in covariant form, although since Einstein made that claim it has been formulated in covariant form by several physicists, including Élie Cartan in 1923 and Kurt Otto Friedrichs in 1927. Charles W. Misner, Kip Thorne, and John Archibald Wheeler, in their textbook Gravitation (1973) Ch. 12 present the covariant version of Newtonian theory. In a letter of 1927 to Karl Försterling Arnold Sommerfeld wrote favorably of Kretschmann's work in relativity and the statistics of electrons, but said that he needed to get a different teaching position (get away from Königsberg) in order to be able to do more research. The issue of whether covariance is a real restriction and if so in what sense appears in various contributions to the philosophical debate concerning Einstein's "hole argument." This argument initially had led Einstein in 1913 for a time to reject generally covariant theories, because a region of space/time without forces would undermine determinism or unique extension of trajectories. He later concluded that space/time points without gravity would not be individuated. It has been claimed also that Kretschmann discovered that the conformal geometry of General Relativity corresponds to the light cone structure, a point rediscovered by and extensively exploited by Hermann Weyl, and since then developed by Jürgen Ehlers and collaborators. Kretschmann's prose is so convoluted and obscure that reception and appreciation of his work was generally delayed. James L. Anderson in the mid-1960s made Kretschmann's work more well-known, though he used it as an object of criticism with respect to Kretschmann's claims concerning the symmetry groups of special and general relativity. Kretschmann published half a dozen less noted papers during the 1920s and early 1930s, the last two in 1934, though he continued to live in Germany for decades. His 1934 paper Beitrag zur Kritik der Blochschen Theorie der Elektrizitätsleitung, although it has only 17 citations as of the end of the year 2021, might be of some historical interest. According to Rudolf Peierls: Major works Kretschmann, Erich. 1915. Über die prinzipielle Bestimmbarkeit der berechtigten Bezugssysteme beliebiger Relativitätstheorien (I), (II). Annalen der Physik 48: 907–942, 943–982. Teil I; Teil II Kretschmann, Erich. 1917. Über den physikalischen Sinn der Relativitätspostulate. A. Einsteins neue und seine ursprüngliche Relativitätstheorie. Annalen der Physik 53: 575–614. Notes References Kretschmann in A. Sommerfeld's correspondence Janssen, Michel, "Einstein’s First Systematic Exposition of General Relativity," on philsci-archive.pitt.edu Norton, John D., "General Covariance and the Foundations of General Relativity: Eight