The signum function is given by

$sgn(x)= \begin{cases} \frac{x}{|x|};x \neq 0& \\ 0;x=0& \end{cases}$

The Fourier series expansion of $sgn (\cos (t) )$ has

1. Only sine terms with all harmonics.
2. Only cosine terms with all harmonics.
3. Only sine terms with even numbered harmonics.
4. Only cosine terms with odd numbered harmonics.

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