an easy simplex-like pivoting algorithm which is provably strongly polynomial for many special cases.

A Computational Framework for Boundary Representation of Solid Sweeps

This paper proposes a robust algorithmic and computational framework to address the problem of modeling the volume obtained by sweeping a solid along a trajectory of rigid motions. The boundary representation (simply brep) of the input solid naturally induces a brep of the swept volume. We show that it is locally similar to the input brep and this serves as the basis of the framework. All the same, it admits several intricacies: (i) geometric, in terms of parametrizations and, (ii) topological, in terms of orientations. We provide a novel analysis for their resolution. More specifically, we prove a non-trivial lifting theorem which allows to locally orient the output using the orientation of the input. We illustrate the framework by providing many examples from a pilot implementation.

Incorporating sharp features in the general solid sweep framework

This paper extends a recently proposed robust computational framework for constructing the boundary representation (brep) of the volume swept by a given smooth solid moving along a one parameter family h of rigid motions. Our extension allows the input solid to have sharp features, and thus it is a significant and useful generalization of that work.

A generalization of the Łoś–Tarski preservation theorem

Abstract We present new parameterized preservation properties that provide for each natural number k, semantic characterizations of the ∃ k ∀ ⁎ and ∀ k ∃ ⁎ prefix classes of first order logic sentences, over the class of all structures and for arbitrary finite vocabularies. These properties, that we call preservation under substructures modulo k-cruxes and preservation under k-ary covered extensions respectively, correspond exactly to the classical properties of preservation under substructures and preservation under extensions, when k equals 0. As a consequence, we get a parameterized generalization of the Łoś–Tarski preservation theorem for sentences, in both its substructural and extensional forms. We call our characterizations collectively the generalized Łoś–Tarski theorem for sentences. We generalize this theorem to theories, by showing that theories that are preserved under k-ary covered extensions are characterized by theories of ∀ k ∃ ⁎ sentences, and theories that are preserved under substructures modulo k-cruxes, are equivalent, under a well-motivated model-theoretic hypothesis, to theories of ∃ k ∀ ⁎ sentences. In contrast to existing preservation properties in the literature that characterize the Σ 2 0 and Π 2 0 prefix classes of FO sentences, our preservation properties are combinatorial and finitary in nature, and stay non-trivial over finite structures as well.

Local Normal Forms for Logics over Traces

We investigate local and global paradigms of reasoning about distributed behaviours, modelled as Mazurkiewicz traces, in the context of first-order and monadic second-order logics. We describe new normal forms for properties expressible in these logics. The first normal form, surprisingly, yields a decomposition of a global property as a boolean combination of local properties. The second normal form strengthens McNaughtons theorem and states that global properties of infinite behaviours may also be described as boolean combinations of recurring properties of finite local histories of the behaviours. We briefly touch upon some of the interesting applications of these normal forms.