Zikai Xiong

I am a postdoctoral fellow at Georgia Tech ISyE and an incoming tenure-track Assistant Professor at Northwestern IEMS.

About Me

I am a postdoctoral fellow at the Georgia Tech H. Milton Stewart School of Industrial and Systems Engineering (ISyE) and the Algorithms and Randomness Center (ARC), working with Prof. Katya Scheinberg. I will join the Northwestern University Department of Industrial Engineering and Management Sciences (IEMS) as a tenure-track Assistant Professor in Fall 2026. I received my Ph.D. from the MIT Operations Research Center (ORC), advised by Prof. Robert Freund. My current research interests are mainly in optimization, with twin interests in theoretical foundations and computational practice.

My research up to now has involved enhancing the scalability of optimization algorithms for solving large-scale linear and convex optimization -- both generically as well as on specific applications. I am delighted that some of my methods have already been implemented in the state-of-art commercial solvers. My work received second place in the George B. Dantzig Dissertation Award, won the INFORMS Optimization Society Student Paper Award, and was a finalist in the George Nicholson Student Paper Competition. My overarching aspiration is to extend the capability of modern optimization methods to effectively solve problems arising in operations research, data science, and machine learning/AI.

For students: I work with a small number of motivated students each year. Northwestern students are welcome to contact me. If you are not yet a Northwestern student and hope to pursue a PhD working with me, please apply to the Northwestern IEMS PhD program. You are also welcome to email a brief note about yourself. I do my best to read all messages, though I may not be able to respond to each one.

News

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Research

(alphabetical authorship marked with *)

PhD Dissertation

• New Theory and New Practical Methods for Solving Large-Scale Linear and Conic Optimization
  PhD Dissertation
  ⭐ Second Place, 2025 George B. Dantzig Dissertation Award

Papers Published or Under Review at Journals

• PDCS: A Primal-Dual Large-Scale Conic Programming Solver
  Zhenwei Lin, Zikai Xiong, Dongdong Ge and Yinyu Ye paper
  Under review
• High-Probability Polynomial-Time Complexity of Restarted PDHG for Linear Programming
  Zikai Xiong paper
  Under revision at Mathematics of Operations Research
• Accessible Theoretical Complexity of the Restarted Primal-Dual Hybrid Gradient Method for Linear Programs with Unique Optima
  Zikai Xiong paper slides
  Under revision at Mathematics of Operations Research
  ⭐ Winner, 2025 INFORMS Optimization Society Student Paper Prize
• The Role of Level-Set Geometry on the Performance of PDHG for Conic Linear Optimization
  Zikai Xiong and Robert M. Freund paper slides
  Under review at Mathematical Programming
  ⭐ Finalist, 2024 George Nicholson Student Paper Competition
• Computational Guarantees for Restarted PDHG for LP based on "Limiting Error Ratios" and LP Sharpness
  Zikai Xiong and Robert M. Freund paper slides additional results
  Under revision at Mathematical Programming
  ⭐ Honorable Mention, 2024 INFORMS Optimization Society Student Paper Prize
• Using Taylor-Approximated Gradients to Improve the Frank-Wolfe Method for Empirical Risk Minimization
  Zikai Xiong and Robert M. Freund paper slides additional results and extensions
  SIAM Journal on Optimization
• Two Sides of "Hidden City Ticketing": Analysis of a Choice-Based Network Revenue Management Model
  Jiacheng Guo*, Bo Jiang*, Zikai Xiong*, and Nanxi Zhang* paper (version at Oct 14, 2025)
  Under review at Operations Research
• From an Interior Point to a Corner Point: Smart Crossover
  Dongdong Ge*, Chengwenjian Wang*, Zikai Xiong*, and Yinyu Ye* papercodesslides
  INFORMS Journal on Computing
• Low-rank Traffic Matrix Completion with Marginal Information
  Zikai Xiong, Renjie Xu, Yanwei Xu, and Yimin Wei paper
  Journal of Computational and Applied Mathematics

Papers Published at Conferences

• Fair Wasserstein Coresets
  Zikai Xiong, Niccolò Dalmasso, Shubham Sharma, Freddy Lecue, Daniele Magazzeni, Vamsi K. Potluru, Tucker Balch, Manuela Veloso paper
  Preliminary version appeared in NeurIPS 2023 Workshop SyntheticData4ML (Oral Presentation)
  Neural Information Processing Systems (NeurIPS 2024)
• FairWASP: Fast and Optimal Fair Wasserstein Pre-processing
  Zikai Xiong, Niccolo Dalmasso, Alan Mishler, Vamsi Potluru, Tucker Balch, Manuela Veloso paper
  AAAI Conference on Artificial Intelligence (AAAI 2024)
• Learning from Multiple Annotator Noisy Labels via Sample-wise Label Fusion
  Zhengqi Gao, Fan-Keng Sun, Mingran Yang, Sucheng Ren, Zikai Xiong, et al. paper codes
  European Conference on Computer Vision (ECCV 2022)
• Interior-Point Methods Strike Back: Solving the Wasserstein Barycenter Problem
  Dongdong Ge*, Haoyue Wang*, Zikai Xiong*, and Yinyu Ye* papercodesslides
  Neural Information Processing Systems (NeurIPS 2020)

Technical Reports

• On the Relation Between LP Sharpness and Limiting Error Ratio and Complexity Implications for Restarted PDHG
  Zikai Xiong and Robert M. Freund technical report

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Experience

• Research Associate Intern, J.P. Morgan Chase AI Research, Summer 2023
  Managers: Niccolò Dalmasso, Vamsi Potluru, and Manuela Veloso
  Algorithmic fairness in machine learning and large-scale optimization.

• Research Assistant, MIT Sloan School of Management, 2020 – 2025
  Supervisor: Robert M. Freund
  Modern optimization methods for huge-scale OR and ML problems.

• Research Assistant, Shanghai University of Finance and Economics, 2019 – 2020
  Supervisors: Yinyu Ye (Stanford), Dongdong Ge (Shanghai Jiao Tong University)
  Optimization for operations research and machine learning.
  

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Teaching and Mentoring

Teaching

• Winter 2024: 15.S60 Computing in Optimization and Statistics
  - Instructor for the advanced optimization sessions of a computing course during MIT January Independent Activities Period. The duties are designing courses, creating teaching materials, and giving lectures.
  - Jupyter Notebook 1 Jupyter Notebook 2

• Fall 2022 and Fall 2023: 15.081 Introduction to Mathematical Programming (Teaching score: 6.8/7 Student feedback])
  - Head TA for the MIT’s doctoral introductory optimization course (for ORC and other MIT PhD programs)
  - Recitation Notes Recitation Solutions (Permission Required)
  ⭐ I was voted as the "Best TA" in the ORC Superlatives voting in 2023 (photo)
  ⭐ "Best TA" again in 2024 (photo)

• Spring 2022: 15.071 The Analytics Edge (Teaching score: 6.4/7 Student feedback])
  - TA for MBA course in business analytics at Sloan School of Management.

• Online Course in Data Science and Business Analytics
  - Designed course materials of a new online course in data science and business analytics.
  - Supervisor: Robert Freund

Mentoring

I informally mentored several students on research projects.

• Zhenwei Lin – PhD student, Shanghai University of Finance and Economics (later became a postdoctoral researcher at Purdue University)
• Qiushi Han – Undergraduate, UIUC (later joined the PhD program at MIT)
• Chenghan Xie – Undergraduate, Fudan University (later joined the PhD program at Stanford)
• Jiacheng Guo – Undergraduate, Fudan University (later joined the PhD program at Princeton)
• Chengwenjian Wang – Undergraduate, Fudan University (later joined the Master program at ETH Zürich and the PhD program at the University of Minnesota Twin Cities)

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Talks

• PDCS: A Primal-Dual Large-Scale Conic Programming Solver with GPU Enhancements
  - INFORMS Annual Meeting, Atlanta, GA, October 2025
  - cuOPT Team, NVIDIA, virtual, July 2025
  - Computational Robotics Group Seminar, Harvard University, Cambridge, MA, May 2025
• New Theory and New Practical Methods for Solving Large-Scale Linear and Conic Optimization slides
  - INFORMS Annual Meeting (Danzig Dissertation Competition Session), Atlanta, GA, October 2025
  - PhD Thesis Defense, MIT, Cambridge, MA, May 2025
• Theoretical Foundations and Practical Improvements for Large-Scale Linear Programming slides
  - Data Science Lab Seminar, MIT, Cambridge, MA, April 2025
  - Georgia Institute of Technology, Atlanta, GA, April 2025
  - Columbia University, New York, NY, February 2025
  - University of Chicago, Chicago, IL, February 2025
  - Cornell University, Ithaca, NY, January 2025
  - Northwestern University, Evanston, IL, January 2025
  - University of British Columbia, Vancouver, Canada, January 2025
  - NC State University, Raleigh, NC, January 2025
  - Chinese University of Hong Kong (CUHK), Hong Kong, January 2025
  - Hong Kong University of Science and Technology (HKUST), Hong Kong, December 2024
• Level-Set Geometry and Improving the Performance of PDHG for Linear Optimization slides
  - OM Seminar, MIT, Cambridge, MA, September 2024
  - Cornell ORIE Young Researchers Workshop, Ithaca, NY, October 2024
  - INFORMS Annual Meeting, Seattle, WA, October 2024
  - INFORMS Annual Meeting (Nicholson Student Paper Competition), Seattle, WA, October 2024
• Theoretical Complexity of PDHG for Linear Programs with Unique Optima slides
  - Conference in honor of Yinyu Ye’s retirement, Shanghai, China, July 2024
  - International Symposium on Mathematical Programming (ISMP), Montréal, Canada, July 2024
  - Modeling and Optimization: Theory and Applications (MOPTA), Lehigh University, PA, August 2024
  - ARC Colloquium, Georgia Tech, Atlanta, GA, September 2025
  - INFORMS Annual Meeting (Optimization Society Award Session), Atlanta, GA, October 2025
• Improving the Geometry of (Conic) Linear Optimization Problems for the Primal-Dual Hybrid Gradient Method (PDHG) slides
  - Workshop on Modern Continuous Optimization, Cambridge, MA, August 2023
  - ORC Student Seminar, MIT, Cambridge, MA, October 2023
  - INFORMS Annual Meeting, Phoenix, AZ, October 2023
  - Invited formal talk at Shanghai University of Finance and Economics, Shanghai, China, September 2023
  - LIDS Conference, MIT, Cambridge, MA, February, 2024
  - INFORMS Optimization Society Conference, Houston, TX, March 2024
  - Modeling and Optimization: Theory and Applications (MOPTA), Lehigh University, PA, August 2024
• Geometric Condition Measures for the Primal-Dual Hybrid Gradient Method for Linear Programming slides
  - SIAM Conference on Optimization (SIOPT), Seattle, WA, June 2023
  - ORC Student Seminar, MIT, Cambridge, MA, March 2023
• Using Taylor-Approximated Gradients to Improve the Frank-Wolfe Method for Empirical Risk Minimization slides
  - International Conference on Continuous Optimization (ICCOPT), Lehigh University, Bethlehem, PA, July 2022
  - ORC Student Seminar, MIT, Cambridge, MA, October 2022
  - INFORMS Annual Meeting, Indianapolis, IN, October 2022
• From an Interior Point to A Corner Point: Smart Crossover slides
  - INFORMS Annual Meeting, Indianapolis, IN, October 2022
• Interior-Point Methods Strike Back: Solving the Wasserstein Barycenter Problem slides
  - INFORMS Annual Meeting, Seattle, WA, October 2019
  - "Heart of the Machine" Seminar, Shanghai, China, October 2019
• Computing the Wasserstein Barycenter Efficiently: A Structured Linear System and Customized Algorithms
  - Seminar of New Advances in Theory and Application of ADMM, Shanghai, China, March 2019

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Service

Reviewer

• Journal: SIAM Journal on Optimization, Mathematical Programming, Journal of Optimization Theory and Applications, Tutorials in Operations Research, Optimization Letters

• Conference: ICML, NeurIPS, AAAI

Session (Co-)Chair

• INFORMS Annual Meeting, Seattle, WA, October 2024
• International Symposium on Mathematical Programming (ISMP), Montréal, Canada, July 2024
• INFORMS Optimization Society Conference, Houston, TX, March 2024
• INFORMS Annual Meeting, Indianapolis, IN, October 2022

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Awards and Honors

• Winner, INFORMS Optimization Society Student Paper Prize, 2025
• Second Place, George B. Dantzig Dissertation Award, 2025
• Finalist, George Nicholson Student Paper Competition, 2024
• Honorable Mention, INFORMS Optimization Society Student Paper Prize, 2024
• Honorable Mention, MIT ORC Best Student Paper Competition, 2024
• First Prize, MIT ORC Common Experience Deep Learning Challenge, 2021
• INFORMS Optimization Society Conference Travel Award, 2024
• SIAM Travel Award, 2021
• MIT Graduate Student Council Travel Grant, 2022
• Star Graduate Award, Fudan University, 2020
• Top Outstanding Undergraduate Student Award, Fudan University, 2019

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My Photo Gallery

See here for some interesting photos from my travels and my life.

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