  An overview of the history of projective representations (= spin
representations) of groups, preceded by the prehistory of studies on the theory
of quaternion due to Rodrigues and Hamilton. Beginning with Schur, we cover
many mathematicians until today, and also physicists Pauli and Dirac. This is a
self translation of Appendix A of my book "Introduction to the theory of
projective representations of groups" in Japanese, 2018, Sugakushobo, and may
serve as an introduction to our paper arXiv: 1804.06063 [math.RT] which will
appear in Kyoto J. Math.
