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We study double ergodic averages with respect to two general commuting transformations and establish a sharp quantitative result on their convergence in the norm. We approach the problem via real harmonic analysis, using recently developed methods for bounding multilinear singular integrals with certain entangled structure. A byproduct of our proof is a bound for a two-dimensional bilinear square function related to the so-called triangular Hilbert transform., Comment: 28 pages

We give an explicit formula for one possible Bellman function associated with the $L^p$ boundedness of dyadic paraproducts regarded as bilinear operators or trilinear forms. Then we apply the same Bellman function in various other settings, to give self-contained alternative proofs of the estimates for several classical operators. These include the martingale paraproducts of Ba\~{n}uelos and Bennett and the paraproducts with respect to the heat flows., Comment: 18 pages, 1 figure, 1 ancillary file (Mathematica notebook); v3: exposition is slightly improved, additional calculations and remarks included

In this paper we introduce a variant of Burkholder's martingale transform associated with two martingales with respect to different filtrations. Even though the classical martingale techniques cannot be applied, we show that the discussed transformation still satisfies some expected $\mathrm{L}^p$ estimates. Then we apply the obtained inequalities to general-dilation twisted paraproducts, particular instances of which have already appeared in the literature. As another application we construct stochastic integrals $\int_{0}^{t}H_s d(X_s Y_s)$ associated with certain continuous-time martingales $(X_t)_{t\geq 0}$ and $(Y_t)_{t\geq 0}$. The process $(X_t Y_t)_{t\geq 0}$ is shown to be a "good integrator", although it is not necessarily a semimartingale, or even adapted to any convenient filtration., 18 pages; v3: accepted for publication, references updated, research support acknowledged

Statistički model i slučajni uzorak osnovni su pojmovi matematičke statistike. Za slučaj beskonačne populacije slučajni uzorak se najčešće definira kao niz nezavisnih i jednako distribuiranih slučajnih veličina u odnosu na svaku vjerojatnost iz pretpostavljenog statističkog modela. Budući da je slučajni uzorak model za niz opažanja određene veličine kao funkcije nekog slučajnog eksperimenta, postavlja se pitanje nije li pretpostavka o nezavisnosti i jednakoj distribuiranosti opažanih pokusa prejaka. Ako jest, koja pretpostavka je slabija od te, a da i dalje povlači poželjne rezultate inferencijalne statistike? Pokazuje se da je to pretpostavka izmjenjivosti.

We prove failure of the natural formulation of a matrix weighted bilinear Carleson embedding theorem, featuring a matrix valued Carleson sequence as well as products of norms for the embedding. We show that assuming an A2 weight is also not sufficient. Indeed, a uniform bound on the conditioning number of the matrix weight is necessary and sufficient to get the bilinear embedding. We show that any improvement of a recent matrix weighted bilinear embedding, featuring a scalar Carleson sequence and inner products instead of norms must fail. In particular, replacing the scalar sequence by a matrix sequence results in failure even when maintaining the formulation using inner products. Any formulation using norms, even in the presence of a scalar Carleson sequence must fail. As a positive result, we prove the so called matrix weighted redundancy condition in full generality., grant info added

When talking about large infrastructure projects one can often hear – “it is a very complex project”. What does it mean? What makes large infrastructure projects complex? This paper is exploring current views on project complexity and its development though history. As part of the research on large infrastructure projects, the perception and elements of complexity was investigated. The research is based on in-depth analysis of large infrastructure projects and project manager's perception of complexity. The result of the paper is proposal of new model of project complexity for large infrastructure projects.