Maybe for the first time in the existing literature, we investigate here the almost periodic type solutions to the abstract Volterra difference equations depending on several variables. We also investigate the generalized almost periodic type sequences and their applications in a rather detailed manner as well as many new important spaces of (metrically) generalized almost periodic type spaces of sequences and functions. We essenitally apply some results from the theory of C-regularized solution operator families to the abstract Volterra integro-differential-difference equations, contributing also to the theory of fractional calculus and fractional differential equations.

The theory of abstract Volterra integro-differential equations and the theory of abstract Volterra difference equations are very attractive fields of research of many authors. The almost periodic features and the asymptotically almost periodic features of solutions to the abstract Volterra differential-difference equations in Banach spaces have been sought in many research articles published by now.

The main aim of this monograph is to continue the work collected in my monographs published with W. de Gruyter recently by providing several new results about the existence and uniqueness of almost periodic type solutions to the abstract Volterra integro-differential-difference equations which could be solvable or unsolvable with respect to the highest derivative (order). We would like to particularly emphasize that this is probably the first research monograph devoted to the study of almost periodic type solutions to the abstract Volterra difference equations depending on several variables. We also consider here many new important spaces of (metrically) generalized almost periodic type spaces of sequences and functions, and their almost automorphic analogues. It is also worth noting that this is probably the first research monograph which concerns the generalized almost periodic type sequences and their applications in a rather detailed manner; for the first time in the existing literature, we also present here some applications of results from the theory of $C$-regularized solution operator families to the abstract Volterra difference equations.

Fractional calculus and discrete fractional calculus are rapidly growing fields of theoretical and applied mathematics, which are incredibly important in modeling of various real phenomena appearing in different fields like aerodynamics, rheology, interval-valued systems, chaotic systems with short memory and image encryption and discrete-time recurrent neural networks. Many important research results regarding the abstract fractional differential equations and the abstract fractional difference equations in Banach spaces have recently been obtained by a great number of authors from the whole world. In this monograph, we also contribute to the theories of (discrete) fractional calculus, fractional differential-difference equations and multi-dimensional Laplace transform.

Although the monograph is far from being complete, we have decided to quote almost eight hundred and fifty research articles which could be of some importance to the interested readers for further developments of the theory established here.