The visibility graph of a set S ⊆ ℝ² is the graph whose vertices are the points of S, with two points x,y connected by an edge if and only if they see each other in S, that is, if the segment xy is contained in S. The edge density of this graph is known as the Beer index of S. Previously, it has been shown that a simply connected set S ⊆ ℝ² of unit Lebesgue measure with Beer index β > 0 contains a convex subset of measure Ω(β); in particular, for visibility graphs of simply connected sets, a positive edge density β > 0 implies the existence of a clique containing an Ω(β)-fraction of all vertices. The simple-connectivity assumption cannot be omitted, as there are non-simply-connected sets with Beer index 1 and no convex subset of positive measure. Nevertheless, in this paper, we extend the above result to non-simply-connected sets, by showing that a visibility graph with large edge density contains a triangle with large convex hull. More precisely, we show that a set S ⊆ ℝ² of unit Lebesgue measure with Beer index β > 0 contains three pairwise visible points whose convex hull has measure Ω(β⁹). If in addition S is an open domain with K holes, then S contains three pairwise visible points with convex hull of measure Ω(β/K) as well as a convex subset of measure Ω(β/K²).

The \emph{genus} $\mathrm{g}(G)$ of a graph $G$ is the minimum $g$ such that $G$ has an embedding on the orientable surface $M_g$ of genus $g$. A drawing of a graph on a surface is \emph{independently even} if every pair of nonadjacent edges in the drawing crosses an even number of times. The \emph{$\mathbb{Z}_2$-genus} of a graph $G$, denoted by $\mathrm{g}_0(G)$, is the minimum $g$ such that $G$ has an independently even drawing on $M_g$. By a result of Battle, Harary, Kodama and Youngs from 1962, the graph genus is additive over 2-connected blocks. In 2013, Schaefer and Štefankovič proved that the $\mathbb{Z}_2$-genus of a graph is additive over 2-connected blocks as well, and asked whether this result can be extended to so-called 2-amalgamations, as an analogue of results by Decker, Glover, Huneke, and Stahl for the genus. We give the following partial answer. If $G=G_1\cup G_2$, $G_1$ and $G_2$ intersect in two vertices $u$ and $v$, and $G-u-v$ has $k$ connected components (among which we count the edge $uv$ if present), then $|\mathrm{g}_0(G)-(\mathrm{g}_0(G_1)+\mathrm{g}_0(G_2))|\le k+1$. For complete bipartite graphs $K_{m,n}$, with $n\ge m\ge 3$, we prove that $\frac{\mathrm{g}_0(K_{m,n})}{\mathrm{g}(K_{m,n})}=1-O(\frac{1}{n})$. Similar results are proved also for the Euler $\mathbb{Z}_2$-genus. We express the $\mathbb{Z}_2$-genus of a graph using the minimum rank of partial symmetric matrices over $\mathbb{Z}_2$; a problem that might be of independent interest.

A drawing of a graph on a surface is independently even if every pair of nonadjacent edges in the drawing crosses an even number of times. The Z_2-genus of a graph G is the minimum g such that G has an independently even drawing on the orientable surface of genus g. An unpublished result by Robertson and Seymour implies that for every t, every graph of sufficiently large genus contains as a minor a projective t x t grid or one of the following so-called t-Kuratowski graphs: K_{3,t}, or t copies of K_5 or K_{3,3} sharing at most 2 common vertices. We show that the Z_2-genus of graphs in these families is unbounded in t; in fact, equal to their genus. Together, this implies that the genus of a graph is bounded from above by a function of its Z_2-genus, solving a problem posed by Schaefer and Stefankovic, and giving an approximate version of the Hanani-Tutte theorem on orientable surfaces.

A binary matrix is a matrix with entries from the set {0,1}. We say that a binary matrix A contains a binary matrix S if S can be obtained from A by removal of some rows, some columns, and changing some 1-entries to 0-entries. If A does not contain S, we say that A avoids S. A k-permutation matrix P is a binary k χ k matrix with exactly one 1-entry in every row and one 1-entry in every column. The Füredi-Hajnal conjecture, proved by Marcus and Tardos, states that for every permutation matrix P, there is a constant cp such that for every n ∊ ℕ, every n × n binary matrix A with at least cpn 1-entries contains P. We show that cp ≤ 2O(κ2/3 log7/3 k/(log log κ)1/3) asymptotically almost surely for a random k-permutation matrix P. We also show that cp < 2(4+o(1))k for every k- permutation matrix P, improving the constant in the exponent of a recent upper bound on cp by Fox. We also consider a higher-dimensional generalization of the Stanley-Wilf conjecture about the number of d-dimensional n-permutation matrices avoiding a fixed d-dimensional k-permutation matrix, and prove almost matching upper and lower bounds of the form (2k)O(n) · (n!)d-1-1/(d-1) and n−O(k)kΩ(n) · (n!)d-1-1/(d-1), respectively.

Permutation Pattern Matching (or PPM) is a decision problem whose input is a pair of permutations π and τ, represented as sequences of integers, and the task is to determine whether τ contains a subsequence order-isomorphic to π. Bose, Buss and Lubiw proved that PPM is NP-complete on general inputs. We show that PPM is NP-complete even when π has no decreasing subsequence of length 3 and τ has no decreasing subsequence of length 4. This provides the first known example of PPM being hard when one or both of π and σ are restricted to a proper hereditary class of permutations. This hardness result is tight in the sense that PPM is known to be polynomial when both π and τ avoid a decreasing subsequence of length 3, as well as when π avoids a decreasing subsequence of length 2. The result is also tight in another sense: we will show that for any hereditary proper subclass c of the class of permutations avoiding a decreasing sequence of length 3, there is a polynomial algorithm solving PPM instances where π is from c and τ is arbitrary. We also obtain analogous hardness and tractability results for the class of so-called skew-merged patterns. From these results, we deduce a complexity dichotomy for the PPM problem restricted to π belonging to Av(α), where Av(α) denotes the class of permutations avoiding a permutation α. Specifically, we show that the problem is polynomial when α is in the set {1,12, 21,132, 213, 231, 312}, and it is NP-complete for any other α.

We consider the following geometric optimization problem: find a convex polygon of maximum area contained in a given simple polygon $P$ with $n$ vertices. We give a randomized near-linear-time $(1-\varepsilon)$-approximation algorithm for this problem: in $O(n( \log^2 n + (1/\varepsilon^3) \log n + 1/\varepsilon^4))$ time we find a convex polygon contained in $P$ that, with probability at least $2/3$, has area at least $(1-\varepsilon)$ times the area of an optimal solution. We also obtain similar results for the variant of computing a convex polygon inside $P$ with maximum perimeter. To achieve these results we provide new results in geometric probability. The first result is a bound relating the area of the largest convex body inside $P$ to the probability that two points chosen uniformly at random inside $P$ are mutually visible. The second result is a bound on the expected value of the difference between the perimeter of any planar convex body $K$ and the perimeter of the convex hull of a uniform random sample inside $K$.

We consider the following geometric optimization problem: find a convex polygon of maximum area contained in a given simple polygon P with n vertices. We give a randomized near-linear-time (1 − ϵ)-approximation algorithm for this problem: in O((n/ϵ6) log2 n log(1/δ)) time we find a convex polygon contained in P that, with probability at least 1 − δ, has area at least (1 − ϵ) times the area of an optimal solution.

The Hanani–Tutte theorem is a classical result proved for the first time in the 1930s that characterizes planar graphs as graphs that admit a drawing in the plane in which every pair of edges not sharing a vertex cross an even number of times. We generalize this result to clustered graphs with two disjoint clusters, and show that a straightforward extension to flat clustered graphs with three or more disjoint clusters is not possible. For general clustered graphs we show a variant of the Hanani–Tutte theorem in the case when each cluster induces a connected subgraph.Di Battista and Frati proved that clustered planarity of embedded clustered graphs whose every face is incident with at most five vertices can be tested in polynomial time. We give a new and short proof of this result, using the matroid intersection algorithm.

Motivated by recent papers of Eppstein, Abel et al. and Barat et al., and by a question of Wood, we investigate properties of planar point sets with no 5-hole (no empty convex pentagon). We answer a question of Wood by showing that the visibility graph of a finite point set with no 5-hole may contain a clique of arbitrary size. This is in contrast with the previous examples of sets with no 5-hole, including the example of the (finite) square lattice. In our construction we use several equivalent local characterizations of (locally) finite planar point sets with no 5-hole which may be of independent interest. Our construction relies on a construction scheme which allows to derive new, non-trivial examples of sets with no 5-hole.

Precise estimation of frequency and phasor has become important in electrical power grids. Knowledge of phasor enables localization of faults, calculation of active and reactive power flows, determination of electrical parameters of system components (lines, transformers), etc. In this paper we present a new method for frequency and phasor assessment. Frequency assessment is done by applying statistical methods such as minimizing standard deviation of moving averages for the window length corresponding to possible frequency. Phasor assessment is done using numerical quadrature. The algorithm has been developed using Wolfram Mathematica®and implemented in development board equipped with a microcontroller.

The VC-dimension of a family P of n-permutations is the largest integer k such that the set of restrictions of the permutations in P on some k-tuple of positions is the set of all k! permutation patterns. Let rk(n) be the maximum size of a set of n-permutations with VC-dimension k. Raz showed that r2(n) grows exponentially in n. We show that and for every t ≥ 1, we have and . We also study the maximum number pk(n) of 1-entries in an n × n (0, 1)-matrix with no (k + 1)-tuple of columns containing all (k + 1)-permutation matrices. We determine that p3(n) = Θ(nα(n)) and for every t ≥ 1. We also show that for every positive s there is a slowly growing function ζs(m) (for example for every odd s ≥ 5) satisfying the following. For all positive integers m, n, B and every m × n (0, 1)-matrix M with ζs(m)Bn 1-entries, the rows of M can be partitioned into s intervals so that some ⌊Bn/m⌋-tuple of columns contains at least B 1-entries in each of the intervals.

We describe our approach in education of the course Digital and Analog Circuits, which belongs to curricula of the Informatics study program. For analysis of analog and simple digital circuits we use computer algebra system Mathematica, which minimizes the amount of routine, handy calculations. This fact enables focusing on the problem and solving more examples, which in turn provides better comprehension of the topic. As Mathematica is later used in subsequent courses, its knowledge is utilized in these courses. Last, but not least, Mathematica provides several programming paradigms, which can be easy demonstrated to students of Informatics study program.

We provide a complete list of 6-critical graphs that can be embedded on the Klein bottle settling a problem of Thomassen [J. Combin. Theory Ser. B, 70 (1997), pp. 67–100, Problem 3]. The list consists of nine nonisomorphic graphs which have altogether 18 nonisomorphic 2-cell embeddings and one embedding that is not 2-cell.