<s>[INST] A Stirling number identity Let $s(n,j)$ denote the signed Stirling numbers of the first kind and $S(n,j)$ the Stirling numbers of the second kind.
I need the following (probably trivial) identity
$\sum\limits_{j = 0}^n {s(n,j)S(m + j,k)}  = 0$ for $k < n$ and $\sum\limits_{j = 0}^n {s(n,j)S(m + j,n)}  = {n^m},$ but don't see how to prove it-
 [/INST] We seek to evaluate (note that this is zero by inspection when $k\gt
n+m$):
$$\sum_{j=0}^n (-1)^{n+j} {n\brack j} {m+j\brace k}$$
where $k\le n.$ Using standard EGFs this becomes
$$n! [z^n] \sum_{j=0}^n (-1)^{n+j}
\frac{1}{j!} \left(\log\frac{1}{1-z}\right)^j
(m+j)! [w^{m+j}] \frac{(\exp(w)-1)^k}{k!}
\\ = (-1)^n n! m!  [z^n] \sum_{j=0}^n (-1)^{j}
{m+j\choose j} \left(\log\frac{1}{1-z}\right)^j
\\ \times \frac{1}{2\pi i} \int_{|w|=\gamma}
\frac{1}{w^{m+j+1}} \frac{(\exp(w)-1)^k}{k!} \; dw
\\ = (-1)^n n! m!  [z^n]
\frac{1}{2\pi i} \int_{|w|=\gamma}
\frac{1}{w^{m+1}} \frac{(\exp(w)-1)^k}{k!}
\\ \times \sum_{j=0}^n (-1)^{j}
{m+j\choose j} \left(\log\frac{1}{1-z}\right)^j \frac{1}{w^j} \; dw.$$
Now $\left(\log \frac{1}{1-z}\right)^j = z^j+\cdots$ so the coefficient
extractor $[z^n]$ enforces the upper limit of the sum:
$$(-1)^n n! m!  [z^n]
\frac{1}{2\pi i} \int_{|w|=\gamma}
\frac{1}{w^{m+1}} \frac{(\exp(w)-1)^k}{k!}
\\ \times \sum_{j\ge 0} (-1)^{j}
{m+j\choose j} \left(\log\frac{1}{1-z}\right)^j
\frac{1}{w^j} \; dw
\\ = (-1)^n n! m! \frac{1}{2\pi i} \int_{|z|=\epsilon}
\frac{1}{z^{n+1}}
\frac{1}{2\pi i} \int_{|w|=\gamma}
\frac{1}{w^{m+1}} \frac{(\exp(w)-1)^k}{k!}
\\ \times \sum_{j\ge 0} (-1)^{j}
{m+j\choose j} \left(\log\frac{1}{1-z}\right)^j
\frac{1}{w^j} \; dw \; dz
\\ = (-1)^n n! m! \frac{1}{2\pi i} \int_{|z|=\epsilon}
\frac{1}{z^{n+1}}
\\ \times \frac{1}{2\pi i} \int_{|w|=\gamma}
\frac{1}{w^{m+1}} \frac{(\exp(w)-1)^k}{k!}
\frac{1}{(1+\frac{1}{w}\log\frac{1}{1-z})^{m+1}} \; dw \; dz
\\ = (-1)^n n! m! \frac{1}{2\pi i} \int_{|z|=\epsilon}
\frac{1}{z^{n+1}}
\\ \times \frac{1}{2\pi i} \int_{|w|=\gamma}
\frac{(\exp(w)-1)^k}{k!}
\frac{1}{(w+\log\frac{1}{1-z})^{m+1}} \; dw \; dz.$$
Now observe that for the geometric series in $j$ to converge we must have
$|\log\frac{1}{1-z}| \lt |w|.$ Note that with $\log \frac{1}{1-z} = z +
\cdots$ the image of $|z|=\epsilon$ makes one turn around the origin, a
circle of radius $\epsilon$ plus additional lower order fluctuations. We
therefore  choose $\epsilon$ to shrink this pseudo-circle to be entirely
contained  in $|w|=\gamma.$ With this choice the pole at
$-\log\frac{1}{1-z}$ is inside the contour in $w.$ We thus require
$$\frac{1}{k! \times m!}
\left(\sum_{q=0}^k {k\choose q} (-1)^{k-q} \exp(qw)\right)^{(m)}
= \frac{1}{k! \times m!}
\sum_{q=0}^k {k\choose q} (-1)^{k-q} q^m \exp(qw).$$
Evaluating the integral in $w$ we find
$$(-1)^n \frac{n!}{k!} \frac{1}{2\pi i} \int_{|z|=\epsilon}
\frac{1}{z^{n+1}}
\sum_{q=0}^k {k\choose q} (-1)^{k-q} q^m (1-z)^q \; dz$$
which is
$$\bbox[5px,border:2px solid #00A000]{
\frac{n!}{k!}
\sum_{q=0}^k {k\choose q} {q\choose n} (-1)^{k-q} q^m.}$$
Now when $k\lt n$ we have ${q\choose n}= 0$ so the entire sum vanishes
as claimed. We get just one term when $k=n$ namely
$$\frac{n!}{n!} {n\choose n} {n\choose n} (-1)^{n-n} n^m = n^m$$
also as claimed. This concludes the argument.
</s>