The book states that: $$2\pi \hat{f}(n) = \frac{-1}{n^{2}} \int_{0}^{2\pi} {f^{\prime \prime}(\theta) e^{-in\theta} d\theta}$$
Therefore, $$2\pi |n|^{2} |\hat{f}(n)| \leq \left| \int_{0}^{2\pi} {f^{\prime \prime}(\theta) e^{-in\theta} d\theta}\right|$$
I do not know how, could anyone explain to me please?
thanks.