Gasanenko, V. A. “On the Asymptotics of the Sojourn Probability of a Poisson Process Between Two Nonlinear Boundaries That Move Away from One Another”.
Ukrains’kyi Matematychnyi Zhurnal, vol. 53, no. 1, Jan. 2001, pp. 14-22,
https://umj.imath.kiev.ua/index.php/umj/article/view/4216.