A continuous-time input signal $x(t)$ is an eigenfunction of an LTI system, if the output is
- $k \: x(t)$, where $k$ is an eigenvalue
- $k \: e^{j \omega t} \: x(t)$, where $k$ is an eigenvalue and $e^{j \omega t}$ is a complex exponential signal
- $x(t) \: e^{j \omega t}$, where $e^{j \omega t}$ is a complex exponential signal
- $k \: H(\omega)$, where $k$ is an eigenvalue and $H(\omega)$ is a frequency response of the system