Vaggos (Evangelos) Chatziafratis where first = vaggos

I am a 4th year PhD student in the Computer Science Department at Stanford University. I am working on approximation algorithms, combinatorial optimization and machine learning theory. Starting Spring 2019, I will be spending some of my time in Columbia University, NYC.

I am thrilled to be advised by Tim Roughgarden and to be part of our wonderful Theory Group. During my time at Stanford, I also had the privilege to work closely with Moses Charikar, Rad Niazadeh and Jan Vondrák.

Prior to Stanford, I received my Diploma in EECS from the National Technical University of Athens, Greece.

Recent News/Talks

  • 15 - 22 April'19: AISTATS and visit to Tokyo Institute of Technology to present our paper to their AI & Web Mining lab.
  • 29 March'19: Excited to present our work at Columbia's Theory Seminar.
  • 21 February - 7 March'19: Visiting Ioannis Panageas at Singapore University of Technology and Design. On the 27th, I will be presenting an overview of recent results on Hierarchical Clustering.
  • 19 February'19: Talk on Hierarchical Clustering in Facebook's Computer Vision group, Menlo Park.
  • 14&20 February'19: Talk on the Computational Power of Online Gradient Descent in Stanford's Theory and ML groups.
  • Excited to join Google New York for an internship next summer under Mohammad Mahdian and Vahab Mirrokni.
  • 12&13 December'18: Talk on Hierarchical Clustering at Google Zurich and ETH Zurich algorithms seminar.

Working Papers

  1. On the Computational Power of Online Gradient Descent. Vaggos Chatziafratis, Tim Roughgarden, Joshua R. Wang. under submission.
  2. On Correlation Clustering with Fairness Objectives. Vaggos Chatziafratis, Neha Gupta. work in progress.
  3. Bilu-Linial stability, certified algorithms and the Independent Set problem. Haris Angelidakis, Pranjal Awasthi, Avrim Blum, Vaggos Chatziafratis, Chen Dan. under submission.


  1. Hierarchical Clustering for Euclidean Data. Moses Charikar, Vaggos Chatziafratis, Rad Niazadeh, Grigory Yaroslavtsev. 22nd International Conference on Artificial Intelligence and Statistics, (AISTATS 2019), Naha, Okinawa (Japan), April 2019.
  2. Hierarchical Clustering better than Average-Linkage. Moses Charikar, Vaggos Chatziafratis, Rad Niazadeh. 30th ACM-SIAM Symposium on Discrete Algorithms (SODA 2019), San Diego (CA), January 2019.
  3. Hierarchical Clustering with Structural Constraints. Vaggos Chatziafratis, Rad Niazadeh, Moses Charikar. 35th International Conference on Machine Learning (ICML 2018), Stockholm (Sweden), July 2018. [10min ICML Video]
  4. Attack vulnerability of power systems under an equal load redistribution model. Talha Cihad Gulcu, Vaggos Chatziafratis, Yingrui Zhang, and Osman Yağan. IEEE/ACM Transactions on Networking (ToN 2018), 26(3): 1306-1319, June 2018. (Journal).
  5. Stability and Recovery for Independence Systems. Vaggos Chatziafratis, Tim Roughgarden, Jan Vondrák. 25th European Symposium on Algorithms (ESA 2017), Vienna (Austria), September 2017.
  6. Approximate Hierarchical Clustering via Sparsest Cut and Spreading Metrics. Moses Charikar, Vaggos Chatziafratis. 28th ACM-SIAM Symposium on Discrete Algorithms (SODA 2017), Barcelona (Spain), January 2017.
  7. On the robustness of power systems: optimal load-capacity distributions and hardness of attacking. Vaggos Chatziafratis, Yingrui Zhang, Osman Yağan. 11th Information Theory and Applications Workshop (ITA 2016), San Diego (CA), February 2016. (Invited).

Professional Experience

  • Summer 2018: Facebook Internship in Core Data Science under Julian Mestre and Nicolas Stier.
  • Summer 2017: Research Assistant at Stanford University under Greg Valiant.
  • Summer 2016: Research Internship at École Polytechnique Federal de Lausanne (EPFL), Switzerland.
  • Summer 2015: Software Engineering Internship at CERN in Switzerland.
  • Winter 2014: Software Engineering Internship at Lappeenranta University of Technology, Finland.

Teaching Experience


  • Leadership at Stanford:
    • President of HELLAS (Hellenic Association at Stanford): Organizing Stanford-wide cultural events.
    • Stanford GSPB (Graduate Student Programming Board): Organizing social events for the grad community.
  • Arts:
    • Music: Diploma in Classical Piano and Music Theory, Greece. Student of Kumaran Arul at Stanford's Music Department.
    • Theater: Playback Theater under Omer Reingold.
  • Volunteering:
    • Mentoring Greek students through Global Prep by raising awareness and providing information about grad school in the US.
    • Organizing annual Educational Trip with the collaboration of MIT, Princeton, UC Berkeley, UC San Diego, Georgia Tech.
    • National Hellenic Student Association: NHSA's Executive Board Member.