The continuous-time sinusoid
x(t) = cos(300ft) is sampled every Ts = 2.0 ms, so that
x[n] = x(0.002n)
For what other values of f in the range 0 Hz to 2.0 KHz does the sinusoid v(t) = cos(2*pi ft) produce the same samples as x(t) (i.e., v[·] = x[·]) when sampled at the same rate?