In the paper, we discuss convergence properties and Voronovskaja type theorem for bivariate $q$-Bernstein polynomials for a function analytic in the polydisc $D_{R_1}\times D_{R_2}=\{z\in C:|z|<R_1\}\times \{z\in C:\left|z\right|<R_1\}$ for arbitrary fixed $q>1$. We give quantitative Voronovskaja type estimates for the bivariate $q$-Bernstein polynomials for $q>1$. In the univariate case the similar results were obtained by S. Ostrovska: $q$-Bernstein polynomials and their iterates. J. Approximation Theory 123 (2003), 232–255. and S. G. Gal: Approximation by Complex Bernstein and Convolution...

MSC 2010: 41A25, 41A35

We obtain modular convergence theorems in modular spaces for nets of operators of the form $\left(T_{w}f\right)\left(s\right)={\int }_H{K}_w(s-h_w\left(t\right),f\left(h_w\left(t\right)\right))d{\mu }_H\left(t\right)$, w > 0, s ∈ G, where G and H are topological groups and ${h_w}_{w>0}$ is a family of homeomorphisms $h_w:H\to h_w\left(H\right)\subset G.$ Such operators contain, in particular, a nonlinear version of the generalized sampling operators, which have many applications in the theory of signal processing.