APA   Click to copy
Einstein, A. (1948). Generalized Theory of Gravitation. Reviews of Modern Physics, 20(1), 35–39.

Chicago/Turabian   Click to copy
Einstein, Albert. “Generalized Theory of Gravitation.” Reviews of Modern Physics 20, no. 1 (January 1948): 35–39.

MLA   Click to copy
Einstein, Albert. “Generalized Theory of Gravitation.” Reviews of Modern Physics, vol. 20, no. 1, Jan. 1948, pp. 35–39.

@article{albert1948a,
  title = {Generalized Theory of Gravitation},
  year = {1948},
  month = jan,
  issue = {1},
  journal = {Reviews of Modern Physics},
  pages = {35-39},
  volume = {20},
  author = {Einstein, Albert},
  month_numeric = {1}
}