Lev Landau (1927). "Das Dämpfungsproblem in der Wellenmechanik (The Damping Problem in Wave Mechanics)". Zeitschrift für Physik45 (5–6): 430–441. doi:10.1007/bf01343064. Bibcode: 1927ZPhy...45..430L. English translation reprinted in: D. Ter Haar, ed. (1965). Collected papers of L.D. Landau. Oxford: Pergamon Press.
Lev Landau (1927). "Das Dämpfungsproblem in der Wellenmechanik (The Damping Problem in Wave Mechanics)". Zeitschrift für Physik45 (5–6): 430–441. doi:10.1007/bf01343064. Bibcode: 1927ZPhy...45..430L. English translation reprinted in: D. Ter Haar, ed. (1965). Collected papers of L.D. Landau. Oxford: Pergamon Press.
During this period Landau visitied Copenhagen three times: 8 April to 3 May 1930, from 20 September to 22 November 1930, and from 25 February to 19 March 1931 (see Landau Lev biography – MacTutor History of Mathematics).