Focused on the latest achievements on the topics of sequence spaces, summability methods, approximation theory and their applications, the aim of this Special Issue is to present new developments in the theory of function spaces, approximation theory along with the theory of sequence spaces and applications in various fields of pure and applied mathematics.

Recently, the Banach sequences spaces are being used to study the solvability of infinite systems of differential and integral equations. In this Special Issue, we particularly welcome submissions that study the solvability of operator equations in Banach function spaces. Besides, several methods of summability are also being used in the study of approximation results by positive linear operators. Approximation theory itself is a vast subject which has a huge number of applications in mathematics, physics and engineering.

We would like to invite original research as well as review articles on the following and related topics.

Potential topics include but are not limited to the following:

• Sequence spaces and their topological and geometric properties
• Special summability methods in the space of functions
• Positive linear operators and approximation methods
• Korovkin’s type approximation
• Measures of noncompactness and their applications in characterizing compact matrix operators
• Applications to differential, integral, functional integral and integro-differential equations in sequence spaces and function spaces