I'm learning Fourier analysis by Elias M. Stein & Rami Shakarchi and stuck on this step on the page 206. How to prove that $\hat{f}( s\gamma ) e^{2\pi ix\cdot \gamma s}$ is an even function so that we could split and double the integral. Or maybe these halves are equal for another reason?
$\hat{f}$ - Fourier transform of $f$
