Q:

How to sum up this series or find a closed expression or argument?

We know the following (Taylor expansion)
$$\sum_{n=0}^\infty \frac{(-1)^n}{n!} (x^2)^n = e^{-x^2}.$$
Which is integrable in $x$ on $\mathbb{R}$. Nevertheless, imagine we instead have:
$$\sum_{n=0}^\infty \frac{(-1)^n}{n!} (x^2)^n \frac{\Gamma(1+n)}{\Gamma(1+a+n)},$$
where $a>0$ is some fixed value. Clearly,
$$\frac{\Gamma(1+n)}{\Gamma(1+a+n)}\leq 1$$
so one should expect that this factor does not contibute to the sum and that the remaining function in $x$ is still integrable on $\mathbb{R}$.
Of course, one can say:
$$\sum_{n=0}^\infty \frac{(-1)^n}{n!} (x^2)^n \frac{\Gamma(1+n)}{\Gamma(1+a+n)}\leq e^{x^2}.$$
Using the inequality, but this implies destroying the $-1$ which is crucial.
Does any one have an idea or argument that can help to either "keep" the $-1$ or to conclude that the remaining function is integrable on $\mathbb{R}$?
Thanks a lot for any ideas you may have! :)

A:

For $a\in\mathbb N$ and $n\in\mathbb N$, we have
$$\frac{\Gamma(1+n)}{\Gamma(1+a+n)}=\frac{n!}{(n+a)!}$$
Notice that
$$\frac{x^n}{n!}\frac{\Gamma(1+n)}{\Gamma(1+a+n)}=\frac{x^n}{n!}\frac{n!}{(n+a)!}=\frac{x^n}{(n+a)!}$$
$$\sum_{n\ge0}\frac{x^n}{n!}\frac{\Gamma(1+n)}{\Gamma(1+a+n)}=\sum_{n\ge0}\frac{x^n}{(n+a)!}=\frac1{x^a}\left(e^x-\sum_{n=0}^{a-1}\frac{x^n}{n!}\right)$$
And one can set bounds on this by seeing that
$$\frac{\Gamma(1+n)}{\Gamma(1+\lceil a\rceil+n)}\le\frac{\Gamma(1+n)}{\Gamma(1+a+n)}\le\frac{\Gamma(1+n)}{\Gamma(1+\lfloor a\rfloor+n)}$$
So your series/function is bounded by
$$\left|\frac1{x^{\lceil a\rceil}}\left(e^x-\sum_{n=0}^{\lceil a\rceil-1}\frac{x^n}{n!}\right)\right|\le|S|\le\left|\frac1{x^{\lfloor a\rfloor}}\left(e^x-\sum_{n=0}^{\lfloor a\rfloor-1}\frac{x^n}{n!}\right)\right|$$
For $x>0$.
And finally let $x\to-x^2$

(Note that the negative sign may cause some problems with the bounds on this.)

