Seminars
25.04.2017 14:15 Michał Ziobro 
Cryptology Introduction to Homomorphic Encryption 
26.04.2017 12:15 Konrad Kalita 
Computer science foundations Java Generics are Turing Complete by Radu Grigore 
This paper describes a reduction from the halting problem of Turing machines to subtype checking in Java. It follows that subtype checking in Java is undecidable, which answers a question posed by Kennedy and Pierce in 2007. It also follows that Java’s type checker can recognize any recursive language, which improves a result of Gil and Levy from 2016. The latter point is illustrated by a parser generator for fluent interfaces. 
26.04.2017 16:15 Marcin Pilipczuk University of Warsaw 
Theoretical computer science Subexponential Parameterized Algorithms for Planar Graphs, ApexMinorFree Graphs and Graphs of Polynomial Growth via Low Treewidth Pattern Covering 
We prove the following theorem. Given a planar graph G and an integer k, it is possible in polynomial time to randomly sample a subset A of vertices of G with the following properties: 1) A induces a subgraph of G of treewidth 2) for every connected subgraph H of G on at most k vertices, the probability that A covers the whole vertex set of H is at least Together with standard dynamic programming techniques for graphs of bounded treewidth, this result gives a versatile technique for obtaining (randomized) subexponential parameterized algorithms for problems on planar graphs, usually with running time bound In the talk I will first focus on the background and motivation, and then highlight the main ideas of the proof by sketching the proof for the case of graph classes of polynomial growth. Based on joint work with Fedor Fomin, Daniel Lokshtanov, Dániel Marx, Michał Pilipczuk, and Saket Saurabh: http://arxiv.org/abs/1604.05999 and http://arxiv.org/abs/1610.07778. 
27.04.2017 16:15 Jakub Szarawski 
Combinatorial Optimization A greedy approach to the Turtle Tower problem 
In the Turtle Tower problem we are given n turtles with a mass and capacity for each of them. We are looking for the highest tower possible, regarding that capacity of every turtle in the tower cannot be exeeded by the sum of the masses of turles it carry. Presented solution is faster than commonly known dynamic one. 
04.05.2017 16:15 Grzegorz Bukowiec 
Combinatorial Optimization Even factors of graphs 
10.05.2017 16:15 Torsten Ueckerdt Karlsruhe Institute of Technology 
Theoretical computer science The kStrong Induced Arboricity of a Graph 
Motivated by a connection to vertexdistinguishing colorings, we initiate the study of a new graph covering parameters: The kstrong induced arboricity. For a graph G and a positive integer k, a kstrong induced forest F in G is an induced forest in which every component has at least k edges. An edge in G is called kvalid if it is contained in at least one kstrong induced forest. The kstrong induced arboricity f_{k}(G) is the smallest number m such that all kvalid edges of G can be covered with m kstrong induced forests in G. 
11.05.2017 16:15 Anna Kobak 
Combinatorial Optimization Lambda number for the direct product of some family of graphs 
17.05.2017 12:15 Jakub Nowak 
Computer science foundations Generic Complexity of Presburger Arithmetic by Alexander N. Rybalov 
Fischer and Rabin proved in that the decision problem for Presburger Arithmetic has at least double exponential worstcase complexity (for deterministic and nondeterministic Turing machines). In paper 4 a theory of genericcase complexity was developed, where algorithmic problems are studied on "most" inputs instead of set of all inputs. An interesting question rises about existing of more efcient (say, polynomial) generic algorithm deciding Presburger Arithmetic on some set of closed formulas of asymptotic density 1 (socalled generic set). We prove, however, that there is not even an exponential generic algorithm working correctly on a set of inputs which (socalled strongly generic set). 
18.05.2017 16:15 Katrzyna Janocha 
Combinatorial Optimization Proper Orientations of Planar Bipartite Graphs 
24.05.2017 12:15 Piotr Wójcik 
Computer science foundations Randombit optimal uniform sampling for rooted planar trees with given sequence of degrees and Applications by O.Bodini, J. David, and Ph. Marchal 
In this paper, we redesign and simplify an algorithm due to Remy et al. for the generation of rooted planar trees that satisfies a given partition of degrees. This new version is now optimal in terms of random bit complexity, up to a multiplicative constant. We then apply a natural process “simulateguessandproof” to analyze the height of a random Motzkin in function of its frequency of unary nodes. When the number of unary nodes dominates, we prove some unconventional height phenomenon. 
31.05.2017 12:15 Grzegorz Bukowiec 
Computer science foundations The Undecidability of the Generalized Collatz Problem by Stuart A. Kurtz and Janos Simon 
The Collatz problem, widely known as the 3x + 1 problem, asks whether or not a certain simple iterative process halts on all inputs. In this paper, we build on work of J. H. Conway to show that a natural generalization of the Collatz problem is $PI^0_2$ complete. 
31.05.2017 16:15 Piotr Micek 
Theoretical computer science Ramsey Theory for Binary Trees and the Separation of Treechromatic Number from Pathchromatic Number 
We propose a Ramsey theory for binary trees and prove that for every rcoloring of "strong copies" of a small binary tree in a huge complete binary tree T, we can find a strong copy of a large complete binary tree in T with all small copies monochromatic. As an application, we construct a family of graphs which have treechromatic number at most 2 while the pathchromatic number is bounded. This construction resolves a problem posed by Seymour. Joint work with Fidel BarreraCruz, Stefan Felsner, Tamás Mészáros, Heather Smith, Libby Taylor, and Tom Trotter. 
14.06.2017 16:15 Piotr Wójcik 
Theoretical computer science On the asymptotic density of valid sentences in firstorder logic about one binary relation 
This study arises from the following question: what is the proportion of tautologies of the given length n among the number of all FO relational sentences of length n? We investigate the simplest language with a fixed signature σ = {r}, where r is a binary relation symbol. The model with four logic symbols and an universal quantifier lead us to discover an unexpected result  the fraction of valid sentences is always greater than a fixed constant and therefore the density, if exists, is positive. The main theorem is derived from the analysis of structural properties of FO formulae, which themselves bear strict resemblance to structural properties of λterms. 
Poprzednie referaty
20.04.2017 16:15 Helena Borak, Zygmunt Łenyk 
Combinatorial Optimization Necklaces, Convolutions, and X + Y, A new upper bound for the online square packing 
Necklaces, Convolutions, and X + Y The necklace alignment problem is to find the optimal rotation of the necklaces to best align the beads, when we have two necklaces given, each with n beads at arbitrary positions. Alignment is measured according to the ℓ_p norm of the vector of distances between pairs of beads from opposite necklaces in the best perfect matching. We show surprisingly different results for p = 1, p even, and p = ∞ and how these problems can be reduced to convolution problems which can be solve in subquadratic time. Besides, we say how the necklace alignment problems, and their corresponding convolution problems, are also intrinsically connected to problems on X + Y matrices. A new upper bound for the online square packing In online square packing problem we try to minimise the height of squares on a plane with width 1. Squares come one by one, they can’t overlap and once set, it’s position can’t be changed. A new upper bound (ratio between algorithm result and optimal packing) is found by applying modified version of previously used First Fit Shelf algorithm. 
19.04.2017 16:15 Lech Duraj, Adam Polak 
Theoretical computer science Longest Common Strictly Increasing Subsequecnce 
The Longest Common Increasing Subsequence problem is a variant of classic Longest Common Subsequence problem, which can be solved in quadratic time with a simple dynamic programming algorithm. During the talk we will show a reduction from the Orthogonal Vectors problem to the Longest Common Increasing Subsequence problem which proves that the latter cannot be solved in strongly subquadratic time unless the SETH is false.
Simple modifications of the reduction prove that the problem for k sequences cannot be solved in O(n^{k}^{ε}) time, that the same lower bounds apply to the Longest Common Weakly Increasing Subsequence, and that the assumption of SETH can be replaced with a weaker statement about satisfiability of nondeterministic branching programs. 
12.04.2017 12:15 Jarek Duda 
Computer science foundations Boundaries for hashing problem, or how many bits ones individuality costs 
I will talk about informationtheoretic boundaries for the hashing problem, the Bloom filter, and generally about informational content of structures like trees and graphs. While the label size has to grow like logarithm of the population size, neglecting information about the order (lg(n!) bits), we get a linear growth of entropy of population, allowing to bound 'the cost of individuality' asymptotically to ~2.33275 bits per object. 
06.04.2017 16:15 Andrzej Głuszyński, Jakub Nowak 
Combinatorial Optimization Local Antimagic Vertex Coloring of a Graph, A short proof of Cayley's tree formula 
Local Antimagic Vertex Coloring of a Graph The edge labelling is called 'local antimagic', if for all vertices sum of labels for incident edges is different for every two adjacent vertices. Such sum induce a correct vertex colouring. The local antimagic chromatic number  X_la(G)  is the minimum number of colours used by any proper local antimagic labelling. In the paper authors present results on this parameter for trees, friendship, wheel and clique graphs. A short proof of Cayley's tree formula Cayley’s tree formula is a very elegant result in Graph Theory. The problem is to find the number of all possible trees on a given set of labeled vertices. For n = 2 and vertex set {v1, v2}, we have only one tree. For n = 3 and vertex set {v1, v2, v3}, we have 3 different trees. Similarly for n = 4, we have 16 trees. We give a short proof of Cayley’s tree formula for counting the number of different labeled trees on n vertices. Alok Bhushan Shukla, A short proof of Cayley's tree formula. 
05.04.2017 12:15 Szymon Stankiewicz 
Computer science foundations CANTOR POLYNOMIALS AND THE FUETERPOLYA THEOREM by MELVYN NATHANSON 
A packing polynomial is a polynomial that maps the set N^2 of lattice points with nonnegative coordinates bijectively onto N. Cantor constructed two quadratic packing polynomials, and Fueter and Polya proved analytically that the Cantor polynomials are the only quadratic packing polynomials. 
04.04.2017 14:15 Mateusz Jachna 
Cryptology Secure Hash Algorithms family and the recently found collision for SHA1 
28.03.2017 14:15 Piotr Wójcik 
Cryptology Quantum Authentication with Key Recycling 
We show that a family of quantum authentication protocols introduced in FOCS 2002 can be used to construct a secure quantum channel and additionally recycle all of the secret key if the message is successfully authenticated, and recycle part of the key if tampering is detected. We give a full security proof that constructs the secure channel given only insecure noisy channels and a shared secret key. We also prove that the number of recycled key bits is optimal for this family of protocols, i.e., there exists an adversarial strategy to obtain all nonrecycled bits. Previous works recycled less key and only gave partial security proofs, since they did not consider all possible distinguishers (environments) that may be used to distinguish the real setting from the ideal secure quantum channel and secret key resource. References: [1] Christopher Portmann, Quantum Authentication with Key Recycling (pdf) 
23.03.2017 16:15 Aleksandra Mędrek, Marcin Muszalski 
Combinatorial Optimization Planning for Fast Connectivity Updates 
Understanding how a single edge deletion can affect the connectivity of a graph amounts to finding the graph bridges. But when faced with d > 1 deletions, can we establish as easily how the connectivity changes? When planning for an emergency, we want to understand the structure of our network ahead of time, and respond swiftly when an emergency actually happens. We describe a linearspace representation of graphs which enables us to determine how a batch of edge updates can impact the graph. Given a set of d edge updates, in time O(d polylg n) we can obtain the number of connected components, the size of each component, and a fast oracle for answering connectivity queries in the updated graph. The initial representation is polynomialtime constructible. 
21.03.2017 14:15 Jan Szczepaniec 
Cryptology Inclusive Block Chain Protocols 
Distributed cryptographic protocols such as Bitcoin and Ethereum use the block chain to synchronize a global log of events between nodes in their network. Previous research has shown that the mechanics of the block chain and block propagation are constrained: if blocks are created at a high rate compared to their propagation time in the network, many conflicting blocks are created and performance suffers greatly.

16.03.2017 16:15 Patryk Urbański 
Combinatorial Optimization Generating Linear Extensions Fast 
One of the most important sets associated with a poset P is its set of linear extensions, E(P). In this paper, we present an algorithm to generate all of the linear extensions of a poset in constant amortized time; that is, in time O(e(P)), where e(P) = E(P). The fastest previously known algorithm for generating the linear extensions of a poset runs in time O(n*e(P)), where n is the number of elements of the poset. Our algorithm is the first constant amortized time algorithm for generating a ``naturally defined'' class of combinatorial objects for which the corresponding counting problem is #Pcomplete. Furthermore, we show that linear extensions can be generated in constant amortized time where each extension differs from its predecessor by one or two adjacent transpositions. The algorithm is practical and can be modified to efficiently count linear extensions, and to compute P(x < y), for all pairs x,y, in time O(n^2 + e(P)). 
16.03.2017 14:15 Jakub Cisło, Grzegorz Jurdziński 
Algorytmika Tight Hardness Results for LCS and other Sequence Similarity Measures 
15.03.2017 16:15 Manuel Bodirsky TU Dresden 
Theoretical computer science The tractability conjecture for finitely bounded homogeneous structures 
Finitely bounded homogeneous structures form a large class of infinite structures that can be seen as a generalisation of the class of all finite structures. Many results about finite structures, in particular in the context of the complexity of constraint satisfaction problems, can be generalised to this larger class. In this talk I will present a reformulation of a tractability conjecture for CSPs for this class in terms of polymorphisms, due to Barto and Pinsker (LICS 2016), and I will present a proof that the condition given in the tractability conjecture is decidable (under some Ramseytheoretic assumptions that might hold in general for all reducts of finitely bounded homogeneous structures). 
15.03.2017 12:15 Łukasz Lachowski 
Computer science foundations Impossibility of Distributed Consensus with One Faulty Process by MICHAEL J. FISCHER, NANCY A. LYNCH AND MICHAEL S. PATERSO 
The consensus problem involves a asynchronous system of processes, some of which may be unreliable.The problem is for the reliable processes to agree on a binary value. In this paper, it is shown that every protocol for this problem has the possibility of nontermination, even with only one faulty process. By way of contrast, solutions are known for the synchronous case, the “Byzantine Generals” problem. 
14.03.2017 14:15 Marcin Briański 
Cryptology NonInteractive Verifiable Computing: Outsourcing Computation to Untrusted Workers 
The talk is based on the paper with the same title by Rosario Gennaro, Craig Gentry and Bryan Parno. Verifiable Computation enables a computationally weak client to "outsource" the computation of a function F on various inputs x_{1}, ..., x_{k} to one or more workers. The workers return the result of the function evaluation, e.g., y_{i} = F(x_{i}), as well as a proof that the computation of F was carried out correctly on the given value x_{i}. The verification of the proof should require substantially less computational effort than computing F(x_{i}) from scratch. We present a protocol that allows the worker to return a computationally sound, noninteractive proof that can be verified in O(m) time, where m is the bitlength of the output of F. The protocol requires a onetime preprocessing stage by the client which takes O(C) time, where C is the smallest Boolean circuit computing F. Our scheme also provides input and output privacy for the client, meaning that the workers do not learn any information about the values x_{i} or y_{i}. 
09.03.2017 16:15 Grzegorz Matecki 
Combinatorial Optimization Boolean dimension of posets 
A boolean dimension bdim(P) of a poset P=(X,<) is a smallest number k for which there is a set l1, l2, ..., lk of labelings X:>N and a boolean formula f(a1, ..., ak) such that the following is true: x < y in P iff f(a1, .., a_k) = 1 where ai =1 iff li(x) < li(x). Generally, it is simple to observe that bdim(P) <= dim(P). Also, it is known that there is a constant c such that bdim(n) <= c log(n) for any poset P of size n. The are few interesting open problems for boolean dimension: 1) Is boolean dimension of the boolean lattice of size n less that n? 2) Is there a constant c such that bdim(P) < c for any planar poset P? 
09.03.2017 14:00 Sylwester Klocek, Wojciech Kruk 
Algorytmika The Alternating Stock Size Problem and the Gasoline Puzzle 
08.03.2017 12:15 Maciej Bendkowski 
Computer science foundations Boltzmann samplers: random generation of combinatorial structures with an application to lambda calculus 
In their seminal paper, Duchon et al. proposed a surprisingly simple, generalpurpose framework of Boltzmann samplers meant for random generation of combinatorial structures. In this talk we revisit their method and discuss its elegant relation with analytic combinatorics as well as important practical implementation details. Finally, we discuss the application of Boltzmann samplers to the random generation of lambda terms used, e.g. in testing functional programming compilers. 
07.03.2017 14:15 Zygmunt Łenyk 
Cryptology Speeding up modular multiplication using Montgomery and Barrett reduction 
In the talk we present Montgomery and Barrett reductions that are used to speed up modular computations. In both reductions some precomputations are made allowing for replacing subsequent expensive divisions by some fixed modulus with much cheaper operations involving a suitable power of 2. This is particularly useful when many modular divisions by the same modulus are performed (for example in finite field arithmetic or in RSA). 
07.03.2017 14:15 Mateusz Twaróg, Łukasz Majcher 
Combinatorial Optimization Combinatorial library core 
Presentation and discussion on core functionalities of the c++ combinatorial library. introduction to classes representing graphs, graph traversing algorithm templates and simple GUI. 
01.03.2017 12:15 Michał Zwonek 
Computer science foundations Wielomianowe kodowania 
Rozważany będzie problem istnienia wielomianowej bijekcji, najniższego stopnia, między N^k, a N. Przedstawione będą także problemy otwarte związane z tą tematyką. Materiały do wystąpienia: 1) Elementarny dowód Twierdzenie FeuterPolya (jedyny kwadratowy i bijektywny wielomian N^2>N to funkcja cantore'a) https://arxiv.org/abs/1512. 2) Praca, w której autorzy pokazują nieistnienie wielomianów 3 i 4 stopnia. http://www.sciencedirect.com/ 3) Praca podobnie tematyczna odnosząca się do problemu istnienia wielomianów bijektywnych z pewnego sektora N^2 w N. (To o czym wspomniałem na koniec, opis tego problemu jest też pod koniec w 1) ). Pod koniec pracy jest opisane 6 problemów otwartych związanych z tą tematyką. https://arxiv.org/abs/1305. 4) W podobnej tematyce. http://www.sciencedirect.com/science/article/pii/0022314X78900355

28.02.2017 Michał Dyrek 
Cryptology LLL algorithm and its applications in Number Theory and Cryptography 
The talk is devoted to the algorithm by A. Lenstra, H. Lenstra and L. Lovász dated 1982 allowing for approximation of Shortest Vector Problem in polynomial time. We will present the idea of the algorithm and highlight its applications such as factoring polynomials over Q, constructing polynomials with small coefficients and connections with attacks on RSA. 
26.01.2017 16:15 Wojciech Kruk, Maciej Woźniak 
Combinatorial Optimization A few open problems 
We mentioned the following open problems in graph theory and discrepancy theory: 1. Erdos discrepancy problem 2. Hoang  Reed conjecture 3. Seagull problem  a consequence of Hadwiger's conjecture 
25.01.2017 16:15 01.03.2017 16:15 Grzegorz Guśpiel 
Theoretical computer science Partial Visibility Representation Extension Problem 
We study a class of graphs that have a special geometric representation. By a bar visibility representation of an undirected graph we mean a function that associates with each vertex of a graph a horizontal line segment in such a way, that between segments representing two ends of an edge there is a vertical strip (of visibility). In case of directed graphs, we additionally assume that the visibility is from the bottom to the top, that is the line segment representing the source of the edge is below the one for the target. Graphs admitting such representations are well understood and can be recognized in linear time, both in the undirected and in the directed case. We work in a more subtle setting, where line segments are already associated with some vertices of a graph, and the question is if this can be extended to a bar visibility representation of an entire graph. We prove some results on complexity of this kind of problems. This is joint work with Steven Chaplick, Grzegorz Gutowski, Tomasz Krawczyk and Giuseppe Liotta. The manuscript is available here: https://arxiv.org/abs/1512.00174 
25.01.2017 12:00 Sylwester Klocek 
Computer science foundations Incompleteness, Undecidability and Automated Proofs by Cristian S. Calude and Declan Thompson 
Incompleteness and undecidability have been used for many years as arguments against automatising the practice of mathematics. The advent of powerful computers and proofassistants – programs that assist the development of formal proofs by humanmachine collaboration – has revived the interest in formal proofs and diminished considerably the value of these arguments. In this paper we discuss some challenges proofassistants face in handling undecidable problems – the very results cited above – using for illustrations the generic proofassistant Isabelle. 
24.01.2017 Kamil Sałaś 
Cryptology Lower Bounds for Discrete Logarithms 
In the talk we will present the computational complexity of the discrete logarithm in the context of "generic algorithms", that is, algorithms which do not exploit any special properties of the encodings of group elements, other than the property that each group element is encoded as unique binary string. For discrete logarithm, any generic algorithm must perform Ω(p^1/2) group operations, where p is the largest prime dividing the order of the group. 
19.01.2017 Paweł Petecki Akademia GórniczoHutnicza 
Combinatorial Optimization Symmetry breaking polynomial 
Let G be a graph, and let Γ= Aut G. A coloring c of G is symmetrybreaking if for every nonidentity automorphism φ in Γ, there is some vertex v of G such that c(v)≠c(φ(v)). There has been a lot of work on the minimum number of colors in a symmetrybreaking coloring of G. We discuss here a different problem: counting the number of kcolorings that are symmetry breaking. The tool, as is frequently the case for problems such as this one, is Möbius inversion. To solve this problem we define symmetry breaking polynomial ψ_{G}. For positive integer k (number of colors), ψ_{G}(k) is the number of kcolorings that break all nontrivial symmetries of the graph G. 
18.01.2017 16:15 Marian Mrozek 
Theoretical computer science The discrete charm of Morse theory 
The lecture will start with recalling P.S. Alexandroff's Theorem (1937) on mutual equivalence of posets and T_{0} topologies on finite sets. Next, we will outline the combinatorial version of the classical Morse Theory presented by R. Forman in 1998. Then, we will elaborate Forman's ideas towards the combinatorial topological dynamics with potential applications in Big Data problems and time series. The topics of the lecture will be expanded in a course for PhD students in the summer semester 2016/17. 
18.01.2017 12:00 Michał Ziobro 
Computer science foundations Inhabitation in SimplyTyped LambdaCalculus through a LambdaCalculus for Proof Search by Jose Espırito Santo, Ralph Matthes, Luıs Pinto 
Kontynuacja seminarium z 23.11.2016 
17.01.2017 Grzegorz Bukowiec 
Cryptology A quasipolynomial algorithm for discrete logarithm in finite fields of small characteristic 
Until recently, all the algorithms for computing discrete logarithm had a subexponential complexity of type L(1/3), similar to the factorization problem. In this talk we'll discuss a heuristic algorithm that provides quasipolynomial complexity for discrete logarithm in finite fields of small characteristic and that even for other cases still surpasses the Function Field Sieve method. References: [1] R. Barbulescu, P. Gaudry, A. Joux, E. Thomé, A quasipolynomial algorithm for discrete logarithm in finite fields of small characteristic (pdf) 
11.01.2017 16:15 Patryk Mikos 
Theoretical computer science Online coloring of intervals with bandwidth 
We study the online interval coloring problem with bandwidth. The input is a sequence of pairs J_{i}= (I_{i},w_{i}) where I_{i} is an interval on the real line and w_{i} is a real number from (0,1]. In this setting a proper coloring is a function f:J_{i }→N such that for each color c and any point p on the real line, the sum of bandwidths of intervals containing p and colored by c does not exceed 1. The best known lower bound on the competitive ratio in this problem is 24/7. We present an explicit strategy for Presenter that increases the competitive ratio ifor this problem to at least 4.1626. 
11.01.2017 12:00 Patryk Mikos 
Computer science foundations ON THE NUMBER OF DISTINCT LANGUAGES ACCEPTED BY FINITE AUTOMATA WITH n STATES by Michael Domaratzki, Derek Kisman and Jeffrey Shallit 
We give asymptotic estimates and some explicit computations for both the number of distinct languages and the number of distinct finite languages over a kletter alphabet that are accepted by deterministic finite automata (resp. nondeterministic finite automata) with n states. 
10.01.2017 Szymon Policht 
Cryptology Faster operations on elliptic curves using Edwards curves 
Elliptic curve cryptography is a broad and commonly used section of modernday cryptography. Because of that, the speed of elliptic curve operations directly impacts the performance of many current systems. In this talk we'll show how to speed up those operations using Edwards curves. References: [1] Bernstein D.J., Lange T. (2007) Faster Addition and Doubling on Elliptic Curves. In: Kurosawa K. (eds) Advances in Cryptology – ASIACRYPT 2007. ASIACRYPT 2007. Lecture Notes in Computer Science, vol 4833. Springer, Berlin, Heidelberg (https://eprint.iacr.org/2007/286.pdf) 
05.01.2017 14:15 Jan Derbisz, Jakub Łabaj 
Algorytmika How to sort by walking on a tree 
We consider a tree with n vertices. On vertex number x there is a box with label p(x), with the function p being a permutation of {1,2,...,n}. A robot is walking on the tree, carrying at most one box at a time. If a box is placed where robot is standing, it can swap this box with the one being carried. The robot's goal is to sort the boxes, placing each one at the vertex with its number. The paper by D. Graf gives an algorithm computing the shortest possible robot's walk in quadratic time, as well as the proof that the problem becomes NPcomplete if planar graphs are considered instead of trees. 
04.01.2017 12:00 Konrad Kalita 
Computer science foundations ANALYTIC MODELS AND AMBIGUITY OF CONTEXTFREE LANGUAGES by Philippe Flajolet 
We establish that several classical contextfree languages are inherently ambiguous by proving that their counting generating functions, when considered as analytic functions, exhibit some characteristic form of transcendental behaviour. To that purpose, we survey some general results on elementary analytic properties and enumerative uses of algebraic functions in relation to formal languages In particular, the paper contains a general density theorem for unambiguous contextfree languages. 
03.01.2017 
Cryptology (Cancelled) 
22.12.2016 16:15 Łukasz Majcher, Jan Szczepaniec 
Combinatorial Optimization Convex ppartitions of bipartite graphs 
A set of vertices X of a graph G is convex if no shortest path between two vertices in X contains a vertex outside X. We prove that for fixed p ≥ 1, all partitions of the vertex set of a bipartite graph into p convex sets can be found in polynomial time. 
21.12.2016 16:15 Maciej Bendkowski 
Theoretical computer science Boltzmann samplers: a framework for random generation of combinatorial structures with an application to lambda calculus 
In their seminal paper, Duchon et al. proposed a surprisingly simple, generalpurpose framework of Boltzmann samplers meant for random generation of combinatorial structures. In this talk we revisit their method and discuss its elegant relation with analytic combinatorics as well as important practical implementation details. Finally, we discuss the application of Boltzmann samplers to the random generation of lambda terms used, e.g. in testing functional programming compilers. 
20.12.2016 12:00 Bartłomiej Puget 
Cryptology An introduction to quantum computing and cryptography II 
15.12.2016 14:15 Michał Glapa, Franciszek Stokowacki 
Algorytmika Maximum matching with algebraic methods 
In 2006, a celebrated result by Mucha and Sankowski stated that the maximum matching problem can be done by Gaussian elimination. The complexity of this algorithm depends on matrix multiplication, but certainly beats O(n^{2.5}) longstanding record of MicaliVazirani algorithm. 
15.12.2016 Anna Kobak 
Combinatorial Optimization Open problems in graph theory concerning minors. 
We mentioned following open problems in graph theory:

14.12.2016 16:15 Grzegorz Matecki 
Theoretical computer science TwoDimensional Irregular Packing Problem 
We present results on packing irregular shapes onto given sheets of material. 
14.12.2016 12:00 Piotr Wójcik 
Computer science foundations Enumeration and random generation of accessible automata by Frederique Bassino and Cyril Nicaud 
We present a bijection between the A_n of deterministic and accessible automata with n states on a kletters alphabet and some diagrams, which can themselves be represented as partitions of a set of kn + 1 elements into n nonempty subsets. This combinatorial construction shows that the asymptotic order of the cardinality of A_n is related to the Stirling number. Our bijective approach also yields an efficient random sampler, for the uniform distribution, of automata with n states, its complexity is O(n^3/2), using the framework of Boltzmann samplers. 
13.12.2016 12:00 Krzysztof Kleiner 
Cryptology An introduction to quantum computing and cryptography I 
In this talk we're going to discuss quantum informatics and its impact on the field of cryptography. We will introduce the basic concepts of quantum computing as well as cryptography based on Quantum Key Distribution scheme, one of the aspects of quantum informatics which already is being used in practice. Then we will present Shor's algorithm for polynomialtime factorization, responsible for the cryptosystems based on the hardness of factorization or discrete logarithm (in abelian groups) being no longer secure against an adversary with access to a quantum computer.

08.12.2016 Lech Duraj 
Algorytmika A short tale of matrix multiplication 
In recent years, some new algorithms for matrix multiplication problem were presented. Each of them is, however, only slightly faster than previous ones, while requiring substantially more complex analysis. Because of this, the longstanding question of optimal matrix multiplication algorithm seems even harder. In my presentation, a short survery of the matrix multiplication algorithm is given. The presentation is based on François Le Gall's survey lecture of 2014. 
08.12.2016 Zygmunt Łenyk 
Combinatorial Optimization Rendezvous on the line. 
This is one of a handful of rendezvous problems where two players must find one another in a certain structured domain. In line case, players are placed on the line with distance 2, without knowing neither on which side is their partner, nor the direction of the line. I'll concentrate on the symmetric case where players must follow a specific (but maybe mixed) strategy. The conjecture is that best expected time of meeting two players equals 4.25. 
07.12.2016 12:00 Jakub Brzeski 
Computer science foundations ENUMERATION OF FORMAL LANGUAGES by Michael Domaratzki 
We survey recent results on the enumeration of formal languages. In particular, we consider enumeration of regular languages accepted by deterministic and nondeterministic finite automata with n states, regular languages generated by regular expressions of a fixed length, and !regular languages accepted by Müller automata. We also survey the uncomputability of enumeration of contextfree languages and more general structures. 
06.12.2016 Marek Rusinowski 
Cryptology Security of instant messaging applications. 
Nowadays billions of people around the world are sharing sensitive information using instant messaging applications. We will look into the current state of IM security, the problems in this area and a few encryption protocolsOTR and Signal Protocol in particularthat provide security features desired by users. 
01.12.2016 Aleksandra Mędrek, Krzysztof Maziarz 
Algorytmika Navigating Central Path with Electrical Flows: from Flows to Matchings, and Back 
The paper by Aleksander Mądry describes a new approach to the maximum flow problem. We define an electrical flow by assigning resistances to every edge and minimizing total energy instead of maximizing flow. Any flow network can be reduced to some electrical flow problem, using auxiliary reductions to some bipartite matching problems. The main result is a new maximum flow algorithm, working in O(m10/7). The presentation will cover a simpler, O(m3/2) version of the algorithm. 
01.12.2016 Patryk Urbański 
Combinatorial Optimization Coloring Ordinary Maps, Maps of Empires and Maps of the Moon 
A short review of generalized map coloring problems:

01.12.2016 Mateusz Twaróg 
Combinatorial Optimization Second Neighborhood via First Neighborhood in Digraphs 
30.11.2016 18:15 Bartosz Walczak 
Theoretical computer science Coloring curves that cross a fixed curve 
A class of graphs is χbounded if the chromatic number of all graphs in the class is bounded by some function of their clique number. String graphs are intersection graphs of curves in the plane. Significant research in combinatorial geometry has been devoted to understanding the classes of string graphs that are χbounded. In particular, it is known since 2012 that the class of all string graphs is not χbounded. We prove that for every integer t≥1, the class of intersection graphs of curves in the plane each of which crosses a fixed curve in at least one and at most t points is χbounded. This result is best possible in several aspects; for example, the upper bound t on the number of crossings with the fixed curve cannot be dropped. As a corollary, we obtain new upper bounds on the number of edges in socalled kquasiplanar topological graphs. This is joint work with Alexandre Rok. 
30.11.2016 12:00 Yauheni Ananchuk 
Computer science foundations ALGEBRAIC FOUNDATIONS FOR QUALITATIVE CALCULI AND NETWORKS by ROBIN HIRSCH, MARCEL JACKSON, AND TOMASZ KOWALSKI 
Binary Constraint Problems have traditionally been considered as Network Satisfaction Problems over some relation algebra. A constraint network is satisfable if its nodes can be mapped into some representation of the relation algebra in such a way that the constraints are preserved. A qualitative representation is like an ordinary representation, but instead of requiring (a ; b) = a j b , as we do for ordinary representations, we only require that. A constraint network is qualitatively satisfable if its nodes can be mapped to elements of a qualitative representation, preserving the constraints. If a constraint network is satisfable then it is clearly qualitatively satisfable, but the converse can fail. However, for a wide range of relation algebras including the point algebra, the Allen Interval Algebra, RCC8 and many others, a network is satisfable if and only if it is qualitatively satisfable. Unlike ordinary composition, the weak composition arising from qualitative representations need not be associative, so we can generalise by considering network satisfaction problems over nonassociative algebras. We prove that computationally, qualitative representations have many advantages over ordinary representations: whereas many finite relation algebras have only infnite representations, every finite qualitatively representable algebra has a finite qualitative representation; the representability problem for (the atom structures of) finite nonassociative algebras is NPcomplete; the network satisfaction problem over a finite qualitatively representable algebra is always ; the validity of equations over qualitative representations is coNPcomplete. On the other hand we prove that there is no finite axiomatisation of the class of qualitatively representable algebra 
29.11.2016 Anna Kobak 
Cryptology Breaking RSA vs Factoring in generic ring model 
In the talk we present results of Aggarwal and Maurer [1], who showed that a generic ring algorithm for breaking RSA with modulus $N$ can be converted into an algorithm for factoring $N$. The results imply that any attempt at breaking RSA without factoring $N$ will be nongeneric and hence will have to manipulate the particular bitrepresentation of the input modulo $N$. This provides new evidence that breaking RSA may be equivalent to factoring the modulus.
References: [1] D. Aggarwal, U. Maurer, Breaking RSA Generically is Equivalent to Factoring, EUROCRYPT 2009 
24.11.2016 Wojciech Łopata 
Combinatorial Optimization Several open problems from game theory, graph theory and combinatorics. 
I'll briefly introduce the audience to two unrelated areas: book embedding and mechanism design, and walk through some open problems in those areas. 
24.11.2016 Dominika Salawa, Jakub Cisło 
Algorytmika Greedy algorithms for Steiner forest 
The paper, resolves a longstanding open question: is the greedy approach for Steiner forest problem optimal up to multiplicative constant? The result by Anupam Gupta and Amit Kumar proves that in fact it is, with constant being at most 96. While some approximation algorithms for this problem, using linear programming, were previously known, this is the first known bound for a greedy algorithm. 
23.11.2016 Piotr Danilewski 
Theoretical computer science Functional Code Building 

23.11.2016 Michał Ziobro 
Computer science foundations Inhabitation in SimplyTyped LambdaCalculus through a LambdaCalculus for Proof Search by Jos´e Espırito Santo, Ralph Matthes, Luıs Pinto 
A new, comprehensive approach to inhabitation problems in simplytyped lambdacalculus is shown, dealing with both decision and counting problems. This approach works by exploiting a representation of the search space generated by a given inhabitation problem, which is in terms of a lambdacalculus for proof search that the authors developed recently. The representation may be seen as extending the CurryHoward representation of proofs by lambdaterms, staying within the methods of lambdacalculus and type systems. Our methodology reveals inductive descriptions of the decision problems, driven by the syntax of the proofsearch expressions, and the end products are simple, recursive decision procedures and counting functions. 
22.11.2016 Aleksandra Nowak 
Cryptology Lattice Cryptography and The Learning With Errors Problem 
17.11.2016 Paweł Kubiak 
Combinatorial Optimization Succinct Data Structures 
17.11.2016 Patryk Gołębiowski, Wojciech Kruk 
Algorytmika Lower Bounds in the Preprocessing and Query Phases of Routing Algorithms 
In the last decade, there has been a substantial amount of research in finding routing algorithms designed specifically to run on realworld graphs. The paper by Colin White analyzes some of these algorithms and gives lower bounds for their complexity. 
16.11.2016 Bartłomiej Bosek 
Theoretical computer science Every digraph is majority 4choosable 
A majority coloring of a digraph is a coloring of its vertices such that for each vertex at most half of its outneighbors has the same color as that vertex. A digraph D is majority kchoosable if for any assignment of color lists of size k to the vertices there is a majority coloring of D from these lists. We prove the statement in the title. This gives a positive answer to a question posed recently in 1. This is a joint work with Marcin Anholcer and Jarosław Grytczuk. 
16.11.2016 Michał Zieliński 
Computer science foundations Most programs stop quickly or never halt by Cristian S. Calude and Michael A. Stay 
The aim of this paper is to provide a probabilistic, but nonquantum, analysis of the Halting Problem. Our approach is to have the probability space extend over both space and time and to consider the probability that a random Nbit program has halted by a random time.We postulate an a priori computable probability distribution on all possible runtimes and we prove that given an integer k >0, we can effectively compute a time bound T such that the probability that an Nbit program will eventually halt given that it has not halted by T is smaller than 2^{−k}. We also show that the set of halting programs (which is computably enumerable, but not computable) can be written as a disjoint union of a computable set and a set of effectively vanishing probability. Finally, we show that “long” runtimes are effectively rare. More formally, the set of times at which an Nbit program can stop after the time 2^{N+constant} has effectively zero density. 
15.11.2016 Piotr Kawałek 
Cryptology Teoretyczne podstawy kryptoanalizy 
Celem referatu jest przedstawienie teoretycznych modeli ataków kryptoanalitycznych oraz tematów pokrewnych wraz z przykładami. 
10.11.2016 Magdalena Gargas, Mateusz Jachna 
Algorytmika Max flows in O(nm) time, or better 
A new maxflow algorithm with O(nm + m16/15 log2 n) time complexity is presented. By combining this with previous results, an O(nm) algorithm may be obtained, resolving a longstanding open question. This work is due to James B. Orlin. 
10.11.2016 Grzegorz Bukowiec 
Combinatorial Optimization On more variants of the Majority Problem 
09.11.2016 26.10.2016 Adam Polak 
Theoretical computer science Open problems in online and approximation algorithms 
During the talk I will present several promising open problems including:

09.11.2016 Wojciech Kruk 
Computer science foundations On the generic undecidability of the Halting Problem for normalized Turing machines by Alexander Rybalov 
Hamkins and Miasnikov presented in 2004 a generic algorithm deciding the classical Halting Problem for Turing machines with oneway tape on a set of asymptotic probability one (on a socalled generic set). Rybalov in 2007 showed that there is no generic algorithm deciding this problem on strongly generic sets of inputs (some subclass of generic sets). In this paper we prove that there is no generic algorithm deciding the Halting Problem for normalized Turing machines on generic sets of inputs. Normalized Turing machines have programs with the following natural restriction: internal states with large indices are not allowed at the beginning of the program. This condition does not reduce the class of computable functions because for every Turing machine there exists a normalized Turing machine which computes the same function. Our proof holds for machines with oneway and twoway tape. It also implies that the HamkinsMiasnikov algorithm is not generic for normalized Turing machines. 
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