4 5 6 D O C U M E N T 5 3 1 M A Y 1 9 2 9 531. From Élie Cartan Le Chesnay (Seine et Oise), 15 May 1929 Dear Sir, Please excuse me for not having answered your letter of 10 May any sooner [1] I found it only today at the Sorbonne! I am returning the two copies of the articles as you requested, that of your last note and that of Weitzenböck’s.[2] The silence of Mr. Weitzenböck with regard to my work is a bit strange, since he lists a note by Bortolotti in his bibliography, in which he referred several times to my work![3] I quite gladly accept your suggestion of writing an annex to your next article for the Zeitschrift für Physik, a brief history of absolute parallelism.[4] Of course it will treat only geometry, leaving aside all of the analytical apparatus. It would seem to me that there would be some interest in demonstrating the principal problems that arise in the subject of absolute parallelism, showing their relationships to certain problems in mechanics and to the theory of continuous groups. Would you see it as inconvenient if I briefly describe the general geometrical theory in which the no- tion of absolute parallelism can be embedded as a special case? I wouldn’t want to send you something that is much longer than you wish. I thank you for sending me the proofs of your articles when you receive them I will read them with great pleasure. With my sincere regards and admiration, E. Cartan