On the simplicity of the full group of ergodic transformations

LetE denote an invertible, non-singular, ergodic transformation of (0, 1). Then the full group ofE is perfect. IfE preserves the Lebesgue measure, then the full group is simple. IfE preserves no measure equivalent to Lebesgue, then the full group is simple. IfE preserves an infinite measure, then there exists a unique normal subgroup. IfT is any invertible transformation preserving the Lebesgue measure, then the full group is simple if and only ifT is ergodic on its support.     

On the discrepancy of some special sequences

We obtain estimates for the discrepancy of the sequence (xs^(d)(q;n))_n=0^∞, where s^(d)(q;n) denotes the sum of the dth powers of the q-ary digits of the nonnegative integer n and x is an irrational number of finite approximation type. Furthermore metric results for a similar type of sequences are given.     

Spectra of ergodic transformations

We study the group properties of the spectrum of a strongly continuous unitary representation of a locally compact Abelian group G implementing an ergodic group of ¹-automorphisms of a von Neumann algebra $\text{R}$. It is shown that in many cases the spectrum equals the dual group of G; e.g. if G is the integers and $\text{R}$ not finite dimensional and Abelian, then the spectrum is the circle group.     

On the pointwise ergodic behaviour of transformations preserving infinite measures

We consider situations in which the asymptotic type of a measure preserving transformation manifests itself in a pointwise manner.     

Rigid factors of ergodic transformations

We prove a theorem concerning cartesian products of ergodic not necessarily measuring preserving transformations, using the notion of rigid factors for such transformations.     

On full groups of measure-preserving and ergodic transformations with uncountable cofinalities

The group of all measure-preserving permutations of the unitinterval and the full group of an ergodic transformation ofthe unit interval are shown to have uncountable cofinality andthe Bergman property. Here, a group G is said to have the Bergmanproperty if, for any generating subset E of G, some boundedpower of EE^–1{1} already covers G. This property arosein a recent interesting paper of Bergman, where it was derivedfor the infinite symmetric groups. We give a general sufficientcriterion for groups G to have the Bergman property. We showthat the criterion applies to a range of other groups, includingsufficiently transitive groups of measure-preserving, non-singular,or ergodic transformations of the reals; it also applies tolarge groups of homeomorphisms of the rationals, the irrationals,or the Cantor set.     

On the kernel and shortest face complexes in the unit cube

Studying the extreme kernel face complexes of a given dimension, we obtain some lower estimates of the number of shortest face complexes in the n-dimensional unit cube. The number of shortest complexes of k-dimensional faces is shown to be of the same logarithm order as the number of complexes consisting of at most 2 ⁿ⁻¹ different k-dimensional faces if 1 ≤ k ≤ c · n and c < 0.5. This implies similar lower bounds for the maximum length of the kernel DNFs and the number of the shortest DNFs of Boolean functions.     

On complexity measures of complexes of faces in the unit cube

Under study is the problem of proving the minimality of complexes of faces in the unit cube. Basing on the ordinal properties of a complexity measure functional and the structural properties of Boolean functions, we formulated some sufficient conditions that can be used to prove that a complex of faces is minimal. This allowed us to expand the set of complexes of faces that were proved to be minimal with respect to the complexity measures with certain properties. The strict inclusion is proved for the sets of complexes of faces: kernel, minimal for an arbitrary complexity measure, and minimal for every complexity measure that is invariant under replacement of faces with isomorphic faces.     

On the discrepancy of random low degree set systems

Motivated by the celebrated Beck‐Fiala conjecture, we consider the random setting where there are n elements and m sets and each element lies in t randomly chosen sets. In this setting, Ezra and Lovett showed an discrepancy bound when n ≤ m and an O(1) bound when n ? m^t. In this paper, we give a tight bound for the entire range of n and m, under a mild assumption that . The result is based on two steps. First, applying the partial coloring method to the case when and using the properties of the random set system we show that the overall discrepancy incurred is at most . Second, we reduce the general case to that of using LP duality and a careful counting argument.     

Polynomial discrepancy of sequences

Generalizing E. Hlawka's concept of polynomial discrepancy we introduce a similar concept for sequences in the unit cube and on the sphere. We investigate the relation of this polynomial discrepancy to the usual discrepancy and obtain lower and upper bounds. In a final section some computational results are established.