Let $\widehat{f}(\xi)$ be Fourier transform of $f$ given by \begin{align} \widehat{f}(\xi)=\int_{\mathbb{R}^n} e^{ix\cdot\xi}f(x)dx. \end{align} Suppose that $\widehat{f}(\xi)$ is nonnegative and locally integrable function, easily seems (by inverse Fourier transform) that \begin{align} \Vert f\Vert_{L^{\infty}} \leq \Vert \widehat{f}\Vert_{L^1}. \end{align} How to show that there is a positive constant $c>0$ such that \begin{align} \Vert \widehat{f}\Vert_{L^1}\leq c \Vert f\Vert_{L^{\infty}}. \end{align}
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If $\hat f$ is nonnegative, then (up to a factor), $$f(0)=\int \hat f=\Vert \hat f \Vert_1 = \Vert f \Vert_\infty.$$ 

