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 Recognitions: Gold Member Homework Help Science Advisor Staff Emeritus I didn't even notice the k's. Since the original problem uses y(x), let's use k to be the variable conjugate to x: \begin{align*} f(x) &= \frac{1}{\sqrt{2\pi}} \int_{-\infty}^{\infty} F(k)e^{ikx}\, dk \\ F(k) &= \frac{1}{\sqrt{2\pi}} \int_{-\infty}^{\infty} f(x)e^{-ikx}\,dx \end{align*} So consider F'(k).