# Papers

Here are some papers I’ve written with a high-level overview of what’s in them. I’ve included a notes section at the bottom of the page to help unpack the jargon.

• Optimal Admission Control for Multiclass Queues with Time-Varying Arrival Rates via State Abstraction
• Short Summary: This paper is about solving the problem of deciding in real-time which of a incoming deluge of tasks one should choose to take on, in the setting where there are $N$ workers available to do the tasks and there are far too many tasks for there to be any hope of doing all of them. Accepted to AAAI 2022.
• Co-authors: Marc Rigter, Parisa Hassanzadeh, Jason Long, Parisa Zehtabi, Daniele Magazzeni.
• Short Summary: In this work we propose a modification to the popular SHAP framework for Machine Learning model explanations, where our modification (Counterfactual Shapley) helps the consumer of the explanations change undesirable outcomes induced by the model. Accepted to FAccT 2022.
• Co-authors: Emanuele Albini, Jason Long, Daniele Magazzeni.
• Tradeoffs in Streaming Binary Classification under Limited Inspection Resources
• Short Summary: This work studies in detail the problem of imbalanced classification, where a machine learning model has to make yes/no decisions but the number of yes instances is very small compared to the number of no instances. We study the trade-offs that occur when you are only allowed to make a restricted number of yes/no decisions and the implications of this on the performance of the system. Accepted to ICAIF 2021.
• Co-authors: Parisa Hassanzadeh, Samuel Assefa, Prashant Reddy, Manuela Veloso.
• Counterfactual Explanations for Arbitrary Regression Models
• Short Summary: Counterfactual Explanations are a fancy phrase for a targeted “what-if?” scenario. The standard example is: if I am denied a loan by some model, what would I have to change about my circumstances to not be denied that loan? We extend this notion to regression models, i.e. those models whose output is a real number, not a simple yes/no. We characterise what is unique about this setting compared to the binary yes/no setting and derive optimal algorithms for finding these explanations.
• Co-authors: Thomas Spooner, Jason Long, Jon Shepard, Jiahao Chen, Daniele Magazzeni.
• Non-Parametric Stochastic Sequential Assignment With Random Arrival Times
• Short Summary: This paper is about solving the problem of deciding in real-time which of a incoming deluge of tasks one should choose to take on, in the setting where we can only choose to take on $n$ tasks over the course of a day and there are far more than $n$ tasks to choose from. Accepted to IJCAI 2021.
• Co-authors: Parisa Hassanzadeh, Samuel Assefa, Prashant Reddy.
• Calibrating Over-Parametrized Simulation Models: A Framework via Eligibility Set
• Short Summary: This paper tackles some technical issues inherent to simulation calibration, namely that if we have a complex simulation and are trying to match its output to observed data, there are many possible ways to do this and still have a convincing output. The paper proposes systematic solutions to this problem.
• Co-authors: Yuanlu Bai, Tucker Balch, Haoxian Chen, Henry Lam, Svitlana Vyetrenko.
• Get Real: Realism Metrics for Robust Limit Order Book Market Simulations
• Short Summary: This paper (written jointly with Svitlana Vyetrenko, David Byrd, Nick Petosa, Mahmoud Mahfouz, Manuela Veloso and Tucker Hybinette Balch) is a realism study of the open-source limit order book simulation ABIDES. The more realistic the simulation is, the more we can trust downstream tasks depending on the simulator, e.g. machine learning models for trading. We show that the simulator follows many stylised facts of real markets, but requires improvement in certain areas.
• Generating synthetic data in finance: opportunities, challenges and pitfalls
• Short Summary: This is a review paper about how the capability of generating synthetic data can be useful for the financial industry and which domains are worth focusing on for the research community.
• Co-authors: Samuel A Assefa, Mahmoud Mahfouz, Robert E Tillman, Prashant Reddy, Manuela Veloso.
• Perfect weak modular product graphs
• Short Summary: This paper is based on the combination of two ideas: i) (by Kozen) finding a clique of a given size in a particular graph product tells you that the two underlying graphs are isomorphic. ii) (by Lovasz) finding cliques in perfect graphs is easy. The corollary is that if this product is perfect, telling if the two graphs are isomorphic is easy. I find all pairs of graphs for which the product is perfect.
• Quantum linear systems algorithms: a primer
• Short summary: This is a review paper (written jointly with Mark Herbster, Peter Mountney, Simone Severini, Naïri Usher and Leonard Wossnig) focusing on algorithms for solving linear systems of equations on a quantum computer. The aim of the paper is to get a researcher in the field of classical linear systems solvers up to speed with the quantum algorithms used for this task.
• For every quantum walk there is a (classical) lifted Markov chain with faster mixing time
• Short Summary: Drawing samples from a probability distribution is an important computational task, used in fields such as randomised algorithms and machine learning. Quantum walks are an approach using a quantum computer that aim to speed up this task. In this paper I show that there is a classical method that is in some sense superior, that is, a random walk that mixes in fewer timesteps, using a technique called lifting.
• Quantum Inspired Algorithms For Graph Matching (Master’s Thesis).
• Short Summary: Graph kernels are a method for recognising patterns in data defined as graphs. In this work I define a graph kernel inspired by the theory of quantum walks, that compares favourably with state of the art* methods in graph classification.

*at the time of writing. This definitely isn’t true anymore in a field that moves so quickly… 😕

• Constructing graphs with limited resources
• Short Summary: This paper (coauthored with Simone Severini and Avinash Mocherla) considers the information inherent in creating graphs. We examine how instructions, memory and randomness can be used top create graphs such as forests and threshold graphs and find bounds on the minimal amounts of these resources needed to create such graphs (amongst others).

## Notes on papers

• Get Real: Realism Metrics for Robust Limit Order Book Market Simulations
• A limit order book simulator is a program that simulates the continuous double auction mechanism found in most large-scale securities markets, wherein traders place buy and sell orders at different prices in an order book continuously over a day.
• Stylised facts are statistical properties that are widely observed but are not defined rigorously, for instance using a $p$-value or a confidence interval. One might ask: why not show more rigorous properties of our simulator? The answer is that inherent noise in the markets mean it is rare to be able to make rigorous statistical statements even about historical market data – the best we have is stylised facts to compare against.
• Perfect weak modular product graphs
• A graph product is a particular way of combining two graphs to form a larger graph, with edges on the larger graph being present only if certain conditions are satisfied by the edges in the two underlying (factor) graphs.
• Graph isomorphism is the problem if determining if two graphs are the same (i.e. have the same edges) up to a relabelling of the vertices. This is an interesting problem because in practise it’s easy, but no one can find a generically ‘fast’ algorithm or prove that such an algorithm doesn’t exist. Usually we know one way or the other.
• A clique in a graph is a subset of vertices that are all connected to one another.
• A perfect graph is one in which (roughly speaking) the size of the largest clique tells you how many colours you can give the vertices such that no adjacent vertices have the same colour.
• Quantum linear systems algorithms: a primer
• A linear system of equations is a collection of two or more linear equations involving the same set of variables. The typical representation for such a system is $A x = b$, where $A$ is a matrix and $x$, $b$ are vectors. $A$ and $b$ are given to us and our job is to find $x$. The solution is $x = A^{-1} b$, where $A^{-1}$ is the inverse of $A$. There are many practical issues to computing this inverse matrix, discussed in the paper.
• For every quantum walk there is a (classical) lifted Markov chain with faster same mixing time
• A Markov chain is another name for a random walk on a graph. You start at some vertex, then with some predefined probability use move along one of the edges of the graph to an adjacent vertex. Continue this process for some time T.
• A quantum walk is the quantum analogue of a discrete-time random walk on a graph. At each timestep a quantum “coin” is flipped and then the walker moves around the vertices of the graph in superposition. After a certain number of timesteps we measure the location of the walker to draw a sample.
• When a random walk (quantum or classical) has mixed, this means that sampling from the distribution is equivalent to sampling from the infinite-time behaviour of the walk. The mixing time is how many timesteps it takes to mix.
• A lifting of a Markov chain is a random walk on a larger, “lifted”, graph. For each vertex in the original graph we assign at least one vertex in the lifted graph and every vertex in the lifted graph is associated to a vertex in the original graph. There can only be an edge between two vertices in the lifted graph if there is an edge in the original graph between the associated vertices.
• Quantum Inspired Algorithms For Graph Matching (Master’s Thesis).
• A graph kernel is a function that takes two graphs and outputs a number that tells you how similar they are.
• A quantum walk describes the motion of a quantum particle on a graph structure and is used in many quantum algorithms. This work is a classical algorithm using the mathematics describing a quantum walk.
• The task of graph classification is a machine learning task where we are given a large number of graphs, each with a known label, say yes or no. Then, given all this data the computer’s job is to classify new graphs as yes/no as quickly and accurately as possible.
• Quantum Inspired Algorithms For Graph Matching (Master’s Thesis).
• A forest graph is the disjoint union of tree graphs, where the disjoint union of two graphs can be thought of as just drawing them next to each other. A tree graph is a graph with no cycles.
• A threshold graph is a graph that can be constructed from a one-vertex graph by repeated applications of the following two operations:
1. Addition of a single isolated vertex to the graph.
1. Addition of a single dominating vertex to the graph, i.e. a single vertex that is connected to all other vertices.