# Upcoming ISE Seminars

We study the design of reliably connected networks. Given a graph with arcs that may fail at random, the goal is to select a minimum cost set of arcs such that a connectivity requirement is met with high probability. We first compare this model with a well-known deterministic model of reliable network design: survivable network design. We demonstrate that, if distributional information on arc failures is known, the chance constraint model can yield a significantly richer set of solutions on the efficient frontier of reliability and cost. We then present two solution approaches for our model, which we formulate as a chance-constrained stochastic integer program. The first approach is based on a formulation that uses binary variables to determine if the connectivity requirement is satisfied in each arc failure scenario, and enforces the connectivity requirement in selected scenarios using scenario-based graph cuts. We derive additional classes of valid inequalities for this formulation and study their facet-inducing properties. The second formulation is based on the idea of probabilistic graph cuts, which is an extension of graph cuts to graphs with random arc failures. Inequalities defined by such cuts are sufficient to define the set of feasible solutions and can be separated efficiently at integer solutions, allowing this formulation to be solved by a branch-and-cut algorithm. Computational results will be presented.

This is joint work with Yongjia Song.

The role of hospital bed management staff and processes has gained increased attention in recent years due to the impact of bed management practices on hospital performance metrics including average boarding time, patient safety, overflow rate, and patient diversions. One of the key tasks of the bed manager is to balance the available capacity with competing requests for beds, accounting for the unique requirements and clinical characteristics of the patients, and expectations about future demand and patient discharges. Due to the constantly changing system state, the process of matching patients with beds is similar to a dynamic assignment problem. We identify structural properties of the bed assignment process, by modeling the hospital system as a tandem queueing network with multiple customer classes and cross-trained server pools. Utilizing these properties we develop new algorithms for making dynamic assignment decisions and test the performance against current hospital bed assignment practices with simulation.

The first part of this talk reviews some modern randomized linear algebra techniques. The goal of these methods is to perform approximate matrix multiplication or matrix factorizations (e.g., SVD) with lower computational cost than conventional methods. We then discuss using these methods inside optimization algorithms. The two main questions are (1) is the randomized approach faster, and (2) does the error from the randomized linear algebra affect the optimization algorithms? There are powerful recent results that bound the error of the linear algebra, but they are rarely applied to obtain rigorous guarantees for an optimization algorithm. We also touch on stochastic gradient descent, and apply all of these topics to some specific matrix recovery problems, such as quantum state tomography and matrix completion of the Netflix dataset.

For all-quadratic problems (without any linear constraints), it is well known that the semidefinite relaxation coincides basically with the Lagrangian dual problem. Here we study a more general case where the constraints can be either quadratic or linear. To be more precise, we include explicit sign constraints on the problem variables, and study both the full Lagrangian dual as well as the Semi-Lagrangian

relaxation. We show that the stronger Semi-Lagrangian dual bounds coincide with the ones resulting from copositive relaxation. This way, we arrive at a full hierarchy of tractable conic bounds stronger than the usual Lagrangian dual (and thus than the SDP) bounds. We also specify sucient conditions for tightness of the Semi-Lagrangian (i.e. copositive) relaxation and show that copositivity of the slack

matrix guarantees global optimality for KKT points of this problem.

For efficient and physically reliable numerical computations for time dependent differential equations it is very important to find positively invariant subsets of the state space and determine time step sizes of the numerical method that guarantee this. In this talk, we present the state-of-the-art theory and novel approaches beyond it to find positively invariant sets and the suitable time step sizes via optimization techniques.

Bayesian statistical models can be used to represent the beliefs of a decision-maker about an uncertain environment. For example, in revenue management, a seller formulates beliefs about customers' willingness to pay; in energy, we may have a belief about the suitability of a candidate location for a new wind farm. Conjugate priors model the evolution of these beliefs over time as new information is collected, either from stochastic simulation or field experiments. However, there are relatively few of these conjugate models, and they simply do not exist in many problems of interest. We show how approximate Bayesian inference can be used to create computationally tractable, "nearly conjugate" models that optimally approximate the actual belief distributions and enable the use of anticipatory information collection and optimization policies. We consider two broad problem classes: simulation selection with unknown correlation structures, and Bayesian learning in logistic regression.

We provide a new proof of the fact that Mathematical Programming is Turing-complete, and show how it can be useful in the analysis of code, by presenting two applications. The first aims to find hard inputs for given programs. The second finds relaxations of the set of values taken by the program variables during execution, without actually executing the code.

Joint work with Fabrizio Marinelli, Universita Politecnica delle Marche.

A major challenge for machine learning in the next decade is the development of methods that continuously predict and learn from a stream of data. The scale of data and the non-stationary nature of the environment make the prevalent "learn from a batch of i.i.d. data" paradigm inadequate. In this talk, we will formally define the problem of sequential prediction as a (computationally difficult) optimization problem. We will present novel techniques for approximately solving it. These techniques recover many prediction algorithms known in the literature, but also yield new and surprising methods with near-optimal performance. The advances are possible due to a confluence of ideas from empirical process theory, game theory, and optimization. The field of sequential prediction is largely unexplored, and we hope to present a framework that might be of interest to the optimization community.

Over the past 10 years there has been a growing body of work at the intersection of mathematical programming as commonly studied in Operations Research and constraint programming (CP) with its origins in Artificial Intelligence (AI) and programming languages. Much of CP's success in solving challenging combinatorial problems comes from the exploitation of inference as embodied in so-called global constraints. This talk provides a brief introduction to CP and then discusses two patterns of hybridization of mixed integer programming (MIP) and CP that have been proposed. I show how these patterns can be used to solve hard combinatorial problems and that they can also inform each other, leading to a more general view of hybridizations centred around the exploitation of problem structure embodied in global constraints. In particular, I propose that the further development of solving techniques associated with global constraints leads to novel research directions in both mathematical and constraint programming.

The pooling problem is a challenging non-convex optimization problem that is motivated by refinery processes in the petroleum industry, and also finds application in other areas, such as waste water treatment, emissions regulation and agricultural industry among others. In this talk, we will present an analysis of mixed integer linear programming (MILP) based techniques to solve the pooling problem. In particular, the talk will focus on three results: (1) We will analyze the quality of dual bounds produced by solving MILP based relaxations of the pooling problem. These MILP relaxations are derived using piecewise-linear approximations for bilinear terms appearing in the model for the pooling problem. (2) We will present an approximation algorithm for the pooling problem and show that unless NP-hard problems have randomized polynomial time algorithms, this algorithm obtains the best possible approximation ratio. (3) Motivated by the approximation algorithm we will present a MILP based heuristic for the pooling problem. This heuristic is guaranteed to provide solutions within a factor of n, where n is the number of output nodes. On medium and large-scale test instances, this MILP based heuristic performs surprisingly well. In particular, it finds the best known solutions for many instances reported in the literature. This is joint work with Akshay Gupte.