Helsinki Institute for Information

HIIT Foundations Friday

HIIT Foundations Friday is a monthly get-together event for the community typically consisting of a tutorial and lunch as well as follow-up activity and discussions. This activity is part of the HIIT focus area on Foundations of Computing. On this page, you will find a brief overview of each talk, past and future as well as information about our usual venue.


In the autumn season we will convene in Fabiankatu 24, Hall 532 on the downtown campus of the University of Helsinki.

For arrival instructions, please refer to the following map for guidance on how to find the correct entrance in the inner courtyard from Fabianinkatu. Once there, take the elevator or the stairs to the 5th floor (Hall 532).

Arrival instructions

Upcoming Talks

Date and Time Speaker and Title Abstract Reference
Friday, 27 Oct 2023, 11:00 Prof. Juho Hirvonen (Aalto University): TBA TBA
Friday, 17 Nov 2023, 11:00 TBA TBA
Friday, 8 Dec 2023, 11:00 TBA TBA

Past Talks

Date and Time Speaker and Title Abstract Reference
Friday, 22 Sep 2023, 11:00 Dr. Tuomas Hakoniemi (University of Helsinki): Pigeons! - Selected topics in Proof Complexity Proof complexity is a field that lives in the intersection of computational complexity theory and mathematical logic. It studies the computational resources needed to prove mathematical statements in formal proof systems with the main emphasis being in proof size lower bounds.In this presentations we discuss some selected themes in proof complexity through few representative results and proof methods. We try to give a broad overview of the field, and see how proof complexity relates to other fields of theoretical computer science and mathematical logic more generally. Slides
Friday, 26 May 2023, 11:00 Prof. Russell W. F. Lai (Aalto University): Information-Theoretic Proofs + Cryptographic Commitments = Succinct Arguments, or “How to verifiably delegate computation?” In cryptography, a succinct argument system allows a prover to convince a verifier that certain NP relations hold with very little communication cost. Succinct arguments are useful, for example, to delegate a costly computation to a powerful server such that the correctness of the computation can be verified efficiently.

Most existing constructions of succinct arguments can be seen as compilations of the following building blocks:

  1. an information-theoretic proof system, e.g. a probabilistically checkable proof (PCP), and
  2. a cryptographic commitment scheme with special openings, e.g. a vector commitment scheme.

In this overview talk, we will look at selected examples of the above objects, highlight recent advances, and point out open research directions.

Friday, 21 April 2023, 11:00 am Dr. Melissa Antonelli (University of Helsinki): On Counting Propositional Logic and Wagner's Hierarchy We will introduce an extension of classical propositional logic with counting quantifiers, intuitively expressing that a formula is true in a certain portion of its possible interpretations. Beyond admitting a sound and complete proof system, this logic is shown to be related with counting complexity classes. In particular, we will consider the counting hierarchy - a hierarchy of complexity classes introduced by Wagner in 1984/86 as a generalization of Meyer and Stockmeyer's one and able to express the complexity of many natural problems in which counting is involved - and prove that the complexity of deciding the validity for our counting formulas in (a special) prenex normal form perfectly matches the corresponding level of Wagner's hierarchy. (Original results are all part of a joint work with Ugo Dal Lago and Paolo Pistone.) Handout
Friday, 24 March 2023, 11:00 am Prof. Chris Brzuska (Aalto University): Cryptographic (r)evolutions: From MiniCrypt to Obfustopia In 1995, Impagliazzo [1] conceived of several worlds to structure the landscape of average-case hardness. For cryptographers of biggest interest were MiniCrypt and CryptoMania. MiniCrypt is a world, where one-way functions (OWFs) exist, functions that are (1) easy to compute, but (2) hard to invert, on the average over a random input, but more "fancy" cryptography does not exist. CryptoMania is a world where one can generate a one-way function even with a trapdoor so that (1) computing the public function is easy, (2) inverting the function is hard on the average, but (3) inverting is easy when knowing the trapdoor. While such trapdoor functions were originally controversial, they soon became a solid building block of our internet security infrastructure. Moreover, the cryptographic community explored even more adventurous assumptions, first discovering the possibility of fully homomorphic encryption in 2009 and then indistinguishability obfuscation in 2013. Fully homomorphic encryption allows computation on encrypted data without knowing the key---and indistinguishability obfuscation allows to turn any program into an unintellegible version of it. We start our talk with a discussion of MiniCrypt. One-way functions are far more powerful than one might think at first sight and even allow us to build pseudorandom generators, pseudorandom functions and symmetric-key encryption schemes. We then move to the very adventurious world of obfustopia and explore the conceptual implications of indistinguishability obfuscation (iO). Clickbait version: 5 odd facts about indistinguishability obfuscation---you won't believe the 1st and the 5th. :-)

Preparation (optional): Section 2 of Impagliazzo's essay [1] is a very fun 6.5-pages long introduction to average-case hardness. I turned Section 2 into a popular science talk [2] a couple of years ago, where I explain P vs. NP, average-case hardness, MiniCrypt and CryptoMania in 15 minutes.

[2] The talk starts at min 2:30, the discussion of Impagliazzo's 5 worlds starts at 8:30 and ends at around 23:00

Friday, 17 February 2023, 11:00 am Prof. Nikolaj Tatti (University of Helsinki): The gentle art of submodular minimisation Submodular optimisation is a well-studied topic. While there is a surfeit of works studying submodular maximisation problems, the counterpart problem---submodular minimisation---is often forgotten. In this talk we revisit a classic iterative algorithm, based on a minimum norm theorem, for solving a generic submodular minimisation problem. While not being, strictly speaking, a polynomial algorithm, it is a more approachable and a more practical algorithm. We will show how submodular minimisation is related to a certain quadratic program, and demonstrate how this problem can be solved with a greedy method. Slides
Friday, 20 January 2023, 11:00 am Prof. Antti Honkela (University of Helsinki): Privacy accounting: evaluating privacy of compositions of differential privacy Differential privacy (DP) provides a firm theoretical foundation for privacy-preserving computation and is widely deployed (US Census 2020; Apple, Google, Meta, Microsoft and many other companies). One of the main strengths of DP is composability: releasing the outputs of multiple DP mechanisms, called composition, is also DP. In my talk I will address the question of privacy accounting: how private such compositions really are. The field has progressed from increasingly complex analytical bounds to numerical methods capable of evaluating the exact cumulative privacy loss. Further open problems remain in accurate accounting for fully adaptive compositions, i.e. when privacy parameters and execution of future mechanisms depend on outcomes of previous mechanisms.
Friday, 9 Dezember 2022, 11:00 am Prof. Jukka Suomela (Aalto University): Locality in online, dynamic, sequential, and distributed graph algorithms I will discuss the notion of locality in the context of graph problems, from four different perspectives: online graph algorithms, dynamic graph algorithms, sequential distributed algorithms, and parallel distributed algorithms. I will use the graph coloring problem as a running example, and I will explore settings like this:
  • Online graph algorithms: The adversary reveals the graph one node at a time. How far do you need to see around a node until it is safe to pick its color?
  • Dynamic graph algorithms: The adversary constructs the graph one edge at a time, and you need to maintain a valid coloring after each addition. How far around the new edge do you need to modify the solution?
  • Distributed graph algorithms: Each node has to choose its own color simultaneously in parallel based on its local neighborhood. How far does it need to see?
While these are different questions in general, we can show that there are families of graph problems for which all these notions are equal to each other.

This talk is based on joint work with Amirreza Akbari, Navid Eslami, Henrik Lievonen, Darya Melnyk, and Joona Särkijärvi, and there is a manuscript available at

Friday, 17 November 2022, 11:00 am Prof. Mikko Koivisto (University of Helsinki): Energy-efficient algorithms – theoretical perspectives I will briefly review approaches to energy-efficient computing, focusing on algorithms for power management and semi-reversible computing.
Friday, 14 October 2022, 11:00 am Prof. Alexandru Paler (Aalto University): QRAM: Functionality, Models and Feasibility Outlook Quantum algorithms and machine learning methods require access to data which has to be stored in the memory of the quantum computer. Assuming a von Neumann architecture for quantum computers, then the QRAM/QROM is the quantum version of a classical computer's RAM/ROM. QRAM is using quantum bits (qubits) for storing classical information (classical bits) and is, as a consequence, an expensive component of a quantum computation. In this talk, I review the different types of QRAM/QROM and discuss their short- and long-term feasibility for practical quantum computations.
Friday, 9 September 2022, 11:00 am Prof. Kalle Kytölä (Aalto University): Formalization and interactive/automated theorem-proving in mathematics Short tutorial on using the Lean theorem prover.
Friday, 1 April 2022, 12:15 am Janne H. Korhonen (Aalto University): Distributed matrix multiplication Tutorial.
Friday, 4 March 2022, 11:00 am Sándor Kisfaludi-Bak (Aalto University): Hyperbolic intersection graphs & (quasi-)polynomial time Tutorial.
Friday, 4 February 2022, 11:30 am Miika Hannula (University of Helsinki): On consistent query answering over primary and foreign keys A database is called consistent if it satisfies all of its integrity constraints, and otherwise inconsistent. In consistent query answering (CQA), one asks whether a Boolean query holds true in every repair of an inconsistent database instance. The data complexity of CQA over primary key constraints and self-join free Boolean conjunctive queries is known to exhibit a complete complexity trichotomy. In this talk we consider extending this problem with unary foreign keys, and present a complexity dichotomy concerning first-order definability.
Friday, 23 December 2021, 10:15 am Prof. Petteri Kaski (Aalto University): Equations Tutorial followed by an Open Problems corner.
Friday, 12 November 2021, 12:00 am Prof. Petteri Kaski (Aalto University): Algebraic fingerprinting in algorithmics Short tutorial.