TeX source:

\| \hat{X}(t) \| = \| \frac{\sin(\pi f T_{c})}{\pi f T_{c}} \sin(2 \pi f t - \pi f T_{c}) \| = \| \sinc(f T_{c}) \sin(2 \pi f t - \pi f T_{c}) \| \le \| \sinc(f T_{c}) \|