Even rational Parseval integral (source code)

= Even rational Parseval integral
{c}

Applying the Parseval identity to the repeated exponential convolution gives
$$
\int_{-\infty}^{\infty}\frac{dk}{(1+k^2)^{n+1}}
=\frac{\pi(2n)!}{2^{2n}(n!)^2}
=\frac\pi{4^n}\binom{2n}{n}.
$$