For a 2-port network to be reciprocal

Relative to a given fixed tree of a network

Of the four networks, N₁,N₂, N₃ and N₄ of figure, the networks having identical driving point functions are

A low pass signal x(t) has a spectrum given by $$X(f) = \left\{ {\matrix{ {1 - \left| f \right|/2000,} & {for\,\,\left| f \right|\, \le \,2000\,Hz} \cr {0,} & {elsewhere} \cr } } \right.$$
Assuming that x(t) is ideally sampled at a sampling frequency of 3 kHz, sketch
(i) x(f), and
(ii) the spectrum of the sampled signal for $${\,\left| f \right|\, \le \,\,3\,kHz}$$.