TITLE: Inverse Laplace Transform of $(s+1)/z^s$
QUESTION [5 upvotes]: I'm trying to compute this ILT
$$\mathcal{L}^{-1}\left\{\frac{s+1}{z^s}\right\},$$
where $|z|>1$. However, I'm not sure this is possile? Any help would be appreciated.

REPLY [2 votes]: This is an odd one.  I worked the actual Bromwich integral directly because the residue theorem is no help here.  You get something in terms of $c$, the offset from the imaginary axis of the integration path.  So I appealed to something more basic.  Consider
$$\hat{f}(s) = \int_0^{\infty} dt \: f(t) e^{-s t}$$
Just the plain Laplace transform of some function $f$.  Let's throw away any conditions on continuity, etc. on $f$, and consider
$$f(t) = \delta(t-\log{z})$$
for some $z$.  Then
$$\hat{f}(s) = z^{-s}$$
Interesting. Now consider $f(t) = \delta'(t-\log{z})$; then
$$\hat{f}(s) = s z^{-s} + \delta(-\log{z})$$
It follows that
$$\mathcal{L}\left\{\delta(t-\log{z})+ \delta'(t-\log{z})\right\} = \frac{s+1}{z^s} + \delta(-\log{z})$$
Therefore
$$\mathcal{L}^{-1}\left\{\frac{s+1}{z^s}\right\} = \delta(t-\log{z})+ \delta'(t-\log{z}) - \delta(-\log{z}) \delta(t)$$