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A list of all the posts and pages found on the site. For you robots out there is an XML version available for digesting as well.
Pages
Posts
New Project, New Challenges
Published:
After 6 years of working in the AI research group, I have taken up a research fellow postdoc position in the real-time and distributed systems group, specifically working on the SCHEME project (safety-critical harsh environment micro-processor evolution). I am applying many of the techniques from previous work such as feature engineering, algorithm selection, constraint programming in order to solve problems in robotics system.
Two Departments, One Building, Let’s Play
Published:
In the summer of 2025, the maths department moved in alongside computer science into the Ian Wand building. The table tennis table in our courtyard stood mostly unused, as did the table-football in the communal pod. I organised a table sports tournament to encourage people from both departments to have some fun getting to know each other over a game.
Co-chairing the 25th Edition of ModRef
Published:
ModRef has been an excellent venue for presenting cutting edge research, at the work in progress stage for researchers interested in constraint programming. As a PhD student, I found it a valuable stepping stone, enabling me to polish and improve my work before submitting it for publication.
Going to Glasgow for CP 2025
Published:
Glasgow, here we come! It was very exciting to be a co-author on two papers accepted to CP2025 in Glasgow. One paper presents a constraint model used by the European Southern Observatory to schedule observations on its very large telescope system. The other paper uses the Klondike solitaire game to showcase some modelling techniques and some CP-based algorithm scheduling approaches.
Fun with LinkedIn Queens and Constraints
Published:
I’ve enjoyed solving the LinkedIn Queens puzzle most mornings since they introduced it earlier this year. The problem requires the player to place the queens on a board according to a very brief set of constraints (or rules). For a bit of fun, I wanted to solve the puzzle automatically (or as close as possible) using constraint programming.
PhD Graduation Ceremony
Published:
It was a wonderful occasion to reflect on and celebrate the PhD experience with colleagues at a very special graduation ceremony organised by the University of York. Having missed my original BSc graduation back in 2000, it was great to finally wear the gown and receive the award on stage. I will always be grateful to all those people who made it possible for me to enage in these years of study, training and research.
PhD Thesis Published
Published:
After minor corrections, my thesis dissertation was approved and is now live at the White Rose E-theses Repository. This completes a very eventful, challenging and rewarding 5 years since stepping out of my career as a school teacher to pursue a long-held ambition to learn more and engage in cutting edge research. I am very grateful to my supervisors for all their help and to my family for all their support.
Teaching on AI Course
Published:
I’m getting the chance to be back at the front of a class! I’ve been employed as a part-time lecturer, in order to teach part of a Masters level course on AI: Problem Solving with Search here at the University of York. I will be co-teaching with the module leader, and it involves everything from writing lectures, labs and quizzes to setting, marking and moderating the exams.
First First-author Journal Article
Published:
Our paper Learning to select SAT Encodings for pseudo-Boolean and linear integer constraints has now been published in the Constraints journal. This paper was an extended version of the work presented at CP2022, with more encodings, more analysis and the first detailed description of Savile Row’s Tree SAT encoding.
PhD Thesis Submitted
Published:
Four eventful years after stepping out of school as a teacher back into university as a student I submitted by PhD Thesis on the topic of Learning SAT Encodings for Constraint Satisfaction Problems. What a relief! I’m looking forward to picking up some of the research projects which were on hold while the write-up was completed. And of course awaiting the viva!
YPAD leading to Associate Fellowship of the HEA
Published:
After completing the York Professional and Academic Development scheme (YPAD) and submitting a report of my teaching-related experience I was awarded Associate Fellowship of the Higher Education Academy. As part of my report I carried out a brief investigation into using summative assessment results to inform teaching and assessment in the next cohort. Per-question breakdown of the marks allows some rudimentary statistical analysis which can shed light on how questions could be improved or which concepts and skills might be particularly difficult for students.
The Bookshelves Problem
Published:
It was good fun to submit an optimisation problem to the CSPLib web site. This was a real-life problem (or opportunity?) to use some discarded planks of wood to build a set of bookshelves. The idea was to create some bookshelves which would accommodate books of a mininum height and would maximise the shelf space given the collection of planks I was left with.
GTA of the Year Award
Published:
I was very honoured to receive the “Graduate Teaching Assistant of the Year” award after “GTA-ing” in a number of contexts, both in-person and remotely during the pandemic.
portfolio
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Short description of portfolio item number 1
Portfolio item number 2
Short description of portfolio item number 2
publications
SAT encodings for Pseudo-Boolean constraints together with at-most-one constraints
Published in Artificial Intelligence, 2021
A thorough presentation of SAT encodings for pseudo-boolean constraints optimised for the existence of at-most-one constraints on the variables in their scope. The first paper I was able to contribute to during my PhD.
Recommended citation: Miquel Bofill, Jordi Coll, Peter Nightingale, Josep Suy, Felix Ulrich-Oltean, Mateu Villaret, "SAT encodings for Pseudo-Boolean constraints together with at-most-one constraints", Artificial Intelligence, Volume 302, 2022, 103604, ISSN 0004-3702. https://doi.org/10.1016/j.artint.2021.103604
Learning to select SAT encodings for pseudo-Boolean and linear integer constraints
Published in Constraints, 2021
Extended journal version of the work in the first two technical chapters of my PhD thesis
Recommended citation: Felix Ulrich-Oltean, Peter Nightingale & James A. Walker, "Learning to select SAT encodings for pseudo-Boolean and linear integer constraints". Constraints 28, 397–426 (2023). https://doi.org/10.1007/s10601-023-09364-1
Learning When to Use Automatic Tabulation in Constraint Model Reformulation
Published in Proceedings of the Thirty-Second International Joint Conference on Artificial Intelligence , 2023
Using ML strategies to decide whether to reformulate parts of constraint models as table constraints.
Recommended citation: Carlo Cena, Özgür Akgün, Zeynep Kiziltan, Ian Miguel, Peter Nightingale, Felix Ulrich-Oltean, "Learning When to Use Automatic Tabulation in Constraint Model Reformulation", Proceedings of the Thirty-Second International Joint Conference on Artificial Intelligence Main Track. Pages 1902-1910. https://doi.org/10.24963/ijcai.2023/211
Learning SAT Encodings for Constraint Satisfaction Problems
Published in White Rose E-theses Repository, 2024
My PhD thesis on using ML to select SAT encodings for constraints
Recommended citation: Felix Ulrich-Oltean, "Learning SAT Encodings for Constraint Satisfaction Problems", PhD Thesis, University of York, March 2024. https://etheses.whiterose.ac.uk/34581/
IndiCon: Selecting SAT Encodings for Individual Pseudo-Boolean and Linear Integer Constraints
Published in Proceedings of the 36th International Conference on Tools with Artificial Intelligence (ICTAI 2024) , 2024
Learning to select SAT encodings for individual constraints in a CSP instance.
Recommended citation: Felix Ulrich-Oltean, Peter Nightingale, James A. Walker, "IndiCon: Selecting SAT Encodings for Individual Pseudo-Boolean and Linear Integer Constraints", Proceedings of the 36th International Conference on Tools with Artificial Intelligence (ICTAI 2024) https://pure.york.ac.uk/portal/en/publications/indicon-selecting-sat-encodings-for-individual-pseudo-boolean-and
TabID: Automatic Identification and Tabulation of Subproblems in Constraint Models
Published in Journal of Artificial Intelligence Research, 2025
One of the first papers I was able to contribute to during my PhD, conducting extensive experiments, and presenting complex results concisely.
Recommended citation: Akgün, Özgür, Ian Gent, Christopher Jefferson, Zeynep Kiziltan, Ian Miguel, Peter Nightingale, András Salamon, and Felix Ulrich-Oltean. "TabID: automatic identification and tabulation of subproblems in constraint models." Journal of Artificial Intelligence Research 82 (2025): 1999-2056. https://doi.org/10.1613/jair.1.17032
Scheduling Telescope Observations for the European Southern Observatory
Published in 31st International Conference on Principles and Practice of Constraint Programming (CP 2025), 2025
Fascinating application of constraint programming to scheduling scientific observation on an array of telescopes which could be rearranged according to complex constraints.
Recommended citation: Michael Prümm, Peter Nightingale, Felix Ulrich-Oltean, "Scheduling Telescope Observations for the European Southern Observatory", 31st International Conference on Principles and Practice of Constraint Programming (CP 2025) https://doi.org/10.4230/LIPIcs.CP.2025.43
Constraint Models for Klondike
Published in 31st International Conference on Principles and Practice of Constraint Programming (CP 2025), 2025
Fun with patience games; trying to solve thousands of Klondike Solitaire instances using a variety of constraint models which relax some of the rules, as well as using schedules of algorithms to maximise the number of solutions found.
Recommended citation: Nguyen Dang, Ian P. Gent, Peter Nightingale, Felix Ulrich-Oltean, Jack Waller, "Constraint Models for Klondike", 31st International Conference on Principles and Practice of Constraint Programming (CP 2025) https://doi.org/10.4230/LIPIcs.CP.2025.9
Verifying Properties of State-Based Models Using Constraint Programming
Published in Rigorous State-Based Methods: 12th International Conference (ABZ 2026), 2026
We used constraint programming to complement existing verification methods, being able to tackle much larger instances of some problems.
Recommended citation: Victoria Johnson, Pedro Ribeiro, Simon Foster, Peter Nightingale, Felix Ulrich-Oltean, "Verifying Properties of State-Based Models Using Constraint Programming", Rigorous State-Based Methods: 12th International Conference (ABZ 2026), https://doi.org/10.1007/978-3-032-26752-8_6 https://eprints.whiterose.ac.uk/id/eprint/239752/
talks
Literature Review Seminar
Published:
This is my first presentation as a PhD research student, given in month 3 of the programme. The presentation introduced the field of constraint programming, the idea of encoding to Boolean SAT and how some of the choices involved in the process could be made using machine learning.
Research Topic Intro for EEML2020
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I’m very pleased to be attending Eastern European Machine Learning summer school in July (shame it couldn’t be in the flesh in Warsaw). As part of the application / registration process I prepared a short video presentation to summarise my research interest. Watch it here.
Learning to Select SAT Encodings for Constraints
Published:
As part of the doctoral programme at the CP2020 conference on Constraint Progamming, I submitted a short paper to describe my research so far and took part in peer-review with other PhD students, going through the EasyChair platform to submit, review, discuss.
Presenting IndiCon at ICTAI2024
Published:
I am presenting our paper IndiCon: Selecting SAT Encodings for Individual Pseudo-Boolean and Linear Integer Constraints at ICTAI2024. This paper is a concise preview of the final chapter of my PhD thesis, attempting to select different encodings for each individual constraint in a combinatorial problem. It will be exciting to make my first ever trip to the USA too.
teaching
A-level Maths and Further Maths
High School, Allerton High School, 2019
I taught maths to high-school students of ages 11-19 for 14 years (2005 to 2019).
Graduate Teaching Assistant for various AI & Data Science courses
GTA on UG Programme, Department of Computer Science, University of York, 2020
During my PhD, I was a graduate teaching assistant (GTA) in a number of courses - my work ranged from providing support to students in person during a practical lab session, to leading zoom-based online lab sessions during the Covid19 pandemic, to marking exams and even carrying out one-to-one maths support sessions for students who had gaps in A-level topics due to schooling missed during the lockdowns.
Foundations of AI and ML (FOAM)
Masters Course, Department of Computer Science, University of York, 2023
I gave a guest 1 hour lecture on Logics in week 3 of this Master’s level course, and assisted in the 1 hour seminar and 2 hour lab. The week introduced ideas around:
AI Problem Solving with Search and Logic (AIPS)
Masters Course, Department of Computer Science, University of York, 2024
I co-taught this module which covered search algorithms, game theory and constraint programming. I was responsible for writing and delivering half of the material, ranging across lectures, labs, formative quizzes and final exams.
Foundations of AI and ML (FOAM)
Masters Course, Department of Computer Science, University of York, 2024
I presented the whole week on Logics for this Master’s level course, including the lecture, seminar and lab. As in last year’s version, this week focussed on:
AI Problem Solving with Search and Logic (AIPS)
Masters Course, Department of Computer Science, University of York, 2025
I co-taught this module once again, covering search algorithms, game theory and constraint programming. I was responsible for writing and delivering half of the material, ranging across lectures, labs, formative quizzes and final exams.