Pinyan Lu

Asymptotically optimal strategy-proof mechanisms for two-facility games

We consider the problem of locating facilities in a metric space to serve a set of selfish agents. The cost of an agent is the distance between her own location and the nearest facility. The social cost is the total cost of the agents. We are interested in designing strategy-proof mechanisms without payment that have a small approximation ratio for social cost. A mechanism is a (possibly randomized) algorithm which maps the locations reported by the agents to the locations of the facilities. A mechanism is strategy-proof if no agent can benefit from misreporting her location in any configuration. This setting was first studied by Procaccia and Tennenholtz [21]. They focused on the facility game where agents and facilities are located on the real line. Alon et al. studied the mechanisms for the facility games in a general metric space [1]. However, they focused on the games with only one facility. In this paper, we study the two-facility game in a general metric space, which extends both previous models. We first prove an Ω(n) lower bound of the social cost approximation ratio for deterministic strategy-proof mechanisms. Our lower bound even holds for the line metric space. This significantly improves the previous constant lower bounds [21, 17]. Notice that there is a matching linear upper bound in the line metric space [21]. Next, we provide the first randomized strategy-proof mechanism with a constant approximation ratio of 4. Our mechanism works in general metric spaces. For randomized strategy-proof mechanisms, the previous best upper bound is O(n) which works only in the line metric space.

Holographic algorithms: from art to science

We develop the theory of holographic algorithms. We definea basis manifold and give characterizations of algebraic varieties of realizable symmetric generators and recognizers on this manifold. We present a polynomial time decision algorithm for the simultaneous realizability problem. Using the general machinery we are able to giveunexpected holographic algorithms for some counting problems, modulo certain Mersenne type integers. These counting problems are P-complete without the moduli. Going beyond symmetric signatures, we define d-admissibility and d-realizability for general signatures, and give a characterizationof 2-admissibility.

Holant problems and counting CSP

We propose and explore a novel alternative framework to study the complexity of counting problems, called Holant Problems. Compared to counting Constrained Satisfaction Problems (CSP), it is a refinement with a more explicit role for the function constraints. Both graph homomorphism and CSP can be viewed as special cases of Holant Problems. We prove complexity dichotomy theorems in this framework. Because the framework is more stringent, previous dichotomy theorems for CSP problems no longer apply. Indeed, we discover surprising tractable subclasses of counting problems, which could not have been easily specified in the CSP framework. The main technical tool we use and develop is holographic reductions. Another technical tool used in combination with holographic reductions is polynomial interpolations. The study of Holant Problems led us to discover and prove a complexity dichotomy theorem for the most general form of Boolean CSP where every constraint function takes values in the complex number field {C}.

Tighter Bounds for Facility Games

In one dimensional facility games, public facilities are placed based on the reported locations of the agents, where all the locations of agents and facilities are on a real line. The cost of an agent is measured by the distance from its location to the nearest facility. We study the approximation ratio of social welfare for strategy-proof mechanisms, where no agent can benefit by misreporting its location. In this paper, we use the total cost of agents as social welfare function. We study two extensions of the simplest version as in [9]: two facilities and multiple locations per agent. In both cases, we analyze randomized strategy-proof mechanisms, and give the first lower bound of 1.045 and 1.33, respectively. The latter lower bound is obtained by solving a related linear programming problem, and we believe that this new technique of proving lower bounds for randomized mechanisms may find applications in other problems and is of independent interest. We also improve several approximation bounds in [9], and confirm a conjecture in [9].

Graph Homomorphisms with Complex Values: A Dichotomy Theorem

Each symmetric matrix

Holographic Algorithms by Fibonacci Gates and Holographic Reductions for Hardness

\mathbf{A}

Holographic Algorithms with Matchgates Capture Precisely Tractable Planar_#CSP

over

Computational Complexity of Holant Problems

\mathbb{C}

Holographic algorithms: From art to science

defines a graph homomorphism function

Bases Collapse in Holographic Algorithms

Z_{\bf A}(\cdot)