FRIEDMANN, Aleksandr Aleksandrovich. & Albert Einstein. Über die Krümmung des Raumes [and] Über die Möglichkeit einer Welt mit konstanter negativer Krümmung des Raumes [and Einstein's two remarks to the first paper:] Bemerkung ... [and] Notiz ...
Berlin: Julius Springer, 1922-1924. All first editions.

In 'Über die Krümmung des Raumes' Friedmann derived the non-stationary solutions to Einstein's field equations. Einstein quickly responded in a short comment ('Bemerkung'), in which he expressed his suspicion of such a model of the Universe and apparently pointed out an error in Friedmann's calculations. However, Friedmann then wrote a letter to Einstein, in which he enclosed his full calculations. Shortly after this, Einstein submitted a short notice ('Notiz'), in which he admitted that he himself had performed a calculation error and that Friedmann's solutions, which shed new light on the matter, were valid. Friedmann's expanding universe model was corroborated by Edwin Hubble's red-shift observations in 1929. In 'Über die Möglichkeit einer Welt mit konstanter negativer Krümmung des Raumes' Friedmann derived the Friedmann-equations and demonstrated that he had command of all three Friedmann-models describing positive, zero and negative curvature respectively, nearly a decade before the independent discoveries of the same models by Lemaître, Robertson, and Walker.

Extracted from: Zeitschrift für Physik, Vol.10, pp.377-86), Vol.11, p.326, Vol. 16, p.228, Vol. 21, pp.326-332. 8vo: 228x156mm. The four papers bound with their corresponding volume-titlepage in a recent marbled boards.