we can write all the alias frequencies as positive values: f N ( f ) ≜ ≜ | f + N f s | . For example, a snapshot of the lower right frame of Fig.2 shows a component at the actual frequency f and another component at alias f − − 1 ( f ) . As f increases during the animation, f − − 1 ( f ) decreases. The point at which they are equal ( f = f s / 2 ) is an axis of symmetry called the folding frequency, also known as Nyquist frequency.