Linear Filters
A filter which [see page 3, obeys] the principle of super position.
Principle of super position: the input is linearly proportional to the output.
\begin{align*}
x\_1[n] &\longrightarrow y\_1[n] \\\\
x\_1[n] + x\_2[n] &\longrightarrow y\_1[n] + y\_2[n] \\\\a * x_1[n] + b * x_2[n] &\longrightarrow a * y_1[n] + b * y_2[n] \end{align*}