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 and Eigenvalues of an LTI Systems

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

LTI System

Input	Output	Reason
$\delta (t)$	$h(t)$	Given
$\delta (t-\lambda )$	$h(t-\lambda )$	Time invariance	$x(\lambda )\delta (t-\lambda )$	$x(\lambda )h(t-\lambda )$	Proportionality	$\int _{-}\infty ^{\infty }x(\lambda )\delta (t-\lambda )d\lambda$	$\int _{-}\infty ^{\infty }x(\lambda )h(t-\lambda )d\lambda$	Superposition