Linear Time Invariant System

Linear Time Invariant Systems (LTI Systems)

A linear time invariant system is one that is linear (superposition and proportionality apply) and one that doesn't change with time. For example a circuit with fixed capacitors, resistors, and inductors having an input and an output is linear and time invariant. If a capacitor changed value with time, then it would not be time invariant.

Eigenfunctions of an LTI System

It is an interesting exercise to show that $e^{j\omega t}$ are eigenfunctions of any LTI system. The eigenvalues are $\omega$.