EINSTEIN, Albert. Autograph inscription, a two-line formula in ink. New York, 1950. 1 page, 12mo, on a single sheet of Plaza Hotel imprinted notepaper.
EINSTEIN WRITES OUT THE FORMULA FOR THE RIEMANN CURVATURE TENSOR
An intriguing inscription of a formula basic to Einstein's work on general relativity. In 1950 Einstein must still have had general relativity very much on his mind. The fundamental mathematical object from which his field equations for the inertio-gravitational field in general relativity are extracted is known as the Riemann curvature tensor. Einstein had begun to seek these field equations as early as 1912, then decided they were physically unacceptable, but finally, in November 1915, returned to them to complete his theory of general relativity. Here, only four years before his death, Einstein once again writes out the fundamental formula of the Riemann curvature tensor, evidently during a stay at New York's famed Plaza Hotel.
John D. Norton, "How Einstein found his field equations, 1912-1915." Historical Studies in the Physical Sciences 14 (1984): 253-316. Reprinted in: Don Howard and John Stachel (eds.), Einstein Studies. Vol. 1. Einstein and the History of General Relativity. Boston: Birkhäuser, 1989, pp. 101-159.
Jürgen Renn, Tilman Sauer, Michel Janssen, John Norton, and John Stachel, The Genesis of General Relativity: Sources and Interpretations. Vol. 1. General Relativity in the Making: Einstein's Zurich Notebook. Dordrecht: Kluwer, forthcoming 
John Stachel, "Einstein's Search for General Covariance, 1912-1915." In: Don Howard and John Stachel (eds.), Einstein Studies. Vol. 1. Einstein and the History of General Relativity. Boston: Birkhäuser, 1989, pp. 63-100.