4 6 0 D O C U M E N T S 5 3 5 , 5 3 6 M A Y 1 9 2 9 535. From Norbert Wiener and Manuel  Sandoval Vallarta Cambridge, Massachusetts, 23 May 1929 Dear Professor, The article that we recently published in the Proceedings of the National Academy[1] concerning your unified field theory, contains an unfortunate method- ical error, as is demonstrated in the enclosed communication,[2] which will soon ap- pear in the same journal however, that has no effect on the main results. The introduction of polar coordinates is, regarding the n-Bein frames themselves, not an easy matter, and it is readily understandable how we strayed from the path. One of us will soon send you a new mathematical communication that is some- what relevant to Levi-Civita’s considerations. A preliminary version will soon be published in Nature.[3] With our best regards, your Norbert Wiener M. S. Vallarta 536. From Élie Cartan Le Chesnay (Seine et Oise) 27 avenue Montespan, 24 May 1929 Dear Sir, I am enclosing here the historical summary of absolute parallelism.[1] I would be grateful if you would have a look at it and let me know of any changes, whether in content or in form, which you would judge to be desirable. If there are editorial changes to be made, you need not send me back the manuscript I have a copy here. With my sincere regards and admiration, E. Cartan I would be grateful if you would address your letters, when the occasion arises, preferably to Chesnay