Marinov, A., J. Jeliazkov, and T. Ignatov. “About the First Crossing of the Poisson Process With a Curved Upper Boundary”. Ann. Sofia Univ. Fac. Math. And Inf., vol. 91, Dec. 1999, pp. 97-105, https://ftl5.uni-sofia.bg/index.php/fmi/article/view/272.