The significance of the concept of summability has been evidently demonstrated in various contexts. For instance, the concept of summability is being applied in analytic continuation, quantum mechanics, probability theory, Fourier analysis, approximation theory, and fixed-point theory. The methods of almost summability and statistical summability have become an active area of research in recent years.

The aim of this Special Issue is to focus on recent developments and achievements in summability theory such as sequence spaces and their geometry, statistical summability and statistical approximation, almost summability, fuzzy sequence spaces, matrix summability, compact matrix operators between sequence spaces, and infinite systems of differential and integral equations in sequence spaces, and various applications. We invite authors to submit original research and review articles describing new methods and applications which are directly or indirectly related to summability, function spaces, and sequence spaces. We also welcome research including fixed-point theory, operator theory, statistical summability, and approximation theory.

Potential topics include but are not limited to the following:

• Matrix and non-matrix methods of summability
• Approximation and statistical approximation with applications in computer aided design and optimization
• Polynomials and linear positive operators
• Classical sequence spaces and their matrix domains
• Dual summability methods
• Analytic continuation
• Fixed point theory and applications
• Topological and geometric properties of sequence spaces
• Fuzzy sequence spaces
• Summability methods for q-operators
• Difference sequence spaces and their applications
• Infinite systems of differential, integral, and integro-differential equations on sequence spaces
• Function spaces