Lattice points problem, equidistribution and ergodic theorems for certain arithmetic spheres

Abstract

We establish an asymptotic formula for the number of lattice points in the sets $$\begin{aligned} {\textbf{S}}_{h_1, h_2, h_3}(\lambda ): =\left\{ x\in {\mathbb {Z}}_+^3:\lfloor h_1(x_1)\rfloor +\lfloor h_2(x_2)\rfloor +\lfloor h_3(x_3)\rfloor =\lambda \right\} \quad \text {with}\quad \lambda \in {\mathbb {Z}}_+; \end{aligned}$$ where functions $$h_1, h_2, h_3$$ are constant multiples of regularly varying functions of the form $$h(x):=x^c\ell _h(x)$$ , where the exponent $$c>1$$ (but close to 1) and a function $$\ell _h(x)$$ is taken from a certain wide class of slowly varying functions. Taking $$h_1(x)=h_2(x)=h_3(x)=x^c$$ we will also derive an asymptotic formula for the number of lattice points in the sets $$\begin{aligned} {\textbf{S}}_{c}^3(\lambda ) := \{x \in {\mathbb {Z}}^3 : \lfloor |x_1|^c \rfloor + \lfloor |x_2|^c \rfloor + \lfloor |x_3|^c \rfloor = \lambda \} \quad \text {with}\quad \lambda \in {\mathbb {Z}}_+; \end{aligned}$$ which can be thought of as a perturbation of the classical Waring problem in three variables. We will use the latter asymptotic formula to study, the main results of this paper, norm and pointwise convergence of the ergodic averages $$\begin{aligned} \frac{1}{\#{\textbf{S}}_{c}^3(\lambda )}\sum _{n\in {\textbf{S}}_{c}^3(\lambda )}f(T_1^{n_1}T_2^{n_2}T_3^{n_3}x) \quad \text {as}\quad \lambda \rightarrow \infty ; \end{aligned}$$ where $$T_1, T_2, T_3:X\rightarrow X$$ are commuting invertible and measure-preserving transformations of a $$\sigma $$ -finite measure space $$(X, \nu )$$ for any function $$f\in L^p(X)$$ with $$p>\frac{11-4c}{11-7c}$$ . Finally, we will study the equidistribution problem corresponding to the spheres $${\textbf{S}}_{c}^3(\lambda )$$ .