**Introduction**

So far, I have found (p. 5) the following generating functions of the unsigned Stirling numbers of the first kind:

begin{equation} tag{1} label{1} sum_{l=1}^{n} |S_{1}(n,l)|z^{l} = (z)_{n} = prod_{k=0}^{n-1} (z+k) =: g(z) , end{equation} and begin{equation} tag{2} label{2} sum_{n=l}^{infty} frac{|S_{1}(n,l)|}{n!}z^{n} = (-1)^{l} frac{ln^{l}(1-z)}{l!} .end{equation}

Instead of summing the latter expression over $n$, I’m curious whether there is a simpler or different expression of the latter generating sum when summed over the other indices:

begin{equation} tag{3} label{3} f(z) := sum_{k=1}^{n} frac{|S_{1}(n,k)|}{k!}z^{k} . end{equation} (One could also take the sum from $k=1$ to $k=infty$, as $|S_{1}(n,k)| = 0$ when $k>n$.)

**Work so far**

*Approach 1*

We see that $(3)$ is the egf version of the ogf in $(1)$. So one of the ways I’ve tried to find $f(cdot)$ is by converting the first equation to the third one by applying the inverse Laplace transform to $g(1/s)/s$.

From $(1)$, we see that $g(1/s)/s = frac{(1/s)_{n}}{s}$. The tricky part of find the inverse Laplace transform lies in the numerator. Therefore, I tried to express it in terms of other functions of which the inverse Laplace transform might be known.

For instance, note that the Chu-Vandermonde identity states: begin{equation} tag{4} label{4} _2F_1left(-n,b;c;1right) = frac{(c-b)_{n}}{(c)_{n}} . end{equation}

Now, set $c=1$ and $b = 1 – 1/s$. Then:

begin{equation} tag{5} label{5} _2F_1left(-n,1-1/s;1;1right) = frac{(1/s)_{n}}{n!} =: h(s) . end{equation}

If the inverse Laplace transform of $h(cdot)$ would be known, then I would only have to multiply by $n!$ and convolve it with $ { mathcal{L}^{-1} (1/s) } (t) = u(t) $, where $u(t)$ is the unit step function.

However, I did not find an expression of the inverse Laplace transform of the hypergeometric function in $(5)$ in the “Tables of Laplace Transforms” by Oberhettinger and Badii (1973).

*Approach 2*

Another approach I tried is to note that begin{equation} tag{6} label{6} g(1/s) = (1/s)_{n} = frac{Gamma(1/s + n)}{Gamma(1/s)} . end{equation}

Unfortunately, the inverse laplace transform of begin{equation} tag{7} label{7} q(s) := frac{Gamma(s+a)}{Gamma(s+b)} end{equation} is only given by Oberhettinger and Badii (p. 308) in the case when $Re(b-a) >0$, which is not the case here.

*Approach 3*

Finally, I tried rewriting $g(1/s)/s$ as follows:

begin{align} g(1/s)/s &= frac{1}{s^{2}} cdot frac{1+s}{s} cdot frac{1+2s}{s} dots frac{1+ns}{s} \

&= frac{ prod_{k=1}^{n} (1+ks) }{s^{n+2}} \

&= frac{n! prod_{k=1}^{n}Big{(}s+frac{1}{k} Big{)} }{s^{n+2}}. end{align}

Observe that ${ mathcal{L}^{-1} s^{-(n+2)} }(t) = ( (n+1)! )^{-1} t^{n+1} $, so we are left with finding the inverse Laplace transform of the numerator (and convolve afterwards). However, I have not been able to do so thusfar.

**Questions**

- Is the inverse Laplace transform of $frac{(1/s)_{n}}{s}$ known, or can it be calculated somehow?
- Are there already any other expressions known for $f(cdot)$ that can be found by other means than the ones I laid out so far?

N.B. this is a more elaborate version of a question I asked earlier on MSE.