APA (7th ed.) Citation

Pogorelov, A. V., Kra, I., & Zaustinskiy, E. (1979). Hilbert's fourth problem.

Chicago Style (17th ed.) Citation

Pogorelov, A. V., Irwin Kra, and Eugene Zaustinskiy. Hilbert's Fourth Problem. Washington, D.C, 1979.

MLA (9th ed.) Citation

Pogorelov, A. V., et al. Hilbert's Fourth Problem. 1979.