Supongamos que buscamos para comprobar que
$$\sum_{k=0}^n {n\elegir k}
{pn-n\elegir k} {pn+k\elegir k} = {pn\elegir n}^2.$$
Utilizamos las integrales
$${pn-n\elegir k} =
\frac{1}{2\pi i}
\int_{|z|=\epsilon} \frac{(1+z)^{pn-n}}{z^{k+1}} \; dz$$
y
$${pn+k\elegir k} =
\frac{1}{2\pi i}
\int_{|w|=\epsilon} \frac{(1+w)^{pn+k}}{w^{k+1}} \; ps.$$
Esto produce por la suma
$$\frac{1}{2\pi i}
\int_{|z|=\epsilon} \frac{(1+z)^{pn-n}}{z}
\frac{1}{2\pi i}
\int_{|w|=\epsilon} \frac{(1+w)^{pn}}{w}
\sum_{k=0}^n {n\elegir k} \frac{(1+w)^k}{z^k w^k}
\; dw \; dz
\\ = \frac{1}{2\pi i}
\int_{|z|=\epsilon} \frac{(1+z)^{pn-n}}{z}
\frac{1}{2\pi i}
\int_{|w|=\epsilon} \frac{(1+w)^{pn}}{w}
\left(1+\frac{1+w}{zw}\right)^n
\; dw \; dz
\\ = \frac{1}{2\pi i}
\int_{|z|=\epsilon} \frac{(1+z)^{pn-n}}{z^{n+1}}
\frac{1}{2\pi i}
\int_{|w|=\epsilon} \frac{(1+w)^{pn}}{w^{n+1}}
(1+w+zw)^n
\; dw \; dz.$$
La expansión de la binomial en el interior de la suma obtenemos
$$\sum_{q=0}^n {n\choose q} w^q (1+z)^q$$
que los rendimientos de
$$\sum_{q=0}^n {n\elegir q}
\frac{1}{2\pi i}
\int_{|z|=\epsilon} \frac{(1+z)^{pn-n+p}}{z^{n+1}}
{pn\elegir n-q} \; dz
\\ = \sum_{q=0}^n {n\elegir q}
{pn-n+p\elegir n}
{pn\elegir n-q}.$$
El interior término es
$${n\elegir q}
{pn-n+p\elegir n}
{pn\elegir pn-n+p}
\\ =
\frac{(pn)!}{p!\veces (n-p)! \times
(pn-2n+p)! \times
(n-p)!}
\\ = {pn\elegir n}
\frac{n! \times (pn-n)!}{p!\veces (n-p)! \times
(pn-2n+p)! \times
(n-p)!}
\\ = {pn\elegir n}
{n\elegir q} {pn-n\elegir n-q}.$$
Así es para mostrar que
$$\sum_{q=0}^n {n\choose q} {pn-n\choose n-q} = {pn\choose n}.$$
Esto se puede hacer de forma combinatoria o el uso de la integral
$$\frac{1}{2\pi i}
\int_{|v|=\epsilon} \frac{(1+v)^{pn-n}}{v^{n+1}}
\sum_{q=0}^n {n\elegir q} v^q \; dv
\\ = \frac{1}{2\pi i}
\int_{|v|=\epsilon} \frac{(1+v)^{pn-n}}{v^{n+1}}
(v+1)^n \; dv
\\ = \frac{1}{2\pi i}
\int_{|v|=\epsilon} \frac{(1+v)^{pn}}{v^{n+1}}
= {pn\elegir n}.$$