Questions on periodic functions, functions $f(x)$ that satisfy the identity $f(x+c)=f(x)$, for some nonzero $c$.
A periodic function is a function that repeats itself in regular intervals, ie, one satisfying $f(x+P)=f(x).$ Here, $P$ is the period.
Graphically, you can see periodicity through translational symmetry. You can see this most easily with trigonometric functions like $\sin$ and $\cos$, which have period $2\pi$. Another example of a periodic function is the nonlinear sawtooth function.