= Solution
Uniform samples at $t_n=n/f_s$ cannot distinguish <frequencies> separated by integer multiples of $f_s$:
$$
e^{2\pi i(f+kf_s)t_n}=e^{2\pi ift_n},\qquad k\in\mathbb Z.
$$
For a real <cosine>, opposite <frequencies> also coincide. Thus an unknown high-frequency component can appear in the sampled record at its folded <frequency> $f_{\rm alias}\in[0,f_s/2]$. For example, \b[2 Hz and 8 Hz <cosines> give identical samples at 10 samples per second]. This ambiguity is <aliasing>, rather than <uniform quantization error>.
\Image[/past-exam-of-the-physics-a-course-of-the-university-of-cambridge/2011/paper-2-aliasing.png]
{title=Two signals, identical samples}
The <Nyquist–Shannon sampling theorem> permits reconstruction of an ideally <bandlimited function> of <bandwidth> $B$ when $f_s>2B$. Use an analogue <anti-aliasing filter> before conversion and allow a transition band between the desired passband and the <frequencies> that fold into it. Sampling faster relaxes the filter requirement. Digital filtering after undersampling cannot separate already identical sample sequences. Finite observation time, <clock jitter>, <noise> and imperfect filter rejection impose further practical limits.
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