Hau Chan

Make it or break it: Manipulating robustness in large networks

The function and performance of networks rely on their robustness, defined as their ability to continue functioning in the face of damage (targeted attacks or random failures) to parts of the network. Prior research has proposed a variety of measures to quantify robustness and various manipulation strategies to alter it. In this paper, our contributions are twofold. First, we critically analyze various robustness measures and identify their strengths and weaknesses. Our analysis suggests natural connectivity, based on the weighted count of loops in a network, to be a reliable measure. Second, we propose the first principled manipulation algorithms that directly optimize this robustness measure, which lead to significant performance improvement over existing, ad-hoc heuristic solutions. Extensive experiments on real-world datasets demonstrate the effectiveness and scalability of our methods against a long list of competitor strategies.

Truthful Multi-unit Procurements with Budgets

We study procurement games where each seller supplies multiple units of his item, with a cost per unit known only to him. The buyer can purchase any number of units from each seller, values different combinations of the items differently, and has a budget for his total payment. For a special class of procurement games, the bounded knapsack problem, we show that no universally truthful budget-feasible mechanism can approximate the optimal value of the buyer within ln n, where n is the total number of units of all items available. We then construct a polynomial-time mechanism that gives a 4(1 + ln n)-approximation for procurement games with concave additive valuations, which include bounded knapsack as a special case. Our mechanism is thus optimal up to a constant factor. Moreover, for the bounded knapsack problem, given the well-known FPTAS, our results imply there is a provable gap between the optimization domain and the mechanism design domain. Finally, for procurement games with sub-additive valuations, we construct a universally truthful budget-feasible mechanism that gives an \(O(\frac{\log^2 n}{\log \log n})\)-approximation in polynomial time with a demand oracle.

External Evaluation of Topic Models: A Graph Mining Approach

Given a topic and its top-k most relevant words generated by a topic model, how can we tell whether it is a low-quality or a high-quality topic? Topic models provide a low-dimensional representation of large document corpora, and drive many important applications such as summarization, document segmentation, word-sense disambiguation, etc. Evaluation of topic models is an important issue, since low-quality topics potentially degrade the performance of these applications. In this paper, we develop a graph mining and machine learning approach for the external evaluation of topic models. Based on the graph-centric features we extract from the projection of topic words on the Wikipedia page-links graph, we learn models that can predict the human-perceived quality of topics (based on human judgments), and classify them as high or low quality. Experiments on four real-world corpora show that our approach boosts the prediction performance up to 30% over three baselines of various complexities, and demonstrate the generality of our method to diverse domains. In addition, we provide an interpretation of our models and outline the discriminating characteristics of topic quality.

POMDPs for Assisting Homeless Shelters – Computational and Deployment Challenges

This paper looks at challenges faced during the ongoing deployment of HEALER, a POMDP based software agent that recommends sequential intervention plans for use by homeless shelters, who organize these interventions to raise awareness about HIV among homeless youth. HEALER’s sequential plans (built using knowledge of social networks of homeless youth) choose intervention participants strategically to maximize influence spread, while reasoning about uncertainties in the network. In order to compute its plans, HEALER (i) casts this influence maximization problem as a POMDP and solves it using a novel planner which scales up to previously unsolvable real-world sizes; (ii) and constructs social networks of homeless youth at low cost, using a Facebook application. HEALER is currently being deployed in the real world in collaboration with a homeless shelter. Initial feedback from the shelter officials has been positive but they were surprised by the solutions generated by HEALER as these solutions are very counter-intuitive. Therefore, there is a need to justify HEALER’s solutions in a way that mirrors the officials’ intuition. In this paper, we report on progress made towards HEALER’s deployment and detail first steps taken to tackle the issue of explaining HEALER’s solutions.

An FPTAS for Computing Nash Equilibrium in Resource Graph Games

We consider the problem of computing a mixedstrategy Nash equilibrium (MSNE) in resource graph games (RGGs), a compact representation for games with an exponential number of strategies. At a high level, an RGG consists of a graphical representation of utility functions together with a representation of strategy spaces as convex polytopes. RGGs are general enough to capture a wide variety of games studied in literature, including congestion games and security games. In this paper, we provide the first Fully Polytnomial Time Approximation Scheme (FPTAS) for computing an MSNE in any symmetric multilinear RGG where its constraint moralized resource graph has bounded treewidth. Our FPTAS can be generalized to compute optimal MSNE, and to games with a constant number of player types. As a consequence, our FPTAS provides new approximation results for security games, network congestion games, and bilinear games.

Maximizing Awareness about HIV in Social Networks of Homeless Youth with Limited Information

This paper presents HEALER, a software agent that recommends sequential intervention plans for use by homeless shelters, who organize these interventions to raise awareness about HIV among homeless youth. HEALER’s sequential plans (built using knowledge of social networks of homeless youth) choose intervention participants strategically to maximize influence spread, while reasoning about uncertainties in the network. While previous work presents influence maximizing techniques to choose intervention participants, they do not address two real-world issues: (i) they completely fail to scale up to real-world sizes; and (ii) they do not handle deviations in execution of intervention plans. HEALER handles these issues via two major contributions: (i) HEALER casts this influence maximization problem as a POMDP and solves it using a novel planner which scales up to previously unsolvable real-world sizes; and (ii) HEALER allows shelter officials to modify its recommendations, and updates its future plans in a deviationtolerant manner. HEALER was deployed in the real world in Spring 2016 with considerable success.

Game-Theoretic Goal Recognition Models with Applications to Security Domains

Motivated by the goal recognition (GR) and goal recognition design (GRD) problems in the artificial intelligence (AI) planning domain, we introduce and study two natural variants of the GR and GRD problems with strategic agents, respectively. More specifically, we consider game-theoretic (GT) scenarios where a malicious adversary aims to damage some target in an (physical or virtual) environment monitored by a defender. The adversary must take a sequence of actions in order to attack the intended target. In the GTGR and GTGRD settings, the defender attempts to identify the adversary’s intended target while observing the adversary’s available actions so that he/she can strengthens the target’s defense against the attack. In addition, in the GTGRD setting, the defender can alter the environment (e.g., adding roadblocks) in order to better distinguish the goal/target of the adversary.

Using social networks to raise HIV awareness among homeless youth

Many homeless shelters conduct interventions to raise awareness about HIV (human immunodeficiency virus) infection among homeless youth. Because of human and financial resource shortages, these shelters need to choose intervention attendees strategically, to maximize awareness through the homeless youth social network. In this work, we propose HEALER (hierarchical ensembling-based agent, which plans for effective reduction in HIV spread), an agent that recommends sequential intervention plans for use by homeless shelters. HEALERs sequential plans (built using knowledge of homeless youth social networks) strategically select intervention participants to maximize influence spread, by solving POMDPs (partially observable Markov decision processes) on social networks using heuristic ensemble methods. In this paper, we explore the motivations behind the design of HEALER and analyze the performance of HEALER in simulations on real-world networks. First, we provide a theoretical analysis of the DIME (dynamic influence maximization under uncertainty) problem, the main computational problem that HEALER solves. HEALER relies on heuristic methods for solving the DIME problem due to its computational hardness. Second, we explain why heuristics used within HEALER work well on real-world networks. Third, we present results comparing HEALER to baseline algorithms augmented by the heuristics of HEALER. HEALER is currently being tested in real-world pilot studies with homeless youth in Los Angeles, California.

Interdependent Defense Games with Applications to Internet Security at the Level of Autonomous Systems

We propose interdependent defense (IDD) games, a computational game-theoretic framework to study aspects of the interdependence of risk and security in multi-agent systems under deliberate external attacks. Our model builds upon interdependent security (IDS) games, a model by Heal and Kunreuther that considers the source of the risk to be the result of a fixed randomized-strategy. We adapt IDS games to model the attacker’s deliberate behavior. We define the attacker’s pure-strategy space and utility function and derive appropriate cost functions for the defenders. We provide a complete characterization of mixed-strategy Nash equilibria (MSNE), and design a simple polynomial-time algorithm for computing all of them for an important subclass of IDD games. We also show that an efficient algorithm to determine whether some attacker’s strategy can be a part of an MSNE in an instance of IDD games is unlikely to exist. Yet, we provide a dynamic programming (DP) algorithm to compute an approximate MSNE when the graph/network structure of the game is a directed tree with a single source. We also show that the DP algorithm is a fully polynomial-time approximation scheme. In addition, we propose a generator of random instances of IDD games based on the real-world Internet-derived graph at the level of autonomous systems (≈27 K nodes and ≈100 K edges as measured in March 2010 by the DIMES project). We call such games Internet games. We introduce and empirically evaluate two heuristics from the literature on learning-in-games, best-response gradient dynamics (BRGD) and smooth best-response dynamics (SBRD), to compute an approximate MSNE in IDD games with arbitrary graph structures, such as randomly-generated instances of Internet games. In general, preliminary experiments applying our proposed heuristics are promising. Our experiments show that, while BRGD is a useful technique for the case of Internet games up to certain approximation level, SBRD is more efficient and provides better approximations than BRGD. Finally, we discuss several extensions, future work, and open problems.