Statistics - (Periodicity|Periodic phenomena|Cycle)

About

Periodic phenomena such as:

sound and light waves,
the position and velocity of harmonic oscillators,
sunlight intensity and day length,
and average temperature variations throughout the year. See Time Serie - Seasonality (Cycle detection)

Trigonometric functions are important in modeling periodic phenomena:

All periodic functions can be exactly represented by Fourier Series

A function is periodic, with fundamental period T, if the following is true for all t: <MATH> f(t+T)=f(t) </MATH>