  In this work, we aim to show that there are generally four possible mapping
functions that can be used to map the time-domain or frequency-domain
representations of an applied voltage input to the resulting time-domain or
frequency-domain electrical charge output; i.e. when the capacitive device is
voltage-charged. Alternatively, there are four more possible combinations when
the device is current-charged. The dual relationship between each pair of
functions for the case of voltage or charge input are provided in terms of
single or double Fourier transforms. All eight system functions coincide with
each other if and only if a constant time- and frequency-independent
capacitance is considered.
