Lambek, J. (1958). Goursats Theorem and the Zassenhaus Lemma. Canadian Journal of Mathematics, 10, 45–56. https://doi.org/10.4153/cjm-1958-005-6 
@article{lambek_1958,
  title = {Goursats Theorem and the Zassenhaus Lemma},
  author = {Lambek, Joachim},
  year = {1958},
  journal = {Canadian Journal of Mathematics},
  publisher = {Cambridge University Press},
  volume = {10},
  pages = {45–56},
  doi = {10.4153/cjm-1958-005-6}
}