A focal index theorem for null geodesics
The authors use the recent approach of \textit{D. Kalish} [Trans. Am. Math. Soc. 308, No.1, 341-348 (1988; Zbl 0652.58016)] to obtain a focal Morse index theorem for a null geodesic initially and terminally perpendicular to space-like submanifolds of arbitrary co-dimension in a general space- time.
scientific article; zbMATH DE number 4165947