Periodic functions with variable period and their main properties

The examples of periodic signals with variable period are provided, as one of the models of such signals the definition of periodic function with variable period is given. The system of orthogonal trigonometric functions with variable period is constructed. It is believed that in the future the theory of periodic functions with variable period will be properly developed and the functions will be applied in the research tasks of transient modes of technical systems, in the problems of the patient´s functional state assessment, in the evaluation of rehabilitation processes, in sport and space medicine.