l x x I N T R O D U C T I O N T O V O L U M E 1 6 Theory” (Doc. 368), which began by crediting both of them for pointing out an error in his earlier paper. The “Continuation” remained only one of several extant drafts of revisions (Doc. 367) and pertinent calculations (Docs. 396, 434). Further correspondence with Müntz (Docs. 377, 448, 451, 455) led to the com- position of a draft (Doc. 452) and final version (Doc. 459) of another paper in which Einstein reverted to his original approach of deriving the teleparallel field equations from a Hamiltonian principle. It was submitted to the Academy on 21 March 1929 and published under the title “Unified Field Theory and Hamilton’s Principle” (Einstein 1929dd [Doc. 459]). This would be the last published state- ment on the teleparallel approach by Einstein completed in the time period covered by this volume. In addition to dominating the intense correspondence with an inner circle of close collaborators such as Grommer, Müntz, and Lanczos, the teleparallel theory also met with considerable interest from contemporary mathematicians and trig- gered several epistolary exchanges. Soon after the publication of his first two notes on the subject (Docs. 216, 219), Einstein received a letter from the mathematician Roland Weitzenböck (Doc. 246) who pointed out that the mathematics of the teleparallel approach was not at all new. In particular, he had published on the teleparallel affine connection, and listed a number of mathematical papers by himself and others: Giuseppe Vitali, George Griss, Machgielis Euwe, Enio Bortolotti, and Luther Pfahler Eisenhart. Their pa- pers dealt with the theory of parallel displacement and the differential invariants to be constructed from a tetrad field. He also made some technical comments on Einstein’s papers and suggested publishing a note. In his response, Einstein admit- ted that he had not been aware of this literature and agreed to present a note by Weitzenböck for publication in the Prussian Academy Proceedings “to clarify this point” (Doc. 248). Weitzenböck sent his note, writing that he had included more than he had “originally intended” (Doc. 251). In his response, Einstein accepted Weitzenböck’s paper (Weitzenböck 1928) but also refuted Weitzenböck’s sugges- tion that Einstein had missed one of the relevant invariants. He also adopted, for some time, Weitzenböck’s notation for the tetrads. When Einstein’s new approach to unified theory became known to a wider public, it became clear that the mathematics of teleparallel geometry was anything but new to the mathematicians. Already in October 1928, Leopold Infeld had alerted Einstein to the fact that the geometry was treated as a special case in