Inverse Laplace Transform of $e^{-\sqrt{s^2+s}}$.

I will illustrate a slightly different, more rigorous derivation of the ILT result here, which is

$$\frac1{i 2 \pi} \int_{c-i \infty}^{c+i \infty} ds \, e^{-\sqrt{s (1+s)}} \, e^{s t} = \frac12 e^{-t/2} \frac{I_1 \left ( \frac12 \sqrt{t^2-1} \right )}{\sqrt{t^2-1}} H(t-1) $$

where

$$H(t-1) = \begin{cases} 1 \quad t \gt 1 \\ 0 \quad t \le 1 \end{cases} $$

To demonstrate where this result comes from, I am going to define the following contour integral:

$$ \oint_C dz \, e^{-\sqrt{z (1+z)}} \, e^{t z} $$

where we will figure out the allowed values of $t$ and the contour $C$ is as follows:

where the radius of the large arc is $R$ and the radii of the small arcs are each $\epsilon$. By Cauchy's theorem, this contour integral is zero. On the other hand, we can write out the contour integral in the limit as $\epsilon \to 0$ as follows:

$$ \int_{c-i \sqrt{R^2-c^2}}^{c+i \sqrt{R^2-c^2}} ds \, e^{-\sqrt{s (1+s)}} \, e^{s t} + i R \int_{\pi/2 - \arcsin{(c/R)}}^{\pi} d\theta \, e^{i \theta} \, e^{-R e^{i \theta} \left (1+\frac1{R e^{i \theta}} \right )^{1/2}} e^{R t e^{i \theta}} \\+ e^{i \pi} \int_R^{1} dx \, e^{\sqrt{x (x-1)}} e^{-x t} + e^{i \pi} \int_1^0 dx \, e^{-i \sqrt{x (1-x)}} e^{-x t} \\ + e^{-i \pi} \int_0^1 dx \, e^{i \sqrt{x (1-x)}} e^{-x t} + e^{-i \pi} \int_1^R dx \, e^{\sqrt{x (x-1)}} e^{-x t} \\ + i R \int_{\pi}^{3 \pi/2 + \arcsin{(c/R)}} d\theta \, e^{i \theta} \, e^{-R e^{i \theta} \left (1+\frac1{R e^{i \theta}} \right )^{1/2}} e^{R t e^{i \theta}}$$

It should be clear that the third and sixth integrals cancel. Further, we can deduce the values of $t$ for which the ILT is defined by combining the second and seventh integrals:

$$\begin{align} i R \int_{\pi/2-\arcsin{(c/R)}}^{3 \pi/2+\arcsin{(c/R)}} d\theta \, e^{i \theta} \, e^{-R e^{i \theta} \left (1+\frac1{R e^{i \theta}} \right )^{1/2}} e^{R t e^{i \theta}} &= R \int_{\frac{c}{R} + i \sqrt{1-\frac{c^2}{R^2}}}^{\frac{c}{R} - i \sqrt{1-\frac{c^2}{R^2}}} d\zeta \, e^{-R \zeta \left ( 1 + \frac1{R \zeta} \right)^{1/2}} e^{R t \zeta} \\ &=_{R \to \infty} R e^{-1/2} \int_{\frac{c}{R}+i}^{\frac{c}{R}-i} d\zeta \, e^{R (t-1) \zeta} \\ &= -i 2 e^{c (t-1)} e^{-1/2} \frac{\sin{R (t-1)}}{t-1} \\ &=_{R \to \infty} -i 2 \pi e^{c (t-1)} e^{-1/2} \delta(t-1) \end{align} $$

It should be noted that the above integrals converge as $R \to \infty$ only when $t \gt 1$. Accordingly, there will be a $H(t-1)$ term in the final result. Because $\lim_{x \to 0} H(x) \delta(x) = 0$, we may ignore the delta function contribution in this case. (When $t \lt 1$, the ILT is zero.)

Now, as $R \to \infty$ we are down to three integrals which we can relate by Cauchy's theorem:

$$ \int_{c-i \infty}^{c+i \infty} ds \, e^{-\sqrt{s (1+s)}} \, e^{s t} = \int_0^1 dx \, e^{i \sqrt{x (1-x)}} e^{-x t} - \int_0^1 dx \, e^{-i \sqrt{x (1-x)}} e^{-x t}$$

so that the ILT for $t \gt 1$ is now in terms of a single, real integral:

$$\frac1{i 2 \pi} \int_{c-i \infty}^{c+i \infty} ds \, e^{-\sqrt{s (1+s)}} \, e^{s t} = \frac1{\pi} \int_0^1 dx \, \sin{\left ( \sqrt{x (1-x)} \right )} e^{-t x} H(t-1)$$

We may now focus on evaluating this real integral. We can put the integral into a more familiar form with the substitution $x=\sin^2{(\theta/2)}$:

$$\begin{align}\frac1{\pi} \int_0^1 dx \, \sin{\left ( \sqrt{x (1-x)} \right )} e^{-t x} &= \frac1{2 \pi} e^{-t/2} \int_0^{\pi} d\theta \, \sin{\theta} \, \sin{\left ( \frac12 \sin{\theta} \right )} e^{(t/2) \cos{\theta}} \\ &= \frac1{4 \pi} e^{-t/2} \operatorname{Im} \int_{-\pi}^{\pi} d\theta \, \sin{\theta} \, e^{i \frac12 \sin{\theta}} e^{(t/2) \cos{\theta}} \end{align} $$

We may evaluate the integral on the right by expressing it as a complex integral and taking it from there...

$$\begin{align} \int_{-\pi}^{\pi} d\theta \, \sin{\theta} \, e^{i \frac12 \sin{\theta}} e^{(t/2) \cos{\theta}} &= -i \oint_{|z|=1} \frac{dz}{z} \left ( \frac{z-z^{-1}}{i 2} \right ) e^{(t/2) \left (\frac{z+z^{-1}}{2} \right )} e^{(i/2) \left (\frac{z-z^{-1}}{i 2} \right )} \\ &= -\frac12 \oint_{|z|=1} \frac{dz}{z} \left ( z-z^{-1} \right ) e^{\frac14 (t+1) z + \frac14 (t-1) z^{-1}} \end{align} $$

The contour integral is equal to $i 2 \pi$ times the sum of the residues of the poles inside of the unit circle. In this case, the integrand has an essential singularity at $z=0$. To compute the residue at $z=0$, we expand the exponential in a Laurent series and we need only determine the coefficient of $z^{-1}$ in that expansion. In short, we wish to compute the coefficient of $z^0$ in the Laurent expansion of

$$\left ( z-z^{-1} \right ) e^{a z + b z^{-1}} $$

where $a=\frac14 (t+1)$ and $b=\frac14 (t-1)$. This expansion looks like

$$\begin{align} \left ( z-z^{-1} \right ) e^{a z + b z^{-1}} &= \left ( z-z^{-1} \right ) \sum_{k=0}^{\infty} \frac1{k!} \left ( a z + b z^{-1} \right )^k \\ &= \left ( z-z^{-1} \right ) \sum_{k=0}^{\infty} \frac1{k!} \sum_{m=0}^k \binom{k}{m} (a z)^m \left ( b z^{-1} \right )^{k-m} \\ &= \left ( z-z^{-1} \right ) \sum_{k=0}^{\infty} b^k \sum_{m=0}^k \frac1{m! (k-m)!} \left ( \frac{a}{b} \right )^m z^{2 m-k} \end{align} $$

To simplify things a bit, we note that only odd values of $k$ will result in a nonzero coefficient of $z^0$. We can then rewrite that last sum as

$$ \left ( z-z^{-1} \right ) e^{a z + b z^{-1}} = \left ( z-z^{-1} \right ) \sum_{k=0}^{\infty} b^{2 k+1} \sum_{m=0}^k \frac1{m! (2 k+1-m)!} \left ( \frac{a}{b} \right )^m z^{2 m-2 k - 1} $$

We have nonzero coefficients of $z^0$ when $m=k$ or $m=k+1$. Therefore, by the residue theorem, the integral above is equal to

$$\int_{-\pi}^{\pi} d\theta \, \sin{\theta} \, e^{i \frac12 \sin{\theta}} e^{(t/2) \cos{\theta}} = -\frac12 i 2 \pi (b-a) \sum_{k=0}^{\infty} \frac{(a b)^k}{k! (k+1)!} = -i \pi \frac{b-a}{\sqrt{a b}} I_1 \left ( 2 \sqrt{a b} \right )$$

Plug in $a=\frac14 (t+1)$ and $b=\frac14 (t-1)$ and we find that

$$\frac1{\pi} \int_0^1 dx \, \sin{\left ( \sqrt{x (1-x)} \right )} e^{-t x} = \frac12 e^{-t/2} \frac{I_1 \left ( \frac12 \sqrt{t^2-1} \right )}{\sqrt{t^2-1}} $$

and the ILT is as asserted above.