Some signals have specific properties which makes computations simpler. For example,

## Even and Odd

A signal is called even (or symmetric) if

Flipping an even signal around amplitude axis results in the same signal. Examples of even signals are and a quadratic signal (see e.g., this Figure).

On the other hand, a signal is called odd (or anti-symmetric) if

An odd signal has symmetry around the origin. Examples of an odd signal are and .

## Periodic and Aperiodic

A signal is periodic if it repeats itself after a certain period .

Both and are examples of periodic signals if is a rational number.

If a signal does not repeat itself forever, there is no value of that satisfies the above periodicity equation. Such a signal is known as aperiodic, an example of which is a unit step signal and most other signals encountered in this text.