Braunschweig & Berlin, Germany: Friedr. Vieweg & Sohn/Julius Springer, 1922, 1923, 1924. Hardcover. First Edition. Volume 10: iv, 413 pp. 8vo; Volume 16: iv, 409,  pp. 8vo; Volume 21: iv, 382 pp, 8vo. Each volume is ex-library with brown patterned paper boards, lighter brown cloth spines and corner tips; gold embossed titling to spine. Library stamps within, including stamps on title page of each volume, very clean with card pocket remaining on the rear pastedown of each volume. All text in German. Very Good.
Alexander Alexandrovich Friedmann (1888-1925) was a Russian mathematician and physicist who built upon Einstein's theory of relativity and further expanded his own theories that the universe has both homogeneous (looks the same from every location) and isotropic (looks the same in every direction). The following articles expand upon these theories: “Über die Krümmung des Raumes” (“On the Curvature of Space“) by Alexander Friedmann (Zeitschrift für Physik 10, pp. 377-386, 1922). “Notiz du der Arbeit von A. Friedmann ‘Über die Krümmung des Raumes’” (“Note on the work of A. Friedmann ‘On the Curvature of Space’”) by Albert Einstein (Zeitschrift für Physik 16, p. 228, 1923). “Über die Möglichkeit einer Welt mit konstanter negativer Krümmung des Raumes” (“On the possibility of a world with constant negative curvature of space”) by Alexander Friedmann (Zeitschrift für Physik 21 pp. 326-332, 1924).