On some generalizations of the Pollachek–Khinchine formula

For the skip-free Poisson process $\xi(t) \ (t\geq0, \xi(0)=0),$
$$ \xi(t)=at+S(t), \ a<0, \ S(t)=\sum_{k\leq\nu(t)}\xi_k, \ \xi_k>0, \xi(0)=0, $$
where $\nu(t)$ is a simple Poisson process with intensity $\lambda>0,$ the moment generating function (m.g.f.) of $\xi^+=\sup_{0\leq t<\infty}\xi(t)$ is defined by the well-known Pollachek–Khinchine formula under the condition $m=E\xi (1)<0$ (see [1-3]).
For a homogeneous process $\xi(t)$ with bounded variation, we establish prelimit and limit generalizations of this formula, which define the m.g.f. of
$$ \xi^+(\theta_s)=\sup_{0\leq t\leq\theta_s}\xi (t), \ \xi^+=\lim_{s\to0}\xi^+(\theta_s) \left(P\{\theta_s>t\}=e^{-st}, \ s>0\right). $$
These generalizations are essentially based on the condition $P\{\tau ^+(0)= \gamma ^+(0)=0\}=0,$ where $(\tau ^+(0),\gamma ^+(0))$ is the initial ladder point of $\xi (t)\ (t\geq0, \xi(0)=0)$.
Some another relations for the m.g.f. of $\xi^+(\theta_s)$ and $\xi^+$ are established for the general lower semicontinuous process $\xi(t)$ on the base of results in [3-5].