Decomposition of Spaces of Periodic Functions into Subspaces of Periodic Functions and Subspaces of Antiperiodic Functions

Abstract: In this paper we prove that the space $ \mathbb{P}p $ of all periodic function of fundamental period $ p $ is a direct sum of the space $ \mathbb{P}{p/2} $ of periodic functions of fundamental period $ p/2 $ and the space $ \mathbb{AP}{p/2} $ of antiperiodic functions of fundamental anti period $ p/2 $. The decomposition can be continued by applying the decomposition process to the successively raising periodic subspaces. It is shown that, under certain condition, a periodic function can be written as a convergent infinite series of anti periodic functions of distinct fundamental anti periods. In addition, we characterize the space of all periodic functions of period $ p \in \mathbb{N} $ in terms of all its periodic and antiperiodic subspaces of integer periods (or anti periods). We show that the elements of a subspace of such a space of periodic functions take a specific form (not arbitrary) of linear combinations of the shifts of the elements of the given space. Lastly, we introduce a lattice diagram named-periodicity diagram for a space of periodic function of a fixed period $ p \in \mathbb{N} $. As a particular example, the periodicity diagram of $ \mathbb{P}{12} $ is shown.