Ciaglia, Florio M. Max Planck Institute for Mathematics in the Sciences, Max Planck Society;
Ciaglia, F. M., Ibort, A., & Marmo, G. (2019). Schwinger's picture of quantum mechanics I: Groupoids. International journal of geometric methods in modern physics, 16(8): 1950119. doi:10.1142/S0219887819501196.